chapter

T he oxidation of long-chain fatty acids to acetyl-CoAis a central energy-yielding pathway in many organ-

isms and tissues. In mammalian heart and liver, forexample, it provides as much as 80% of the energeticneeds under all physiological circumstances. The elec-trons removed from fatty acids during oxidation passthrough the respiratory chain, driving ATP synthesis;the acetyl-CoA produced from the fatty acids may becompletely oxidized to CO2 in the citric acid cycle, re-sulting in further energy conservation. In some speciesand in some tissues, the acetyl-CoA has alternative fates.In liver, acetyl-CoA may be converted to ketone bod-ies—water-soluble fuels exported to the brain and othertissues when glucose is not available. In higher plants,acetyl-CoA serves primarily as a biosynthetic precursor,only secondarily as fuel. Although the biological role offatty acid oxidation differs from organism to organism,the mechanism is essentially the same. The repetitivefour-step process, called � oxidation, by which fattyacids are converted into acetyl-CoA is the main topic ofthis chapter.

In Chapter 10 we described the properties of tria-cylglycerols (also called triglycerides or neutral fats)

that make them especially suitable as storage fuels. Thelong alkyl chains of their constituent fatty acids are es-sentially hydrocarbons, highly reduced structures withan energy of complete oxidation (~38 kJ/g) more thantwice that for the same weight of carbohydrate or pro-tein. This advantage is compounded by the extreme insolubility of lipids in water; cellular triacylglycerols aggregate in lipid droplets, which do not raise the osmolarity of the cytosol, and they are unsolvated. (Instorage polysaccharides, by contrast, water of solvationcan account for two-thirds of the overall weight of thestored molecules.) And because of their relative chem-ical inertness, triacylglycerols can be stored in largequantity in cells without the risk of undesired chemicalreactions with other cellular constituents.

The properties that make triacylglycerols good stor-age compounds, however, present problems in their roleas fuels. Because they are insoluble in water, ingestedtriacylglycerols must be emulsified before they can bedigested by water-soluble enzymes in the intestine, andtriacylglycerols absorbed in the intestine or mobilizedfrom storage tissues must be carried in the blood boundto proteins that counteract their insolubility. To over-come the relative stability of the COC bonds in a fattyacid, the carboxyl group at C-1 is activated by attach-ment to coenzyme A, which allows stepwise oxidationof the fatty acyl group at the C-3, or �, position—hencethe name � oxidation.

We begin this chapter with a brief discussion of thesources of fatty acids and the routes by which they travelto the site of their oxidation, with special emphasis onthe process in vertebrates. We then describe the chem-ical steps of fatty acid oxidation in mitochondria. Thecomplete oxidation of fatty acids to CO2 and H2O takesplace in three stages: the oxidation of long-chain fattyacids to two-carbon fragments, in the form of acetyl-CoA(� oxidation); the oxidation of acetyl-CoA to CO2 in the citric acid cycle (Chapter 16); and the transfer of

FATTY ACID CATABOLISM17.1 Digestion, Mobilization, and Transport

of Fats 632

17.2 Oxidation of Fatty Acids 637

17.3 Ketone Bodies 650

Jack Sprat could eat no fat,His wife could eat no lean,And so between them both you see,They licked the platter clean.

—John Clarke, Paroemiologia Anglo-Latina(Proverbs English and Latin), 1639

17

631

electrons from reduced electron carriers to the mitochon-drial respiratory chain (Chapter 19). In this chapter wefocus on the first of these stages. We begin our discus-sion of � oxidation with the simple case in which a fullysaturated fatty acid with an even number of carbonatoms is degraded to acetyl-CoA. We then look briefly at the extra transformations necessary for the degrada-tion of unsaturated fatty acids and fatty acids with anodd number of carbons. Finally, we discuss variations on the �-oxidation theme in specialized organelles—peroxisomes and glyoxysomes—and two less commonpathways of fatty acid catabolism, � and � oxidation. Thechapter concludes with a description of an alternativefate for the acetyl-CoA formed by � oxidation in verte-brates: the production of ketone bodies in the liver.

17.1 Digestion, Mobilization, and Transport of FatsCells can obtain fatty acid fuels from three sources: fatsconsumed in the diet, fats stored in cells as lipiddroplets, and fats synthesized in one organ for exportto another. Some species use all three sources undervarious circumstances, others use one or two. Verte-brates, for example, obtain fats in the diet, mobilize fatsstored in specialized tissue (adipose tissue, consistingof cells called adipocytes), and, in the liver, convert ex-cess dietary carbohydrates to fats for export to othertissues. On average, 40% or more of the daily energy re-quirement of humans in highly industrialized countriesis supplied by dietary triacylglycerols (although mostnutritional guidelines recommend no more than 30% ofdaily caloric intake from fats). Triacylglycerols providemore than half the energy requirements of some organs,particularly the liver, heart, and resting skeletal muscle.Stored triacylglycerols are virtually the sole source ofenergy in hibernating animals and migrating birds. Pro-tists obtain fats by consuming organisms lower in thefood chain, and some also store fats as cytosolic lipiddroplets. Vascular plants mobilize fats stored in seedsduring germination, but do not otherwise depend on fatsfor energy.

Dietary Fats Are Absorbed in the Small Intestine

In vertebrates, before ingested triacylglycerols can beabsorbed through the intestinal wall they must be con-verted from insoluble macroscopic fat particles to finelydispersed microscopic micelles. This solubilization iscarried out by bile salts, such as taurocholic acid (p.355), which are synthesized from cholesterol in the liver,stored in the gallbladder, and released into the small intestine after ingestion of a fatty meal. Bile salts areamphipathic compounds that act as biological deter-gents, converting dietary fats into mixed micelles of bile salts and triacylglycerols (Fig. 17–1, step 1 ).

Micelle formation enormously increases the fraction oflipid molecules accessible to the action of water-solublelipases in the intestine, and lipase action converts tria-cylglycerols to monoacylglycerols (monoglycerides) anddiacylglycerols (diglycerides), free fatty acids, and glyc-erol (step 2 ). These products of lipase action diffuseinto the epithelial cells lining the intestinal surface (theintestinal mucosa) (step 3 ), where they are recon-verted to triacylglycerols and packaged with dietarycholesterol and specific proteins into lipoprotein aggre-gates called chylomicrons (Fig. 17–2; see also Fig.17–1, step 4 ).

Apolipoproteins are lipid-binding proteins in theblood, responsible for the transport of triacylglycerols,phospholipids, cholesterol, and cholesteryl esters be-tween organs. Apolipoproteins (“apo” means “detached”or “separate,” designating the protein in its lipid-freeform) combine with lipids to form several classes oflipoprotein particles, spherical aggregates with hy-drophobic lipids at the core and hydrophilic protein sidechains and lipid head groups at the surface. Variouscombinations of lipid and protein produce particles ofdifferent densities, ranging from chylomicrons and very-low-density lipoproteins (VLDL) to very-high-densitylipoproteins (VHDL), which can be separated by ultra-centrifugation. The structures of these lipoprotein par-ticles and their roles in lipid transport are detailed inChapter 21.

The protein moieties of lipoproteins are recognizedby receptors on cell surfaces. In lipid uptake from theintestine, chylomicrons, which contain apolipoproteinC-II (apoC-II), move from the intestinal mucosa into thelymphatic system, and then enter the blood, which car-ries them to muscle and adipose tissue (Fig. 17–1, step5 ). In the capillaries of these tissues, the extracellular

enzyme lipoprotein lipase, activated by apoC-II, hy-drolyzes triacylglycerols to fatty acids and glycerol (step6 ), which are taken up by cells in the target tissues

(step 7 ). In muscle, the fatty acids are oxidized for en-ergy; in adipose tissue, they are reesterified for storageas triacylglycerols (step 8 ).

The remnants of chylomicrons, depleted of most oftheir triacylglycerols but still containing cholesterol andapolipoproteins, travel in the blood to the liver, wherethey are taken up by endocytosis, mediated by recep-tors for their apolipoproteins. Triacylglycerols that en-ter the liver by this route may be oxidized to provideenergy or to provide precursors for the synthesis of ke-tone bodies, as described in Section 17.3. When the dietcontains more fatty acids than are needed immediatelyfor fuel or as precursors, the liver converts them to triacylglycerols, which are packaged with specificapolipoproteins into VLDLs. The VLDLs are transportedin the blood to adipose tissues, where the triacylglyc-erols are removed and stored in lipid droplets withinadipocytes.

Chapter 17 Fatty Acid Catabolism632

17.1 Digestion, Mobilization, and Transport of Fats 633

Fatty acids are oxidizedas fuel or reesterified

for storage.

8

Lipoprotein lipase,activated by

apoC-II in the capillary,converts triacylglycerols

to fatty acids and glycerol.

6

5 Chylomicrons movethrough the lymphatic

system andbloodstream

to tissues.

7

Triacylglycerols are incorporated,with cholesterol and apolipoproteins,into chylomicrons.

4

Fats ingestedin diet

Gallbladder

Bile salts emulsifydietary fats in thesmall intestine, forming mixed micelles.

Smallintestine

Intestinal lipasesdegrade triacylglycerols.

Fatty acids and other breakdownproducts are taken up by the intestinal mucosa and convertedinto triacylglycerols.

Intestinalmucosa

Capillary

Chylomicron

Lipoprotein lipase

Fatty acids enter cells.

ApoC-II

Myocyte oradipocyte

ATP

CO2

1

2

3

FIGURE 17–1 Processing of dietary lipids in vertebrates. Digestionand absorption of dietary lipids occur in the small intestine, and thefatty acids released from triacylglycerols are packaged and deliveredto muscle and adipose tissues. The eight steps are discussed in thetext.

FIGURE 17–2 Molecular structure of a chylomicron. The surface isa layer of phospholipids, with head groups facing the aqueous phase.Triacylglycerols sequestered in the interior (yellow) make up more than80% of the mass. Several apolipoproteins that protrude from the sur-face (B-48, C-III, C-II) act as signals in the uptake and metabolism ofchylomicron contents. The diameter of chylomicrons ranges fromabout 100 to 500 nm.

Apolipoproteins

B-48

C-III

C-II

Phospholipids

Triacylglycerols andcholesteryl esters

Cholesterol

Hormones Trigger Mobilization of Stored Triacylglycerols

Neutral lipids are stored in adipocytes (and in steroid-synthesizing cells of the adrenal cortex, ovary, andtestes) in the form of lipid droplets, with a core of sterolesters and triacylglycerols surrounded by a monolayerof phospholipids. The surface of these droplets is coatedwith perilipins, a family of proteins that restrict accessto lipid droplets, preventing untimely lipid mobilization.When hormones signal the need for metabolic energy,triacylglycerols stored in adipose tissue are mobilized(brought out of storage) and transported to tissues(skeletal muscle, heart, and renal cortex) in which fatty

acids can be oxidized for energy production. The hor-mones epinephrine and glucagon, secreted in responseto low blood glucose levels, activate the enzyme adenylylcyclase in the adipocyte plasma membrane (Fig. 17–3),which produces the intracellular second messengercyclic AMP (cAMP; see Fig. 12–13). Cyclic AMP–dependent protein kinase (PKA) phosphorylates perilipin A, and the phosphorylated perilipin causes hormone-sensitive lipase in the cytosol to move tothe lipid droplet surface, where it can begin hydrolyz-ing triacylglycerols to free fatty acids and glycerol. PKA also phosphorylates hormone-sensitive lipase, doublingor tripling its activity, but the more than 50-fold increasein fat mobilization triggered by epinephrine is due pri-marily to perilipin phosphorylation. Cells with defectiveperilipin genes have almost no response to increases incAMP concentration; their hormone-sensitive lipasedoes not associate with lipid droplets.

As hormone-sensitive lipase hydrolyzes triacylglyc-erol in adipocytes, the fatty acids thus released (free

fatty acids, FFA) pass from the adipocyte into theblood, where they bind to the blood protein serum al-

bumin. This protein (Mr 66,000), which makes up abouthalf of the total serum protein, noncovalently binds asmany as 10 fatty acids per protein monomer. Bound tothis soluble protein, the otherwise insoluble fatty acidsare carried to tissues such as skeletal muscle, heart, andrenal cortex. In these target tissues, fatty acids dissoci-ate from albumin and are moved by plasma membranetransporters into cells to serve as fuel.

About 95% of the biologically available energy of tri-acylglycerols resides in their three long-chain fatty acids;only 5% is contributed by the glycerol moiety. The glyc-erol released by lipase action is phosphorylated by glyc-

erol kinase (Fig. 17–4), and the resulting glycerol 3-phosphate is oxidized to dihydroxyacetone phosphate.The glycolytic enzyme triose phosphate isomerase con-verts this compound to glyceraldehyde 3-phosphate,which is oxidized via glycolysis.

Fatty Acids Are Activated and Transported into Mitochondria

The enzymes of fatty acid oxidation in animal cells arelocated in the mitochondrial matrix, as demonstrated in1948 by Eugene P. Kennedy and Albert Lehninger. Thefatty acids with chain lengths of 12 or fewer carbons enter mitochondria without the help of membrane trans-porters. Those with 14 or more carbons, which consti-tute the majority of the FFA obtained in the diet or released from adipose tissue, cannot pass directlythrough the mitochondrial membranes—they must firstundergo the three enzymatic reactions of the carnitine

shuttle. The first reaction is catalyzed by a family ofisozymes (different isozymes specific for fatty acids hav-ing short, intermediate, or long carbon chains) present

Chapter 17 Fatty Acid Catabolism634

Bloodstream

PP

PPP

P

Adipocyte Myocyte

Lipiddroplet

Perilipin

cAMP

PKA

ATP

Hormone

CO2

Serumalbumin

ATP

Fatty acidtransporter

Adenylylcyclase

Fatty acids

1

2

6

8

7

5

3

4

b oxidation,citric acid cycle,respiratory chain

Triacyl-glycerol

P

Hormone-sensitive

lipase

ReceptorG5

FIGURE 17–3 Mobilization of triacylglycerols stored in adipose tis-sue. When low levels of glucose in the blood trigger the release ofglucagon, 1 the hormone binds its receptor in the adipocyte mem-brane and thus 2 stimulates adenylyl cyclase, via a G protein, to produce cAMP. This activates PKA, which phosphorylates 3 the hormone-sensitive lipase and 4 perilipin molecules on the surfaceof the lipid droplet. Phosphorylation of perilipin permits hormone-sensitive lipase access to the surface of the lipid droplet, where 5 ithydrolyzes triacylglycerols to free fatty acids. 6 Fatty acids leave theadipocyte, bind serum albumin in the blood, and are carried in theblood; they are released from the albumin and 7 enter a myocytevia a specific fatty acid transporter. 8 In the myocyte, fatty acids areoxidized to CO2, and the energy of oxidation is conserved in ATP,which fuels muscle contraction and other energy requiring metabo-lism in the myocyte.

in the outer mitochondrial membrane, the acyl-CoA

synthetases, which promote the general reaction

Fatty acid � CoA � ATP 8zy8 fatty acyl–CoA � AMP � PPi

Thus, acyl-CoA synthetases catalyze the formation of athioester linkage between the fatty acid carboxyl groupand the thiol group of coenzyme A to yield a fatty

acyl–CoA, coupled to the cleavage of ATP to AMP andPPi. (Recall the description of this reaction in Chapter13, to illustrate how the free energy released by cleav-age of phosphoanhydride bonds in ATP could be cou-pled to the formation of a high-energy compound; p.XXX.) The reaction occurs in two steps and involves afatty acyl–adenylate intermediate (Fig. 17–5).

Fatty acyl–CoAs, like acetyl-CoA, are high-energycompounds; their hydrolysis to FFA and CoA has a large,negative standard free-energy change (�G�� ≈ �31kJ/mol). The formation of a fatty acyl–CoA is made morefavorable by the hydrolysis of two high-energy bonds inATP; the pyrophosphate formed in the activation reactionis immediately hydrolyzed by inorganic pyrophosphatase(left side of Fig. 17–5), which pulls the preceding activa-tion reaction in the direction of fatty acyl–CoA formation.The overall reaction is

Fatty acid � CoA � ATP Onfatty acyl–CoA � AMP � 2Pi (17–1)

�G�� � �34 kJ/mol

Fatty acyl–CoA esters formed at the cytosolic sideof the outer mitochondrial membrane can be trans-ported into the mitochondrion and oxidized to produceATP, or they can be used in the cytosol to synthesize

17.1 Digestion, Mobilization, and Transport of Fats 635

CH2

HO

C

H

P

O�

CH2OH

HO C H

CH2OH

O

O�O

CH2

C

P

O�

CH2OH

O

O�O

O

CH2

C

P

O�

O

O�O

C OH OH

H

Glycerol

ATP

ADP

NADH � H�

NAD�

D-Glyceraldehyde3-phosphate

Dihydroxyacetonephosphate

L-Glycerol3-phosphate

glycerolkinase

Glycolysis

triose phosphateisomerase

glycerol 3-phosphatedehydrogenase

CH2OH

FIGURE 17–4 Entry of glycerol into the glycolytic pathway.

2Pi

P�O

O

O

O�

P

O

O

O�

P

O

O

O�

Adenosine

R

O�

O

C Fatty acyl–CoA

fatty acyl–CoAsynthetase

Fatty acid

P OO Adenosine

O�

O

OR C

�

�O P

O

O

O�

P

O

O�

O�

AMPinorganicpyrophosphatase

OR C Fatty acyl–adenylate

(enzyme-bound)

S-CoA

CoA-SHPyrophosphate

fatty acyl–CoAsynthetase

ATP

�G�� � �19 kJ/mol �G�� � �15 kJ/mol(for the two-step process)

1

2

MECHANISM FIGURE 17–5Conversion of a fatty acid to a fattyacyl–CoA. The conversion is catalyzedby fatty acyl–CoA synthetase and inorganic pyrophosphatase. Fatty acidactivation by formation of the fattyacyl–CoA derivative occurs in twosteps. In step �1 , the carboxylate iondisplaces the outer two (� and �)phosphates of ATP to form a fattyacyl–adenylate, the mixed anhydrideof a carboxylic acid and a phosphoricacid. The other product is PPi, an excellent leaving group that is immediately hydrolyzed to two Pi,pulling the reaction in the forward direction. In step �2 , the thiol group ofcoenzyme A carries out nucleophilicattack on the enzyme-bound mixedanhydride, displacing AMP and formingthe thioester fatty acyl–CoA. The overall reaction is highly exergonic.

membrane lipids. Fatty acids destined for mitochondrialoxidation are transiently attached to the hydroxyl groupof carnitine to form fatty acyl–carnitine—the secondreaction of the shuttle. This transesterification is cat-alyzed by carnitine acyltransferase I (Mr 88,000), inthe outer membrane. Either the acyl-CoA passesthrough the outer membrane and is converted to thecarnitine ester in the intermembrane space (Fig. 17–6),or the carnitine ester is formed on the cytosolic face ofthe outer membrane, then moved across the outer mem-brane to the intermembrane space—the current evi-dence does not reveal which. In either case, passage intothe intermembrane space (the space between the outerand inner membranes) occurs through large pores(formed by the protein porin) in the outer membrane.The fatty acyl–carnitine ester then enters the matrix byfacilitated diffusion through the acyl-carnitine/carni-

tine transporter of the inner mitochondrial membrane(Fig. 17–6).

In the third and final step of the carnitine shuttle,the fatty acyl group is enzymatically transferred fromcarnitine to intramitochondrial coenzyme A by carni-

tine acyltransferase II. This isozyme, located on theinner face of the inner mitochondrial membrane, re-generates fatty acyl–CoA and releases it, along with freecarnitine, into the matrix (Fig. 17–6). Carnitine reen-ters the intermembrane space via the acyl-carnitine/car-nitine transporter.

CH2 CH CH2

CH3

N�

CH3

COO�

Carnitine

CH3

OH

This three-step process for transferring fatty acidsinto the mitochondrion—esterification to CoA, transes-terification to carnitine followed by transport, and trans-esterification back to CoA—links two separate pools ofcoenzyme A and of fatty acyl–CoA, one in the cytosol,the other in mitochondria. These pools have differentfunctions. Coenzyme A in the mitochondrial matrix islargely used in oxidative degradation of pyruvate, fattyacids, and some amino acids, whereas cytosolic coen-zyme A is used in the biosynthesis of fatty acids (seeFig. 21–10). Fatty acyl–CoA in the cytosolic pool can beused for membrane lipid synthesis or can be moved intothe mitochondrial matrix for oxidation and ATP pro-duction. Conversion to the carnitine ester commits thefatty acyl moiety to the oxidative fate.

The carnitine-mediated entry process is the rate-limiting step for oxidation of fatty acids in mitochondriaand, as discussed later, is a regulation point. Once in-side the mitochondrion, the fatty acyl–CoA is acted uponby a set of enzymes in the matrix.

SUMMARY 17.1 Digestion, Mobilization,and Transport of Fats

■ The fatty acids of triacylglycerols furnish alarge fraction of the oxidative energy inanimals. Dietary triacylglycerols are emulsifiedin the small intestine by bile salts, hydrolyzedby intestinal lipases, absorbed by intestinalepithelial cells, reconverted intotriacylglycerols, then formed into chylomicronsby combination with specific apolipoproteins.

Chapter 17 Fatty Acid Catabolism636

Cytosol

R CO

R CO

Matrix

Carnitine

Transporter

Carnitine S-CoA

CoA-SH

Carnitineacyltransferase II

Carnitine

CarnitineR C

O

R CO

S-CoA

CoA-SH

Carnitineacyltransferase I

Intermembranespace

Inner mitochondrialmembrane

Outer mitochondrialmembrane

FIGURE 17–6 Fatty acid entry into mitochondria via the acyl-carnitine/carnitine transporter. After fatty acyl–carnitine is formed at the outermembrane or in the intermembrane space, it moves into the matrixby facilitated diffusion through the transporter in the inner membrane.In the matrix, the acyl group is transferred to mitochondrial coenzyme

A, freeing carnitine to return to the intermembrane space through thesame transporter. Acyltransferase I is inhibited by malonyl-CoA, thefirst intermediate in fatty acid synthesis (see Fig. 21–1). This inhibitionprevents the simultaneous synthesis and degradation of fatty acids.

■ Chylomicrons deliver triacylglycerols to tissues,where lipoprotein lipase releases free fattyacids for entry into cells. Triacylglycerolsstored in adipose tissue are mobilized by ahormone-sensitive triacylglycerol lipase. Thereleased fatty acids bind to serum albumin andare carried in the blood to the heart, skeletalmuscle, and other tissues that use fatty acidsfor fuel.

■ Once inside cells, fatty acids are activated atthe outer mitochondrial membrane byconversion to fatty acyl–CoA thioesters. Fattyacyl–CoA to be oxidized enters mitochondria inthree steps, via the carnitine shuttle.

17.2 Oxidation of Fatty AcidsAs noted earlier, mitochondrial oxidation of fatty acidstakes place in three stages (Fig. 17–7). In the firststage—� oxidation—fatty acids undergo oxidative re-moval of successive two-carbon units in the form ofacetyl-CoA, starting from the carboxyl end of the fattyacyl chain. For example, the 16-carbon palmitic acid(palmitate at pH 7) undergoes seven passes through theoxidative sequence, in each pass losing two carbons asacetyl-CoA. At the end of seven cycles the last two car-bons of palmitate (originally C-15 and C-16) remain asacetyl-CoA. The overall result is the conversion of the16-carbon chain of palmitate to eight two-carbon acetylgroups of acetyl-CoA molecules. Formation of eachacetyl-CoA requires removal of four hydrogen atoms(two pairs of electrons and four H�) from the fatty acylmoiety by dehydrogenases.

In the second stage of fatty acid oxidation, theacetyl groups of acetyl-CoA are oxidized to CO2 in thecitric acid cycle, which also takes place in the mito-chondrial matrix. Acetyl-CoA derived from fatty acidsthus enters a final common pathway of oxidation withthe acetyl-CoA derived from glucose via glycolysis andpyruvate oxidation (see Fig. 16–1). The first two stagesof fatty acid oxidation produce the reduced electron car-riers NADH and FADH2, which in the third stage donateelectrons to the mitochondrial respiratory chain,through which the electrons pass to oxygen with theconcomitant phosphorylation of ADP to ATP (Fig.17–7). The energy released by fatty acid oxidation isthus conserved as ATP.

We now take a closer look at the first stage of fattyacid oxidation, beginning with the simple case of a sat-urated fatty acyl chain with an even number of carbons,then turning to the slightly more complicated cases ofunsaturated and odd-number chains. We also considerthe regulation of fatty acid oxidation, the �-oxidativeprocesses as they occur in organelles other than mito-chondria, and, finally, two less-general modes of fattyacid catabolism, � oxidation and � oxidation.

The � Oxidation of Saturated Fatty Acids Has FourBasic Steps

Four enzyme-catalyzed reactions make up the first stageof fatty acid oxidation (Fig. 17–8a). First, dehydro-genation of fatty acyl–CoA produces a double bond between the � and � carbon atoms (C-2 and C-3), yield-ing a trans-�2

-enoyl-CoA (the symbol �2 designatesthe position of the double bond; you may want to re-view fatty acid nomenclature, p. 343.) Note that the newdouble bond has the trans configuration, whereas thedouble bonds in naturally occurring unsaturated fattyacids are normally in the cis configuration. We considerthe significance of this difference later.

This first step is catalyzed by three isozymes ofacyl-CoA dehydrogenase, each specific for a range offatty-acyl chain lengths: very-long-chain acyl-CoA de-hydrogenase (VLCAD), acting on fatty acids of 12 to 18

17.2 Oxidation of Fatty Acids 637

NADH, FADH2

Respiratory(electron-transfer)

chain

ATP

H2O

2H� � 1

O2

Stage 1 Stage 2

Stage 3

16CO2

Citricacid cycle

8 Acetyl-CoA

e�

e�

� Pi

2

CH2

64e�

CH3

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

C

CH2

O�

O

� Oxidation

ADP

FIGURE 17–7 Stages of fatty acid oxidation. Stage 1: A long-chainfatty acid is oxidized to yield acetyl residues in the form of acetyl-CoA. This process is called � oxidation. Stage 2: The acetyl groups areoxidized to CO2 via the citric acid cycle. Stage 3: Electrons derivedfrom the oxidations of stages 1 and 2 pass to O2 via the mitochon-drial respiratory chain, providing the energy for ATP synthesis byoxidative phosphorylation.

carbons; medium-chain (MCAD), acting on fatty acidsof 4 to 14 carbons; and short-chain (SCAD), acting onfatty acids of 4 to 8 carbons. All three isozymes are flavo-proteins with FAD (see Fig. 13–18) as a prostheticgroup. The electrons removed from the fatty acyl–CoAare transferred to FAD, and the reduced form of the de-hydrogenase immediately donates its electrons to anelectron carrier of the mitochondrial respiratory chain,the electron-transferring flavoprotein (ETF) (seeFig. 19–8). The oxidation catalyzed by an acyl-CoA de-hydrogenase is analogous to succinate dehydrogenation

in the citric acid cycle (p. XXX); in both reactions theenzyme is bound to the inner membrane, a double bondis introduced into a carboxylic acid between the � and� carbons, FAD is the electron acceptor, and electronsfrom the reaction ultimately enter the respiratory chainand pass to O2, with the concomitant synthesis of about1.5 ATP molecules per electron pair.

In the second step of the �-oxidation cycle (Fig.17–8a), water is added to the double bond of thetrans-�2-enoyl-CoA to form the L stereoisomer of �-hydroxyacyl-CoA (3-hydroxyacyl-CoA). This re-action, catalyzed by enoyl-CoA hydratase, is for-mally analogous to the fumarase reaction in the citricacid cycle, in which H2O adds across an �–� doublebond (p. XXX).

In the third step, L-�-hydroxyacyl-CoA is dehydro-genated to form �-ketoacyl-CoA, by the action of �-hydroxyacyl-CoA dehydrogenase; NAD� is theelectron acceptor. This enzyme is absolutely specific forthe L stereoisomer of hydroxyacyl-CoA. The NADHformed in the reaction donates its electrons to NADH

dehydrogenase, an electron carrier of the respiratorychain, and ATP is formed from ADP as the electrons passto O2. The reaction catalyzed by �-hydroxyacyl-CoA de-hydrogenase is closely analogous to the malate dehy-drogenase reaction of the citric acid cycle (p. XXX).

The fourth and last step of the �-oxidation cycle iscatalyzed by acyl-CoA acetyltransferase, more com-monly called thiolase, which promotes reaction of �-

ketoacyl-CoA with a molecule of free coenzyme A tosplit off the carboxyl-terminal two-carbon fragment ofthe original fatty acid as acetyl-CoA. The other productis the coenzyme A thioester of the fatty acid, now short-ened by two carbon atoms (Fig. 17–8a). This reactionis called thiolysis, by analogy with the process of hy-drolysis, because the �-ketoacyl-CoA is cleaved by re-action with the thiol group of coenzyme A.

The last three steps of this four-step sequence arecatalyzed by either of two sets of enzymes, with the en-zymes employed depending on the length of the fattyacyl chain. For fatty acyl chains of 12 or more carbons,the reactions are catalyzed by a multienzyme complexassociated with the inner mitochondrial membrane, thetrifunctional protein (TFP). TFP is a heterooctamerof �4�4 subunits. Each � subunit contains two activities,the enoyl-CoA hydratase and the �-hydroxyacyl-CoAdehydrogenase; the � subunits contain the thiolase ac-tivity. This tight association of three enzymes may allowefficient substrate channeling from one active site to the

Chapter 17 Fatty Acid Catabolism638

CH2

CH3

O

CH2

(C14) Acyl-CoA(myristoyl-CoA)

S-CoA

�

acyl-CoA CoA-SH

C

C

CH2 S-CoA

C

O Palmitoyl-CoA

acyl-CoA

FADH2

FAD

H2O

R CH2

O trans-�2-Enoyl-CoA

S-CoACC C

H

CH2

H

enoyl-CoA

R CH2

O L-�-Hydroxy-acyl-CoA

S-CoA

�-hydroxyacyl-CoA

NADH � H�

NAD�

CC

OH

H

CH2

O

(C16) R

R CH2

O �-Ketoacyl-CoAS-CoACCCH2

O

S-CoA �

(b)

Acetyl

(C14) R

C12

C10

C8

C6

C14

Acetyl -CoA

Acetyl -CoA

Acetyl -CoA

Acetyl -CoA

Acetyl -CoA

(a)

dehydrogenase

hydratase

dehydrogenase

acetyltransferase(thiolase)

C4

-CoA

Acetyl -CoA

Acetyl -CoA

FIGURE 17–8 The �-oxidation pathway. (a) In each pass through thisfour-step sequence, one acetyl residue (shaded in pink) is removed inthe form of acetyl-CoA from the carboxyl end of the fatty acyl chain—in this example palmitate (C16), which enters as palmitoyl-CoA. (b) Sixmore passes through the pathway yield seven more molecules ofacetyl-CoA, the seventh arising from the last two carbon atoms of the16-carbon chain. Eight molecules of acetyl-CoA are formed in all.

next, without diffusion of the intermediates away fromthe enzyme surface. When TFP has shortened the fattyacyl chain to 12 or fewer carbons, further oxidations arecatalyzed by a set of four soluble enzymes in the matrix.

As noted earlier, the single bond between methyl-ene (OCH2O) groups in fatty acids is relatively stable.The �-oxidation sequence is an elegant mechanism fordestabilizing and breaking these bonds. The first threereactions of � oxidation create a much less stable COCbond, in which the � carbon (C-2) is bonded to two car-bonyl carbons (the �-ketoacyl-CoA intermediate). Theketone function on the � carbon (C-3) makes it a goodtarget for nucleophilic attack by the OSH of coenzymeA, catalyzed by thiolase. The acidity of the � hydrogenand the resonance stabilization of the carbanion gener-ated by the departure of this hydrogen make the termi-nal OCH2OCOOS-CoA a good leaving group, facilitatingbreakage of the �–� bond.

The Four �-Oxidation Steps Are Repeated to YieldAcetyl-CoA and ATP

In one pass through the �-oxidation sequence, one mol-ecule of acetyl-CoA, two pairs of electrons, and four pro-tons (H�) are removed from the long-chain fattyacyl–CoA, shortening it by two carbon atoms. The equa-tion for one pass, beginning with the coenzyme A esterof our example, palmitate, is

Palmitoyl-CoA � CoA � FAD � NAD� � H2O Onmyristoyl-CoA � acetyl-CoA �FADH2 � NADH � H�

(17–2)

Following removal of one acetyl-CoA unit from palmitoyl-CoA, the coenzyme A thioester of the shortened fattyacid (now the 14-carbon myristate) remains. Themyristoyl-CoA can now go through another set of four�-oxidation reactions, exactly analogous to the first, toyield a second molecule of acetyl-CoA and lauroyl-CoA,the coenzyme A thioester of the 12-carbon laurate. Altogether, seven passes through the �-oxidation sequence are required to oxidize one molecule ofpalmitoyl-CoA to eight molecules of acetyl-CoA (Fig.17–8b). The overall equation is

Palmitoyl-CoA � 7CoA � 7FAD � 7NAD� � 7H2O On8 acetyl-CoA � 7FADH2 � 7NADH � 7H� (17–3)

Each molecule of FADH2 formed during oxidation of thefatty acid donates a pair of electrons to ETF of the res-piratory chain, and about 1.5 molecules of ATP are gen-erated during the ensuing transfer of each electron pairto O2. Similarly, each molecule of NADH formed deliv-ers a pair of electrons to the mitochondrial NADH de-hydrogenase, and the subsequent transfer of each pairof electrons to O2 results in formation of about 2.5 mol-ecules of ATP. Thus four molecules of ATP are formedfor each two-carbon unit removed in one pass throughthe sequence. Note that water is also produced in this

process. Transfer of electrons from NADH or FADH2 toO2 yields one H2O per electron pair. Reduction of O2 byNADH also consumes one H� per NADH molecule:NADH � H� �

12

O2 On NAD� � H2O. In hibernatinganimals, fatty acid oxidation provides metabolic energy,heat, and water—all essential for survival of an animalthat neither eats nor drinks for long periods (Box 17–1).Camels obtain water to supplement the meager supplyavailable in their natural environment by oxidation offats stored in their hump.

The overall equation for the oxidation of palmitoyl-CoA to eight molecules of acetyl-CoA, including theelectron transfers and oxidative phosphorylations, is

Palmitoyl-CoA � 7CoA � 7O2 � 28Pi � 28ADP On8 acetyl-CoA � 28ATP � 7H2O (17–4)

Acetyl-CoA Can Be Further Oxidized in the Citric Acid Cycle

The acetyl-CoA produced from the oxidation of fattyacids can be oxidized to CO2 and H2O by the citric acidcycle. The following equation represents the balancesheet for the second stage in the oxidation of palmitoyl-CoA, together with the coupled phosphorylations of thethird stage:

8 Acetyl-CoA � 16O2 � 80Pi � 80ADP On8CoA � 80ATP � 16CO2 � 16H2O (17–5)

Combining Equations 17–4 and 17–5, we obtain theoverall equation for the complete oxidation of palmitoyl-CoA to carbon dioxide and water:

Palmitoyl-CoA � 23O2 � 108Pi � 108ADP OnCoA � 108ATP � 16CO2 � 23H2O (17–6)

Table 17–1 summarizes the yields of NADH, FADH2, and ATP in the successive steps of palmitoyl-CoA oxida-tion. Note that because the activation of palmitate topalmitoyl-CoA breaks both phosphoanhydride bonds inATP (Fig. 17–5), the energetic cost of activating a fattyacid is equivalent to two ATP, and the net gain per mol-ecule of palmitate is 106 ATP. The standard free-energychange for the oxidation of palmitate to CO2 and H2Ois about 9,800 kJ/mol. Under standard conditions, theenergy recovered as the phosphate bond energy of ATPis 106 � 30.5 kJ/mol � 3,230 kJ/mol, about 33% of thetheoretical maximum. However, when the free-energychanges are calculated from actual concentrations of re-actants and products under intracellular conditions (seeBox 13–1), the free-energy recovery is more than 60%;the energy conservation is remarkably efficient.

Oxidation of Unsaturated Fatty Acids Requires Two Additional Reactions

The fatty acid oxidation sequence just described is typ-ical when the incoming fatty acid is saturated (that is,has only single bonds in its carbon chain). However,

17.2 Oxidation of Fatty Acids 639

most of the fatty acids in the triacylglycerols and phos-pholipids of animals and plants are unsaturated, havingone or more double bonds. These bonds are in the cisconfiguration and cannot be acted upon by enoyl-CoAhydratase, the enzyme catalyzing the addition of H2O to

the trans double bond of the �2-enoyl-CoA generatedduring � oxidation. Two auxiliary enzymes are neededfor � oxidation of the common unsaturated fatty acids:an isomerase and a reductase. We illustrate these aux-iliary reactions with two examples.

Chapter 17 Fatty Acid Catabolism640

BOX 17–1 THE WORLD OF BIOCHEMISTRY

Fat Bears Carry Out � Oxidation in Their SleepMany animals depend on fat stores for energy duringhibernation, during migratory periods, and in other sit-uations involving radical metabolic adjustments. Oneof the most pronounced adjustments of fat metabo-lism occurs in hibernating grizzly bears. These animalsremain in a continuous state of dormancy for periodsas long as seven months. Unlike most hibernatingspecies, the bear maintains a body temperature of be-tween 32 and 35 �C, close to the normal (nonhiber-

nating) level. Although expending about 25,000 kJ/day(6,000 kcal/day), the bear does not eat, drink, urinate,or defecate for months at a time.

Experimental studies have shown that hibernat-ing grizzly bears use body fat as their sole fuel. Fatoxidation yields sufficient energy for maintenance ofbody temperature, active synthesis of amino acidsand proteins, and other energy-requiring activities,such as membrane transport. Fat oxidation also re-leases large amounts of water, as described in the text,which replenishes water lost in breathing. The glyc-erol released by degradation of triacylglycerols is con-verted into blood glucose by gluconeogenesis. Ureaformed during breakdown of amino acids is reab-sorbed in the kidneys and recycled, the amino groupsreused to make new amino acids for maintaining bodyproteins.

Bears store an enormous amount of body fat inpreparation for their long sleep. An adult grizzly con-sumes about 38,000 kJ/day during the late spring andsummer, but as winter approaches it feeds 20 hours aday, consuming up to 84,000 kJ daily. This change infeeding is a response to a seasonal change in hormonesecretion. Large amounts of triacylglycerols areformed from the huge intake of carbohydrates duringthe fattening-up period. Other hibernating species, in-cluding the tiny dormouse, also accumulate largeamounts of body fat.

A grizzly bear prepares its hibernation nest, near the McNeil Riverin Canada.

Number of NADH Number of ATP Enzyme catalyzing the oxidation step or FADH2 formed ultimately formed*

Acyl-CoA dehydrogenase 7 FADH2 10.5�-Hydroxyacyl-CoA dehydrogenase 7 NADH 17.5Isocitrate dehydrogenase 8 NADH 20�-Ketoglutarate dehydrogenase 8 NADH 20Succinyl-CoA synthetase 8†

Succinate dehydrogenase 8 FADH2 12Malate dehydrogenase 8 NADH 20

Total 108

TABLE 17–1 Yield of ATP during Oxidation of One Molecule of Palmitoyl-CoA to CO2 and H2O

*These calculations assume that mitochondrial oxidative phosphorylation produces 1.5 ATP per FADH2 oxidized and 2.5 ATP per NADH oxidized.†GTP produced directly in this step yields ATP in the reaction catalyzed by nucleoside diphosphate kinase (p. XXX).

Oleate is an abundant 18-carbon monounsaturatedfatty acid with a cis double bond between C-9 and C-10(denoted �9). In the first step of oxidation, oleate is con-verted to oleoyl-CoA and, like the saturated fatty acids,enters the mitochondrial matrix via the carnitine shut-tle (Fig. 17–6). Oleoyl-CoA then undergoes three passesthrough the fatty acid oxidation cycle to yield three mol-ecules of acetyl-CoA and the coenzyme A ester of a �3,12-carbon unsaturated fatty acid, cis-�3-dodecenoyl-CoA (Fig. 17–9). This product cannot serve as a sub-strate for enoyl-CoA hydratase, which acts only on transdouble bonds. The auxiliary enzyme �3

,�2-enoyl-CoA

isomerase isomerizes the cis-�3-enoyl-CoA to thetrans-�2-enoyl-CoA, which is converted by enoyl-CoAhydratase into the corresponding L-�-hydroxyacyl-CoA(trans-�2-dodecenoyl-CoA). This intermediate is nowacted upon by the remaining enzymes of � oxidation to yield acetyl-CoA and the coenzyme A ester of a 10-carbon saturated fatty acid, decanoyl-CoA. The latterundergoes four more passes through the pathway toyield five more molecules of acetyl-CoA. Altogether,nine acetyl-CoAs are produced from one molecule of the18-carbon oleate.

The other auxiliary enzyme (a reductase) is re-quired for oxidation of polyunsaturated fatty acids—for

example, the 18-carbon linoleate, which has a cis-�9,cis-�12 configuration (Fig. 17–10). Linoleoyl-CoA under-goes three passes through the �-oxidation sequence toyield three molecules of acetyl-CoA and the coenzymeA ester of a 12-carbon unsaturated fatty acid with a cis-�3,cis-�6 configuration. This intermediate cannot beused by the enzymes of the �-oxidation pathway; itsdouble bonds are in the wrong position and have thewrong configuration (cis, not trans). However, the com-bined action of enoyl-CoA isomerase and 2,4-dienoyl-

CoA reductase, as shown in Figure 17–10, allows reen-try of this intermediate into the �-oxidation pathway

17.2 Oxidation of Fatty Acids 641

18 19

S-CoA

O

C

oxidationb(five cycles)

S-CoA

H

C

H O

�3, �2-enoyl-CoA isomerase

S-CoAH

C

OH

� oxidation(three cycles)

3 Acetyl-CoA

6 Acetyl-CoA

Oleoyl-CoA

trans-�2-Dodecenoyl-CoA

cis-�3-Dodecenoyl-CoA

12

12

CoA

oxidation(one cycle, andfirst oxidation

of second cycle)

Acetyl-

NADPH � H�

NADP�

enoyl-CoAisomerase

3

18

C

9

6

12

2(a)

CoA

oxidation(four cycles)

3 Acetyl-

1

42

5

3

2,4-dienoyl-CoAreductase

�3, �2-enoyl-CoAisomerase

oxidation(three cycles)

5 Acetyl-CoA

Linoleoyl-CoA

trans-�2

cis-�9, cis-�12

cis-�3, cis-�6

trans-�2,

,

cis-�6

trans-�2 cis-�4

trans-�3S-CoAC

O

S-CoA

C

O

2(a)5

5

3(b)

3(b)

4

64

C

S-CoA

O

S-CoA

O

1

S-CoAC

O

4

1

2

5

3

S-CoAC

O

4

1

2

12

12

10

10

10

b

b

b

FIGURE 17–9 Oxidation of a monounsaturated fatty acid. Oleic acid,as oleoyl-CoA (�9), is the example used here. Oxidation requires anadditional enzyme, enoyl-CoA isomerase, to reposition the doublebond, converting the cis isomer to a trans isomer, a normal interme-diate in � oxidation.

FIGURE 17–10 Oxidation of a polyunsaturated fatty acid. Theexample here is linoleic acid, as linoleoyl-CoA (�9,12). Oxidation re-quires a second auxiliary enzyme in addition to enoyl-CoA isomerase:NADPH-dependent 2,4-dienoyl-CoA reductase. The combined actionof these two enzymes converts a trans-�2,cis-�4-dienoyl-CoA inter-mediate to the trans-�2-enoyl-CoA substrate necessary for � oxidation.

and its degradation to six acetyl-CoAs. The overall re-sult is conversion of linoleate to nine molecules ofacetyl-CoA.

Complete Oxidation of Odd-Number Fatty AcidsRequires Three Extra Reactions

Although most naturally occurring lipids contain fattyacids with an even number of carbon atoms, fatty acidswith an odd number of carbons are common in the lipidsof many plants and some marine organisms. Cattle andother ruminant animals form large amounts of the three-carbon propionate (CH3OCH2OCOO�) during fer-mentation of carbohydrates in the rumen. The propi-onate is absorbed into the blood and oxidized by theliver and other tissues. And small quantities of propi-onate are added as a mold inhibitor to some breads andcereals, thus entering the human diet.

Long-chain odd-number fatty acids are oxidized inthe same pathway as the even-number acids, beginningat the carboxyl end of the chain. However, the substratefor the last pass through the �-oxidation sequence is afatty acyl–CoA with a five-carbon fatty acid. When thisis oxidized and cleaved, the products are acetyl-CoA andpropionyl-CoA. The acetyl-CoA can be oxidized in thecitric acid cycle, of course, but propionyl-CoA enters adifferent pathway involving three enzymes.

Propionyl-CoA is first carboxylated to form the D

stereoisomer of methylmalonyl-CoA (Fig. 17–11) bypropionyl-CoA carboxylase, which contains the co-factor biotin. In this enzymatic reaction, as in the pyru-vate carboxylase reaction (see Fig. 16–16), CO2 (or itshydrated ion, HCO3

�) is activated by attachment to bi-otin before its transfer to the substrate, in this case thepropionate moiety. Formation of the carboxybiotin in-termediate requires energy, which is provided by thecleavage of ATP to ADP and Pi. The D-methylmalonyl-CoA thus formed is enzymatically epimerized to its L

stereoisomer by methylmalonyl-CoA epimerase (Fig.17–11). The L-methylmalonyl-CoA then undergoes anintramolecular rearrangement to form succinyl-CoA,which can enter the citric acid cycle. This rearrange-ment is catalyzed by methylmalonyl-CoA mutase,

which requires as its coenzyme 5�-deoxyadenosyl-

cobalamin, or coenzyme B12, which is derived fromvitamin B12 (cobalamin). Box 17–2 describes the role ofcoenzyme B12 in this remarkable exchange reaction.

Fatty Acid Oxidation Is Tightly Regulated

Oxidation of fatty acids consumes a precious fuel, andit is regulated so as to occur only when the need for en-ergy requires it. In the liver, fatty acyl–CoA formed inthe cytosol has two major pathways open to it: (1) � ox-idation by enzymes in mitochondria or (2) conversioninto triacylglycerols and phospholipids by enzymes inthe cytosol. The pathway taken depends on the rate of

transfer of long-chain fatty acyl–CoA into mitochondria.The three-step process (carnitine shuttle) by whichfatty acyl groups are carried from cytosolic fattyacyl–CoA into the mitochondrial matrix (Fig. 17–6) israte-limiting for fatty acid oxidation and is an importantpoint of regulation. Once fatty acyl groups have enteredthe mitochondrion, they are committed to oxidation toacetyl-CoA.

Malonyl-CoA, the first intermediate in the cytoso-lic biosynthesis of long-chain fatty acids from acetyl-CoA(see Fig. 21–1), increases in concentration wheneverthe animal is well supplied with carbohydrate; excessglucose that cannot be oxidized or stored as glycogenis converted in the cytosol into fatty acids for storageas triacylglycerol. The inhibition of carnitine acyltrans-ferase I by malonyl-CoA ensures that the oxidation of

Chapter 17 Fatty Acid Catabolism642

H

C

O

HCO3�

Propionyl-CoA

ATP

ADP � Pi

D-Methylmalonyl-CoA

L-Methylmalonyl-CoA Succinyl-CoA

�O

O�

propionyl-CoAbiotin

HC

C

C

H

H

O

H

C

CoA-S O

HC

C

HH

H

H

C

O

HC

C

HH

H

C

OO�

HC

C

HH CO

carboxylase

CO

methylmalonyl-CoAepimerase

CoA-S

CoA-S

CoA-Smethyl-

malonyl-CoAmutase

coenzymeB12

FIGURE 17–11 Oxidation of propionyl-CoA produced by � oxida-tion of odd-number fatty acids. The sequence involves the carboxy-lation of propionyl-CoA to D-methylmalonyl-CoA and conversion ofthe latter to succinyl-CoA. This conversion requires epimerization ofD- to L-methylmalonyl-CoA, followed by a remarkable reaction inwhich substituents on adjacent carbon atoms exchange positions (seeBox 17–2).

fatty acids is inhibited whenever the liver is amply sup-plied with glucose as fuel and is actively making tria-cylglycerols from excess glucose.

Two of the enzymes of � oxidation are also regu-lated by metabolites that signal energy sufficiency.When the [NADH]/[NAD�] ratio is high, �-hydroxyacyl-CoA dehydrogenase is inhibited; in addition, high con-centrations of acetyl-CoA inhibit thiolase (Fig. 17–12).

Genetic Defects in Fatty Acyl–CoA DehydrogenasesCause Serious Disease

Stored triacylglycerols are typically the chiefsource of energy for muscle contraction, and an

inability to oxidize fatty acids from triacylglycerols hasserious consequences for health. The most common ge-netic defect in fatty acid catabolism in U.S. and north-ern European populations is due to a mutation in thegene encoding the medium-chain acyl-CoA dehy-

drogenase (MCAD). Among northern Europeans, the

�OOC CH2 S-CoAC

O

Malonyl-CoA

frequency of carriers (individuals with this recessivemutation on one of the two homologous chromosomes)is about 1 in 40, and about 1 individual in 10,000 hasthe disease—that is, has two copies of the mutant MCADallele and is unable to oxidize fatty acids of 6 to 12 car-bons. The disease is characterized by recurring episodesof a syndrome that includes fat accumulation in the liver,high blood levels of octanoic acid, low blood glucose (hypoglycemia), sleepiness, vomiting, and coma. Thepattern of organic acids in the urine helps in the diag-nosis of this disease: the urine commonly contains highlevels of 6-carbon to 10-carbon dicarboxylic acids (pro-duced by � oxidation) and low levels of urinary ketonebodies (we discuss � oxidation below and ketone bod-ies in Section 17.3). Although individuals may have nosymptoms between episodes, the episodes are very se-rious; mortality from this disease is 25% to 60% in earlychildhood. If the genetic defect is detected shortly after birth, the infant can be started on a low-fat, high-carbohydrate diet. With early detection and careful man-agement of the diet—including avoiding long intervalsbetween meals, to prevent the body from turning to itsfat reserves for energy—the prognosis for these indi-viduals is good.

17.2 Oxidation of Fatty Acids 643

Fatty acidsynthesis

Fatty acidb oxidation

b oxidation

1

2

67

4

8

5

3

Dietarycarbohydrate

High bloodglucose

Insulin

Inactive

Glucagon

PKA

ACC

ACC

Low bloodglucose

Fattyacyl–CoA

Carnitine

Fattyacyl–CoA

CoASH

FADH

NADH

Acetyl-CoA

Fatty acyl–carnitine

Fatty acyl–carnitine

Mitochondrion

Acetyl–CoA Malonyl-CoAGlucose

Fatty acids

carnitineacyl-transferase I

multistepglycolysis,pyruvate

dehydrogenasecomplex

P

phosphatase

Pi

FIGURE 17–12 Coordinated regulation of fatty acid synthesis andbreakdown. When the diet provides a ready source of carbohydrateas fuel, � oxidation of fatty acids is unnecessary and is therefore down-regulated. Two enzymes are key to the coordination of fatty acid metabolism: acetyl-CoA carboxylase (ACC), the first enzyme in thesynthesis of fatty acids (see Fig. 21–1 ), and carnitine acyl transferase I,which limits the transport of fatty acids into the mitochondrial matrixfor � oxidation (see Fig. 17–6). Ingestion of a high-carbohydrate mealraises the blood glucose level and thus 1 triggers the release of in-sulin. 2 Insulin-dependent protein phosphatase dephosphorylatesACC, activating it. 3 ACC catalyzes the formation of malonyl-CoA

(the first intermediate of fatty acid synthesis), and 4 malonyl-CoA in-hibits carnitine acyltransferase I, thereby preventing fatty acid entryinto the mitochondrial matrix.

When blood glucose levels drop between meals, 5 glucagon re-lease activates cAMP-dependent protein kinase (PKA), which 6 phos-phorylates and inactivates ACC. The concentration of malonyl-CoAfalls, the inhibition of fatty acid entry into mitochondria is relieved,and 7 fatty acids enter the mitochondrial matrix and 8 become themajor fuel. Because glucagon also triggers the mobilization of fattyacids in adipose tissue, a supply of fatty acids begins arriving in theblood.

644

BOX 17–2 WORKING IN BIOCHEMISTRY

Coenzyme B12: A Radical Solution to a Perplexing ProblemIn the methylmalonyl-CoA mutase reaction (see Fig.17–11), the group OCOOS-CoA at C-2 of the originalpropionate exchanges position with a hydrogen atomat C-3 of the original propionate (Fig. 1a). CoenzymeB12 is the cofactor for this reaction, as it is for almostall enzymes that catalyze reactions of this general type(Fig. 1b). These coenzyme B12–dependent processesare among the very few enzymatic reactions in biol-ogy in which there is an exchange of an alkyl or sub-stituted alkyl group (X) with a hydrogen atom on anadjacent carbon, with no mixing of the transferred

hydrogen atom with the hydrogen of the solvent,

H2O. How can the hydrogen atom move between twocarbons without mixing with the enormous excess ofhydrogen atoms in the solvent?

Coenzyme B12 is the cofactor form of vitamin B12,which is unique among all the vitamins in that itcontains not only a complex organic molecule but anessential trace element, cobalt. The com-plex corrin ring system of vitamin B12

(colored blue in Fig. 2), to which cobalt(as Co3�) is coordinated, is chemically re-lated to the porphyrin ring system of hemeand heme proteins (see Fig. 5–1). A fifthcoordination position of cobalt is filledby dimethylbenzimidazole ribonucleotide(shaded yellow), bound covalently by its3�-phosphate group to a side chain of thecorrin ring, through aminoisopropanol.The formation of this complex cofactor oc-curs in one of only two known reactions inwhich triphosphate is cleaved from ATP(Fig. 3); the other reaction is the forma-tion of S-adenosylmethionine from ATPand methionine (see Fig. 18–18).

Vitamin B12 as usually isolated is calledcyanocobalamin, because it contains acyano group (picked up during purification)attached to cobalt in the sixth coordinationposition. In 5�-deoxyadenosylcobalamin,

the cofactor for methylmalonyl-CoA mu-tase, the cyano group is replaced by the 5�-deoxyadenosyl group (red in Fig. 2),covalently bound through C-5� to the cobalt.The three-dimensional structure of the co-factor was determined by Dorothy CrowfootHodgkin in 1956, using x-ray crystallo-graphy.

The key to understanding how coen-zyme B12 catalyzes hydrogen exchange liesin the properties of the covalent bond be-tween cobalt and C-5� of the deoxyadeno-

FIGURE 1

N

CH3

N

NN

HO

3�

1�

2�

5�

4�

Co3�N

NN

N

H

H

HH

H

CH3

CH3

CH3

CH2

CH2

CH3

CH2

CH2

CH3

CH3

CH2

C

NH2

C

O

O

CH2

NH

CH2

HC

CH3

O

H

OH

HH

H

CH3

C

O

NH2

CH2

H2N

CH2

CH2

CH2

C

OCH2

O

H OH

H

H

CH3

CH2

O P O�

H

N

N

O

OCH3

OH

Corrinringsystem

Amino-isopropanol

5�-Deoxy-adenosine

Dimethyl-benzimidazoleribonucleotide

NH2

C

OCH2

NH2

C

O

NH2

C

O

NH2

H

O

H

CCO

CO�

H

S-CoA

Succinyl-CoA

H

H

O

C(b)

CO�

H

S-CoA

L-Methylmalonyl-CoA

methylmalonyl-CoAmutase

coenzyme B12

coenzyme B12

O(a)

HC C

H

CC

H

C

X HX

CC

syl group (Fig. 2). This is a relatively weak bond; itsbond dissociation energy is about 110 kJ/mol, com-pared with 348 kJ/mol for a typical COC bond or 414kJ/mol for a COH bond. Merely illuminating the com-pound with visible light is enough to break this CoOCbond. (This extreme photolability probably accountsfor the absence of vitamin B12 in plants.) Dissociationproduces a 5�-deoxyadenosyl radical and the Co2�

FIGURE 2

645

form of the vitamin. Thechemical function of 5�-de-oxyadenosylcobalamin is togenerate free radicals in thisway, thus initiating a seriesof transformations such asthat illustrated in Figure 4—a postulated mechanism forthe reaction catalyzed bymethylmalonyl-CoA mutaseand a number of other coen-zyme B12–dependent trans-formations.

1 The enzyme first breaks the CoOC bond in thecofactor, leaving the coenzyme in its Co2� form andproducing the 5�-deoxyadenosyl free radical. 2 Thisradical now abstracts a hydrogen atom from the sub-strate, converting the substrate to a radical and pro-ducing 5�-deoxyadenosine. 3 Rearrangement of thesubstrate radical yields another radical, in which themigrating group X (OCOOS-CoA for methylmalonyl-CoA mutase) has moved to the adjacent carbon to forma radical that has the carbon skeleton of the eventualproduct (a four-carbon straight chain). The hydrogenatom initially abstracted from the substrate is now partof the OCH3 group of 5�-deoxyadenosine. 4 One ofthe hydrogens from this same OCH3 group (it can bethe same one originally abstracted) is now returned to

the productlike radical, generating the product and regenerating the deoxyadenosyl free radical. 5 Thebond re-forms between cobalt and the OCH2 group ofthe deoxyadenosyl radical, destroying the free radicaland regenerating the cofactor in its Co3� form, readyto undergo another catalytic cycle. In this postulatedmechanism, the migrating hydrogen atom never existsas a free species and is thus never free to exchangewith the hydrogen of surrounding water molecules.

Vitamin B12 deficiency results in serious disease.This vitamin is not made by plants or animals

and can be synthesized only by a few species of mi-croorganisms. It is required by healthy people in onlyminute amounts, about 3 �g/day. The severe diseasepernicious anemia results from failure to absorb vi-tamin B12 efficiently from the intestine, where it is synthesized by intestinal bacteria or obtained from di-gestion of meat. Individuals with this disease do notproduce sufficient amounts of intrinsic factor, a gly-coprotein essential to vitamin B12 absorption. Thepathology in pernicious anemia includes reduced pro-duction of erythrocytes, reduced levels of hemoglobin,and severe, progressive impairment of the central nerv-ous system. Administration of large doses of vitamin B12

alleviates these symptoms in at least some cases. ■

FIGURE 3

N N

NN

P

NH2

O

H

HO

Coenzyme B12

H

H

OH

CH2

Co

�O OP

O

O�

O P

O

O�

O�

H

O

ATP

POO P

O

O�

O P

O

O� O�

O

O�

O�

N N

NN

3�

1�

2�

5�

4�

NH2

O

H

HO

H

H

OH

CH2

H

CobalaminCo

N N

N

CH2

H

C

H

N N

N

H C H

N N

N

H C H

HC

C

X

N N

N

H C

H

HC C

X

5

1

CC

XH

Product

5�-Deoxyadenosylfree radical

Substrateradical

Productlikeradical

2

C

H

C

XSubstrate

34

5�-Deoxy-adenosylfree radical

CoenzymeB12

Co2�

N

Co3�

N

Co2�

N

Co2�

N

N N

NCo2�

N

DeoxyadenosineDeoxyadenosine

Deoxyadenosine

Deoxyadenosine

Deoxyadenosine

radicalrearrangement

Dorothy Crowfoot Hodgkin,1910–1994

MECHANISM FIGURE 4

More than 20 other human genetic defects in fattyacid transport or oxidation have been documented, mostmuch less common than the defect in MCAD. One of themost severe disorders results from loss of the long-chain�-hydroxyacyl-CoA dehydrogenase activity of the tri-functional protein, TFP. Other disorders include defectsin the � or � subunits that affect all three activities ofTFP and cause serious heart disease and abnormalskeletal muscle. ■

Peroxisomes Also Carry Out � Oxidation

The mitochondrial matrix is the major site of fatty acidoxidation in animal cells, but in certain cells other com-partments also contain enzymes capable of oxidizingfatty acids to acetyl-CoA, by a pathway similar to, butnot identical with, that in mitochondria. In plant cells,the major site of � oxidation is not mitochondria butperoxisomes.

In peroxisomes, membrane-enclosed organelles ofanimal and plant cells, the intermediates for � oxidationof fatty acids are coenzyme A derivatives, and theprocess consists of four steps, as in mitochondrial � ox-idation (Fig. 17–13): (1) dehydrogenation, (2) additionof water to the resulting double bond, (3) oxidation ofthe �-hydroxyacyl-CoA to a ketone, and (4) thiolyticcleavage by coenzyme A. (The identical reactions alsooccur in glyoxysomes, as discussed below.)

One difference between the peroxisomal and mito-chondrial pathways is in the chemistry of the first step.In peroxisomes, the flavoprotein acyl-CoA oxidase thatintroduces the double bond passes electrons directly toO2, producing H2O2 (Fig. 17–13). This strong and po-tentially damaging oxidant is immediately cleaved toH2O and O2 by catalase. Recall that in mitochondria,the electrons removed in the first oxidation step passthrough the respiratory chain to O2 to produce H2O, andthis process is accompanied by ATP synthesis. In per-oxisomes, the energy released in the first oxidative stepof fatty acid breakdown is not conserved as ATP, but isdissipated as heat.

A second important difference between mito-chondrial and peroxisomal � oxidation in mam-

mals is in the specificity for fatty acyl–CoAs; the peroxisomal system is much more active on very-long-chain fatty acids such as hexacosanoic acid (26:0) andon branched-chain fatty acids such as phytanic acid andpristanic acid (see Fig. 17–17). These less-common fattyacids are obtained in the diet from dairy products, thefat of ruminant animals, meat, and fish. Their catabo-lism in the peroxisome involves several auxiliary en-zymes unique to this organelle. The inability to oxidizethese compounds is responsible for several serious hu-man diseases. Individuals with Zellweger syndrome

are unable to make peroxisomes and therefore lack allthe metabolism unique to that organelle. In X-linked

adrenoleukodystrophy (XALD), peroxisomes fail to

oxidize very-long-chain fatty acids, apparently for lackof a functional transporter for these fatty acids in theperoxisomal membrane. Both defects lead to accumu-lation in the blood of very-long-chain fatty acids, espe-cially 26:0. XALD affects young boys before the age of10 years, causing loss of vision, behavioral disturbances,and death within a few years. ■

In mammals, high concentrations of fats in the dietresult in increased synthesis of the enzymes of peroxi-somal � oxidation in the liver. Liver peroxisomes do notcontain the enzymes of the citric acid cycle and cannotcatalyze the oxidation of acetyl-CoA to CO2. Instead,

Chapter 17 Fatty Acid Catabolism646

CO

RS-CoA

CoASH CoASH

H2O H2O

Mitochondrion Peroxisome/glyoxysome

FAD

FADH2

H2O2

O2

NAD+

NADH

O2

H2O

Respiratorychain

FAD

ATP

FADH2

O2

H2O

Respiratorychain

NAD+

NADH

H2O � 12O2

R CH2 CH2 CS-CoA

O

CO

CR

H

C

CO

CH2CR

H

OH

CO

CH2C

O

R

CO

CH3

�

NADH exportedfor reoxidation

H

S-CoA

S-CoA

S-CoA

S-CoA

ATP

Citricacidcycle

Acetyl-CoAexported

FIGURE 17–13 Comparison of � oxidation in mitochondria and inperoxisomes and glyoxysomes. The peroxisomal/glyoxysomal systemdiffers from the mitochondrial system in two respects: (1) in the firstoxidative step electrons pass directly to O2, generating H2O2, and(2) the NADH formed in the second oxidative step cannot be reoxi-dized in the peroxisome or glyoxysome, so reducing equivalents areexported to the cytosol, eventually entering mitochondria. The acetyl-CoA produced by peroxisomes and glyoxysomes is also exported; theacetate from glyoxysomes (organelles found only in germinating seeds)serves as a biosynthetic precursor (see Fig. 17–14). Acetyl-CoA pro-duced in mitochondria is further oxidized in the citric acid cycle.

long-chain or branched fatty acids are catabolized toshorter-chain products, such as hexanoyl-CoA, whichare exported to mitochondria and completely oxidized.

Plant Peroxisomes and Glyoxysomes Use Acetyl-CoAfrom � Oxidation as a Biosynthetic Precursor

In plants, fatty acid oxidation does not occur primarilyin mitochondria (as noted earlier) but in the peroxi-somes of leaf tissue and in the glyoxysomes of germi-nating seeds. Plant peroxisomes and glyoxysomes aresimilar in structure and function; glyoxysomes, whichoccur only in germinating seeds, may be considered spe-cialized peroxisomes. The biological role of � oxidationin these organelles is to use stored lipids primarily toprovide but biosynthetic precursors, not energy.

During seed germination, stored triacylglycerols areconverted into glucose, sucrose, and a wide variety ofessential metabolites (Fig. 17–14). Fatty acids releasedfrom the triacylglycerols are first activated to their coen-zyme A derivatives and oxidized in glyoxysomes by thesame four-step process that takes place in peroxisomes(Fig. 17–13). The acetyl-CoA produced is converted viathe glyoxylate cycle (see Fig. 16–20) to four-carbon precursors for gluconeogenesis (see Fig. 14–18). Gly-oxysomes, like peroxisomes, contain high concentra-tions of catalase, which converts the H2O2 produced by� oxidation to H2O and O2.

The �-Oxidation Enzymes of Different OrganellesHave Diverged during Evolution

Although the �-oxidation reactions in mitochondria areessentially the same as those in peroxisomes and gly-oxysomes, the enzymes (isozymes) differ significantlybetween the two types of organelles. The differencesapparently reflect an evolutionary divergence that oc-curred very early, with the separation of gram-positiveand gram-negative bacteria (see Fig. 1–6).

In mitochondria, the four �-oxidation enzymes thatact on short-chain fatty acyl–CoAs are separate, solubleproteins (as noted earlier), similar in structure to theanalogous enzymes of gram-positive bacteria (Fig.17–15a). The gram-negative bacteria have four activitiesin three soluable subunits (Fig. 17–15b), and the eukary-otic enzyme system that acts on long-chain fatty acids—the trifunctional protein, TFP—has three enzyme activ-ities in two subunits that are membrane-associated (Fig.17–15c). The �-oxidation enzymes of plant peroxisomesand glyoxysomes, however, form a complex of proteins,one of which contains four enzymatic activities in a sin-gle polypeptide chain (Fig. 17–15d). The first enzyme,acyl-CoA oxidase, is a single polypeptide chain; the mul-

tifunctional protein (MFP) contains the second andthird enzyme activities (enoyl-CoA hydratase and hydroxyacyl-CoA dehydrogenase) as well as two auxil-iary activities needed for the oxidation of unsaturated

fatty acids (D-3-hydroxyacyl-CoA epimerase and �3,�2-enoyl-CoA isomerase); the fourth enzyme, thiolase, is aseparate, soluble polypeptide.

It is interesting that the enzymes that catalyze es-sentially the reversal of � oxidation in the synthesis offatty acids are also organized differently in prokaryotesand eukaryotes; in bacteria, the seven enzymes neededfor fatty acid synthesis are separate polypeptides, butin mammals, all seven activities are part of a single, hugepolypeptide chain (see Fig. 21–7). One advantage to thecell in having several enzymes of the same pathway en-coded in a single polypeptide chain is that this solvesthe problem of regulating the synthesis of enzymes thatmust interact functionally; regulation of the expressionof one gene ensures production of the same number ofactive sites for all enzymes in the path. When each en-zyme activity is on a separate polypeptide, some mech-anism is required to coordinate the synthesis of all thegene products. The disadvantage of having several ac-tivities on the same polypeptide is that the longer thepolypeptide chain, the greater is the probability of a mis-take in its synthesis: a single incorrect amino acid in thechain may make all the enzyme activities in that chainuseless. Comparison of the gene structures for these pro-teins in many species may shed light on the reasons forthe selection of one or the other strategy in evolution.

The � Oxidation of Fatty Acids Occurs in the Endoplasmic Reticulum

Although mitochondrial � oxidation, in which enzymesact at the carboxyl end of a fatty acid, is by far the most

17.2 Oxidation of Fatty Acids 647

FIGURE 17–14 Triacylglycerols as glucose source in seeds. � Oxi-dation is one stage in a pathway that converts stored triacylglycerolsto glucose in germinating seeds. For more detail, see Figure 16–22.

Seed triacylglycerols

Fatty acids

Acetyl-CoA

Oxaloacetate

GlucoseSucrose,polysaccharides

Amino acids

Energy

Nucleotides

Metabolicintermediates

lipases

� oxidation

glyoxylate cycle

gluconeogenesis

important catabolic fate for fatty acids in animal cells,there is another pathway in some species, including ver-tebrates, that involves oxidation of the � (omega) car-bon—the carbon most distant from the carboxyl group.The enzymes unique to � oxidation are located (invertebrates) in the endoplasmic reticulum of liver andkidney, and the preferred substrates are fatty acids of10 or 12 carbon atoms. In mammals � oxidation is nor-mally a minor pathway for fatty acid degradation, butwhen � oxidation is defective (because of mutation ora carnitine deficiency, for example) it becomes more important.

The first step introduces a hydroxyl group onto the� carbon (Fig. 17–16). The oxygen for this group comesfrom molecular oxygen (O2) in a complex reaction thatinvolves cytochrome P450 and the electron donorNADPH. Reactions of this type are catalyzed by mixed-

function oxidases, described in Box 21–1. Two moreenzymes now act on the � carbon: alcohol dehydro-

genase oxidizes the hydroxyl group to an aldehyde, andaldehyde dehydrogenase oxidizes the aldehyde groupto a carboxylic acid, producing a fatty acid with a car-boxyl group at each end. At this point, either end canbe attached to coenzyme A, and the molecule can en-

Chapter 17 Fatty Acid Catabolism648

Product

Product

Enz1

Enz2

Intermediate

Intermediate

Intermediate

Substrate

(a) Gram-positive bacteria and mitochondrialshort-chain-specific system

(d) Peroxisomal and glyoxysomalsystem of plants

(b) Gram-negative bacteria

(c) Mitochondrial very-long-chain-specific system

Product

Enz4 Enz2

Product

Innermembrane

Enz3

Enz1

Enz1

Enz1

Matrix

Substrate

Enz2

Enz6

Enz5

Enz3

Enz4

Enz4

Enz4

Enz3

Enz3

Enz2

Substrate

Substrate

MFP

FIGURE 17–15 The enzymes of � oxidation. Shown here are the dif-ferent subunit structures of the enzymes of � oxidation in gram-positiveand gram-negative bacteria, mitochondria, and plant peroxisomes andglyoxysomes. Enz1 is acyl-CoA dehydrogenase; Enz2, enoyl-CoA hy-dratase; Enz3, L-�-hydroxyacyl-CoA dehydrogenase; Enz4, thiolase;Enz5, D-3-hydroxyacyl-CoA epimerase, and Enz6, �3,�2-enoyl-CoA iso-merase. (a) The four enzymes of � oxidation in gram-positive bacteriaare separate, soluble entities, as are those of the short-chain-specificsystem of mitochondria. (b) In gram-negative bacteria, the four enzyme

activities reside in three polypeptides; enzymes 2 and 3 are parts of asingle polypeptide chain. (c) The very-long-chain-specific system of mitochondria is also composed of three polypeptides, one of which includes the activities of enzymes 2 and 3; in this case, the system isbound to the inner mitochondrial membrane. (d) In the peroxisomaland glyoxysomal �-oxidation systems of plants, enzymes 1 and 4 areseparate polypeptides, but enzymes 2 and 3, as well as two auxiliaryenzymes, are part of a single polypeptide chain, the multifunctional pro-tein, MFP.

ter the mitochondrion and undergo � oxidation by thenormal route. In each pass through the �-oxidationpathway, the “double-ended” fatty acid yields dicar-boxylic acids such as succinic acid, which can enter thecitric acid cycle, and adipic acid (Fig. 17–16).

Phytanic Acid Undergoes � Oxidation in Peroxisomes

The presence of a methyl group on the � carbonof a fatty acid makes � oxidation impossible, and

these branched fatty acids are catabolized in peroxi-somes of animal cells by � oxidation. In the oxidationof phytanic acid, for example (Fig. 17–17), phytanoyl-CoA is hydroxylated on its � carbon, in a reaction thatinvolves molecular oxygen; decarboxylated to form analdehyde one carbon shorter; and then oxidized to the

17.2 Oxidation of Fatty Acids 649

O�

C(CH2)10

O

CH3

O�

C(CH2)10

O

CH2HO

NADPH, O2mixed-functionoxidase

NADP�

NAD�

alcoholdehydrogenase

NADH

O�

CC (CH2)10

OO

H

O��O

CC (CH2)10

OO

O��OCC (CH2)2

OO

O��OCC (CH2)4

OO

NAD�

aldehydedehydrogenase

� oxidation

NADH

Adipate (adipic acid)Succinate

�

FIGURE 17–16 The � oxidation of fatty acids in the endoplasmicreticulum. This alternative to � oxidation begins with oxidation of thecarbon most distant from the � carbon—the � (omega) carbon. Thesubstrate is usually a medium-chain fatty acid; shown here is lauricacid (laurate). This pathway is generally not the major route for ox-idative catabolism of fatty acids.

COOH�

phytanoil-CoA synthetase AMP, PPi

ATP, CoA-SH

Phytanic acid

Phytanoyl-CoA

phytanoyl-CoA hydroxylase

CO S-CoA

CO2

�-Ketoglutarate, Ascorbate

, SuccinateFe2�

�-Hydroxyphytanoyl-CoA

CO S-CoA

OH

-hydroxyphytanoyl-CoA lyase Formic acidFormyl-CoA

2CO

C

O

H

Pristanal

aldehyde dehydrogenase NAD(P)H

NAD(P)�

COOH Pristanic acid

� oxidation

4,8,12-Trimethyltri-decanoyl-CoAC

O

S-CoA

Propionyl-CoA

�

C

O

S-CoACH CH23

FIGURE 17–17 The � oxidation of a branched-chain fatty acid (phy-tanic acid) in peroxisomes. Phytanic acid has a methyl-substituted �

carbon and therefore cannot undergo � oxidation. The combined action of the enzymes shown here removes the carboxyl carbon ofphytanic acid, to produce pristanic acid, in which the � carbon is unsubstituted, allowing oxidation. Notice that � oxidation of pristanicacid releases propionyl-CoA, not acetyl-CoA. This is further catabo-lized as in Figure 17–11. (The details of the reaction that producespristanal remain controversial.)

corresponding carboxylic acid, which now has no sub-stituent on the � carbon and can be oxidized further by� oxidation. Refsum’s disease, resulting from a geneticdefect in phytanoyl-CoA hydroxylase, leads to very highblood levels of phytanic acid and severe neurologicalproblems including blindness and deafness. ■

SUMMARY 17.2 Oxidation of Fatty Acids

■ In the first stage of � oxidation, four reactionsremove each acetyl-CoA unit from the carboxylend of a saturated fatty acyl–CoA: (1) dehydrogenation of the � and � carbons(C-2 and C-3) by FAD-linked acyl-CoAdehydrogenases, (2) hydration of the resultingtrans-�2 double bond by enoyl-CoA hydratase,(3) dehydrogenation of the resulting L-�-hydroxyacyl-CoA by NAD-linked �-

hydroxyacyl-CoA dehydrogenase, and (4) CoA-requiring cleavage of the resulting �-ketoacyl-CoA by thiolase, to form acetyl-CoAand a fatty acyl–CoA shortened by twocarbons. The shortened fatty acyl–CoA thenreenters the sequence.

■ In the second stage of fatty acid oxidation, theacetyl-CoA is oxidized to CO2 in the citric acidcycle. A large fraction of the theoretical yieldof free energy from fatty acid oxidation isrecovered as ATP by oxidative phosphorylation,the final stage of the oxidative pathway.

■ Malonyl-CoA, an early intermediate of fattyacid synthesis, inhibits carnitine acyltransferaseI, preventing fatty acid entry into mitochondria.This blocks fatty acid breakdown whilesynthesis is occurring.

■ Genetic defects in the medium-chain acyl-CoAdehydrogenase result in serious human disease,as do mutations in other components of the �-oxidation system.

■ Oxidation of unsaturated fatty acids requirestwo additional enzymes: enoyl-CoA isomeraseand 2,4-dienoyl-CoA reductase. Odd-numberfatty acids are oxidized by the �-oxidationpathway to yield acetyl-CoA and a molecule of propionyl-CoA. This is carboxylated tomethylmalonyl-CoA, which is isomerized tosuccinyl-CoA in a reaction catalyzed bymethylmalonyl-CoA mutase, an enzymerequiring coenzyme B12.

■ Peroxisomes of plants and animals, andglyoxysomes of plants, carry out � oxidation infour steps similar to those of the mitochondrialpathway in animals. The first oxidation step,however, transfers electrons directly to O2,generating H2O2. Peroxisomes of animal tissues

specialize in the oxidation of very-long-chainfatty acids and branched fatty acids. Inglyoxysomes, in germinating seeds, � oxidationis one step in the conversion of stored lipidsinto a variety of intermediates and products.

■ The reactions of � oxidation, occurring in theendoplasmic reticulum, produce dicarboxylicfatty acyl intermediates, which can undergo �oxidation at either end to yield shortdicarboxylic acids such as succinate.

17.3 Ketone BodiesIn humans and most other mammals, acetyl-CoA formedin the liver during oxidation of fatty acids can either en-ter the citric acid cycle (stage 2 of Fig. 17–7) or un-dergo conversion to the “ketone bodies,” acetone, ace-

toacetate, and D-�-hydroxybutyrate, for export toother tissues. (The term “bodies” is a historical artifact;the term is occasionally applied to insoluble particles,but these compounds are quite soluble in blood andurine.) Acetone, produced in smaller quantities than the other ketone bodies, is exhaled. Acetoacetate andD-�-hydroxybutyrate are transported by the blood to tis-sues other than the liver (extrahepatic tissues), wherethey are converted to acetyl-CoA and oxidized in thecitric acid cycle, providing much of the energy requiredby tissues such as skeletal and heart muscle and the renal cortex. The brain, which preferentially uses glu-cose as fuel, can adapt to the use of acetoacetate or D-�-hydroxybutyrate under starvation conditions, whenglucose is unavailable. The production and export of ke-tone bodies from the liver to extrahepatic tissues allowcontinued oxidation of fatty acids in the liver whenacetyl-CoA is not being oxidized in the citric acid cycle.

Ketone Bodies, Formed in the Liver, Are Exported to Other Organs as Fuel

The first step in the formation of acetoacetate, occurringin the liver (Fig. 17–18), is the enzymatic condensationof two molecules of acetyl-CoA, catalyzed by thiolase;

AO

G

OH

CH3 C

H O�

OCA

CH2O

O

Acetone

B

OG

CH3

CO�

OC

O

O

O

D-�-Hydroxybutyrate

BCH3OC

O

CH2O

CH3

Acetoacetate

J

J

Chapter 17 Fatty Acid Catabolism650

this is simply the reversal of the last step of � oxidation.The acetoacetyl-CoA then condenses with acetyl-CoA toform �-hydroxy-�-methylglutaryl-CoA (HMG-CoA),

which is cleaved to free acetoacetate and acetyl-CoA.The acetoacetate is reversibly reduced by D-�-hydroxy-butyrate dehydrogenase, a mitochondrial enzyme, toD-�-hydroxybutyrate. This enzyme is specific for the

D stereoisomer; it does not act on L-�-hydroxyacyl-CoAsand is not to be confused with L-�-hydroxyacyl-CoAdehydrogenase of the �-oxidation pathway.

In healthy people, acetone is formed in very smallamounts from acetoacetate, which is easily de-

carboxylated, either spontaneously or by the action ofacetoacetate decarboxylase (Fig. 17–18). Becauseindividuals with untreated diabetes produce large quan-tities of acetoacetate, their blood contains significantamounts of acetone, which is toxic. Acetone is volatileand imparts a characteristic odor to the breath, whichis sometimes useful in diagnosing diabetes. ■

In extrahepatic tissues, D-�-hydroxybutyrate is ox-idized to acetoacetate by D-�-hydroxybutyrate dehy-drogenase (Fig. 17–19). The acetoacetate is activatedto its coenzyme A ester by transfer of CoA from suc-cinyl-CoA, an intermediate of the citric acid cycle (seeFig. 16–7), in a reaction catalyzed by �-ketoacyl-CoA

transferase. The acetoacetyl-CoA is then cleaved bythiolase to yield two acetyl-CoAs, which enter the citricacid cycle. Thus the ketone bodies are used as fuels.

The production and export of ketone bodies by theliver allow continued oxidation of fatty acids with onlyminimal oxidation of acetyl-CoA. When intermediates ofthe citric acid cycle are being siphoned off for glucose

17.3 Ketone Bodies 651

OCH2

O

D- -Hydroxybutyrate

acetoacetate

Acetoacetate

D

M

O

C

NAD�

OCCH3

S-CoA

O

2 Acetyl-CoA

G

JOCCH3

S-CoA�

G

JO

C

CH3

S-CoA

O

Acetoacetyl-CoA

G

J

O

C

B

O

OCH2O O

C

CH3

HMG-CoA

CoA-SH

Acetyl-CoA �H2O

O

C

CH2

S-CoA

O

A GD

M J

O

C

OH

CH3

O

�O

O

CHA

�O

BO

Acetone

thiolase CoA-SH

HMG-CoAlyase Acetyl-CoA

OCH2O

OH

CH2OCA

CH3

O

CH3OCOCH3

O

D

MC

�O

O

NADH� H�

�

�

�

�-Hydroxy- -methylglutaryl-CoA(HMG-CoA)

D- -hydroxybutyratedehydrogenase

CO2

synthase

decarboxylase

B

FIGURE 17–18 Formation of ketone bodies from acetyl-CoA.Healthy, well-nourished individuals produce ketone bodies at a rela-tively low rate. When acetyl-CoA accumulates (as in starvation or un-treated diabetes, for example), thiolase catalyzes the condensation oftwo acetyl-CoA molecules to acetoacetyl-CoA, the parent compoundof the three ketone bodies. The reactions of ketone body formationoccur in the matrix of liver mitochondria. The six-carbon compound�-hydroxy-�-methylglutaryl-CoA (HMG-CoA) is also an intermediateof sterol biosynthesis, but the enzyme that forms HMG-CoA in thatpathway is cytosolic. HMG-CoA lyase is present only in the mito-chondrial matrix.

FIGURE 17–19 D-�-Hydroxybutyrate as a fuel. D-�-Hydroxybutyrate,synthesized in the liver, passes into the blood and thus to other tis-sues, where it is converted in three steps to acetyl-CoA. It is first ox-idized to acetoacetate, which is activated with coenzyme A donatedfrom succinyl-CoA, then split by thiolase. The acetyl-CoA thus formedis used for energy production.

O

CH3 C

�-ketoacyl-CoAtransferase

Succinate

D-�-Hydroxybutyrate

OH

CH2 C

H O�

CH3 CO

Succinyl-CoA

CoA-SHthiolase

Acetoacetate

NADH � H�

NAD�

D-�-hydroxybutyratedehydrogenase

O

CH2 CO�

CH3 CO

� CH3 CS-CoA

O

2 Acetyl-CoA

CH3 CS-CoA

O

CH2 CS-CoA

OAcetoacetyl-CoA

synthesis by gluconeogenesis, for example, oxidation ofcycle intermediates slows—and so does acetyl-CoA oxi-dation. Moreover, the liver contains only a limited amountof coenzyme A, and when most of it is tied up in acetyl-CoA, � oxidation slows for want of the free coenzyme.The production and export of ketone bodies free coen-zyme A, allowing continued fatty acid oxidation.

Ketone Bodies Are Overproduced in Diabetes and during Starvation

Starvation and untreated diabetes mellitus leadto overproduction of ketone bodies, with several

associated medical problems. During starvation, gluco-neogenesis depletes citric acid cycle intermediates, di-verting acetyl-CoA to ketone body production (Fig.17–20). In untreated diabetes, when the insulin level isinsufficient, extrahepatic tissues cannot take up glucoseefficiently from the blood, either for fuel or for conver-sion to fat. Under these conditions, levels of malonyl-CoA (the starting material for fatty acid synthesis) fall,inhibition of carnitine acyltransferase I is relieved, andfatty acids enter mitochondria to be degraded to acetyl-CoA—which cannot pass through the citric acid cyclebecause cycle intermediates have been drawn off for useas substrates in gluconeogenesis. The resulting accu-mulation of acetyl-CoA accelerates the formation of ke-tone bodies beyond the capacity of extrahepatic tissuesto oxidize them. The increased blood levels of acetoac-etate and D-�-hydroxybutyrate lower the blood pH,causing the condition known as acidosis. Extremeacidosis can lead to coma and in some cases death. Ketone bodies in the blood and urine of untreated diabetics can reach extraordinary levels—a blood con-centration of 90 mg/100 mL (compared with a normallevel of �3 mg/100 mL) and urinary excretion of 5,000mg/24 hr (compared with a normal rate of 125 mg/24 hr). This condition is called ketosis.

Individuals on very low-calorie diets, using the fatsstored in adipose tissue as their major energy source,also have increased levels of ketone bodies in their bloodand urine. These levels must be monitored to avoid thedangers of acidosis and ketosis (ketoacidosis). ■

SUMMARY 17.3 Ketone Bodies

■ The ketone bodies—acetone, acetoacetate, andD-�-hydroxybutyrate—are formed in the liver.The latter two compounds serve as fuelmolecules in extrahepatic tissues, throughoxidation to acetyl-CoA and entry into thecitric acid cycle.

■ Overproduction of ketone bodies inuncontrolled diabetes or severely reducedcalorie intake can lead to acidosis or ketosis.

Chapter 17 Fatty Acid Catabolism652

FIGURE 17–20 Ketone body formation and export from the liver.Conditions that promote gluconeogenesis (untreated diabetes, severelyreduced food intake) slow the citric acid cycle (by drawing off ox-aloacetate) and enhance the conversion of acetyl-CoA to acetoacetate.The released coenzyme A allows continued � oxidation of fatty acids.

citricacidcycle

Acetyl-CoA

�

�

ketone bodyformation

�

CoA

Fattyacids

Glucose exportedas fuel for brain and other tissues

Glucose

gluconeogenesis

Hepatocyte

Lipid droplets

oxidation

Oxaloacetate

Acetoacetate, D- -hydroxybutyrate,

acetone Acetoacetate andD- -hydroxybutyrateexported as energy source for heart,skeletal muscle,kidney, and brain

Key Terms

� oxidation XXXchylomicron XXXapolipoprotein XXXlipoprotein XXXperilipins XXXhormone-sensitive

lipase XXXfree fatty acids XXXserum albumin XXX

carnitine shuttle XXXcarnitine acyltransferase

I XXXacyl-carnitine/carnitine

transporter XXXcarnitine acyltransferase

II XXXtrifunctional protein

(TFP) XXX

methylmalonyl-CoAmutase XXX

coenzyme B12 XXXpernicious anemia XXXintrinsic factor XXXmalonyl-CoA XXXmedium-chain acyl-CoA

dehydrogenase(MCAD) XXX

multifunctional protein(MFP) XXX

� oxidation XXXmixed-function

oxidases XXX� oxidation XXXacidosis XXXketosis XXX

Terms in bold are defined in the glossary.

Chapter 17 Problems 653

Further Reading

GeneralBoyer, P.D. (1983) The Enzymes, 3rd edn, Vol. 16: Lipid

Enzymology, Academic Press, Inc., San Diego, CA.

Ferry, G. (1998) Dorothy Hodgkin: A Life, Cold Spring HarborLaboratory Press, Cold Spring Harbor, NY.

Fascinating biography of an amazing woman.

Gurr, M.I., Harwood, J.L., & Frayn; K.N. (2002) Lipid Biochem-

istry: An Introduction, 5th edn, Blackwell Science, Oxford, UK.

Langin, D., Holm, C., & Lafontan, M. (1996) Adipocytehormone-sensitive lipase: a major regulator of lipid metabolism.Proc. Nutr. Soc. 55, 93–109.

Ramsay, T.G. (1996) Fat cells. Endocrinol. Metab. Clin. N. Am.

25, 847–870.A review of all aspects of fat storage and mobilization inadipocytes.

Scheffler, I.E. (1999) Mitochondria, Wiley-Liss, New York.An excellent book on mitochondrial structure and function.

Wang, C.S., Hartsuck, J., & McConathy, W.J. (1992) Structureand functional properties of lipoprotein lipase. Biochim. Biophys.

Acta 1123, 1–17.Advanced-level discussion of the enzyme that releases fattyacids from lipoproteins in the capillaries of muscle and adiposetissue.

Mitochondrial � OxidationBannerjee, R. (1997) The yin-yang of cobalamin biochemistry.Chem. Biol. 4, 175–186.

A review of the biochemistry of coenzyme B12 reactions, includ-ing the methylmalonyl-CoA mutase reaction.

Eaton, S., Bartlett, K., & Pourfarzam, M. (1996) Mammalianmitochondrial �-oxidation. Biochem. J. 320, 345–357.

A review of the enzymology of � oxidation, inherited defects inthis pathway, and regulation of the process in mitochondria.

Eaton, S., Bursby, T., Middleton, B., Pourfarzam, M., Mills,

K., Johnson, A.W., & Bartlett, K. (2000) The mitochondrialtrifunctional protein: centre of a �-oxidation metabolon? Biochem.

Soc. Trans. 28, 177–182. Short, intermediate-level review.

Harwood, J.L. (1988) Fatty acid metabolism. Annu. Rev. Plant

Physiol. Plant Mol. Biol. 39, 101–138.

Jeukendrup, A.E., Saris, W.H., & Wagenmakers, A.J. (1998)Fat metabolism during exercise: a review. Part III: effects of nutri-tional interventions. Int. J. Sports Med. 19, 371–379.

This paper is one of a series that reviews the factors that influ-ence fat mobilization and utilization during exercise.

Kerner, J. & Hoppel, C. (1998) Genetic disorders of carnitine

metabolism and their nutritional management. Annu. Rev. Nutr.

18, 179–206.

Kerner, J. & Hoppel, C. (2000) Fatty acid import into mitochon-dria. Biochim. Biophys. Acta 1486, 1–17.

Kunau, W.H., Dommes, V., & Schulz, H. (1995) �-Oxidation offatty acids in mitochondria, peroxisomes, and bacteria: a century ofcontinued progress. Prog. Lipid Res. 34, 267–342.

A good historical account and a useful comparison of � oxida-tion in different systems.

Rinaldo, P., Matern, D., & Bennett, M.J. (2002) Fatty acidoxidation disorders. Annu. Rev. Physiol. 64, 477–502.

Advanced review of metabolic defects in fat oxidation, includingMCAD mutations.

Sherratt, H.S. (1994) Introduction: the regulation of fatty acidoxidation in cells. Biochem. Soc. Trans. 22, 421–422.

Introduction to reviews (in this journal issue) of various aspectsof fatty acid oxidation and its regulation.

Thorpe, C. & Kim, J.J. (1995) Structure and mechanism ofaction of the acyl-CoA dehydrogenases. FASEB J. 9, 718–725.

Short, clear description of the three-dimensional structure andcatalytic mechanism of these enzymes.

Peroxisomal � OxidationGraham, I.A. & Eastmond, P.J. (2002) Pathways of straight andbranched chain fatty acid catabolism in higher plants. Prog. Lipid

Res. 41, 156–181.

Hashimoto, T. (1996) Peroxisomal �-oxidation: enzymology andmolecular biology. Ann. N. Y. Acad. Sci. 804, 86–98.

Mannaerts, G.P. & van Veldhoven, P.P. (1996) Functions andorganization of peroxisomal �-oxidation. Ann. N. Y. Acad. Sci.

804, 99–115.

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McGarry, J.D. & Foster, D.W. (1980) Regulation of hepatic fattyacid oxidation and ketone body production. Annu. Rev. Biochem.

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1. Energy in Triacylglycerols On a per-carbon basis,where does the largest amount of biologically available en-ergy in triacylglycerols reside: in the fatty acid portions orthe glycerol portion? Indicate how knowledge of the chemi-cal structure of triacylglycerols provides the answer.

2. Fuel Reserves in Adipose Tissue Triacylglycerols,with their hydrocarbon-like fatty acids, have the highest en-ergy content of the major nutrients.

(a) If 15% of the body mass of a 70.0 kg adult consistsof triacylglycerols, what is the total available fuel reserve, in

Problems

Chapter 17 Fatty Acid Catabolism654

both kilojoules and kilocalories, in the form of triacylglyc-erols? Recall that 1.00 kcal � 4.18 kJ.

(b) If the basal energy requirement is approximately8,400 kJ/day (2,000 kcal/day), how long could this person sur-vive if the oxidation of fatty acids stored as triacylglycerolswere the only source of energy?

(c) What would be the weight loss in pounds per dayunder such starvation conditions (1 lb � 0.454 kg)?

3. Common Reaction Steps in the Fatty Acid Oxida-

tion Cycle and Citric Acid Cycle Cells often use thesame enzyme reaction pattern for analogous metabolic con-versions. For example, the steps in the oxidation of pyruvateto acetyl-CoA and of �-ketoglutarate to succinyl-CoA, al-though catalyzed by different enzymes, are very similar. Thefirst stage of fatty acid oxidation follows a reaction sequenceclosely resembling a sequence in the citric acid cycle. Useequations to show the analogous reaction sequences in thetwo pathways.

4. Chemistry of the Acyl-CoA Synthetase Reaction

Fatty acids are converted to their coenzyme A esters in a re-versible reaction catalyzed by acyl-CoA synthetase:

(a) The enzyme-bound intermediate in this reaction hasbeen identified as the mixed anhydride of the fatty acid andadenosine monophosphate (AMP), acyl-AMP:

Write two equations corresponding to the two steps ofthe reaction catalyzed by acyl-CoA synthetase.

(b) The acyl-CoA synthetase reaction is readily re-versible, with an equilibrium constant near 1. How can thisreaction be made to favor formation of fatty acyl–CoA?

5. Oxidation of Tritiated Palmitate Palmitate uniformlylabeled with tritium (3H) to a specific activity of 2.48 � 108

counts per minute (cpm) per micromole of palmitate is addedto a mitochondrial preparation that oxidizes it to acetyl-CoA.The acetyl-CoA is isolated and hydrolyzed to acetate. Thespecific activity of the isolated acetate is 1.00 � 107

cpm/�mol. Is this result consistent with the �-oxidation path-way? Explain. What is the final fate of the removed tritium?

6. Compartmentation in � Oxidation Free palmitateis activated to its coenzyme A derivative (palmitoyl-CoA) inthe cytosol before it can be oxidized in the mitochondrion. Ifpalmitate and [14C]coenzyme A are added to a liver ho-mogenate, palmitoyl-CoA isolated from the cytosolic fractionis radioactive, but that isolated from the mitochondrial frac-tion is not. Explain.

7. Comparative Biochemistry: Energy-Generating

Pathways in Birds One indication of the relative impor-

tance of various ATP-producing pathways is the Vmax of cer-tain enzymes of these pathways. The values of Vmax of sev-eral enzymes from the pectoral muscles (chest muscles usedfor flying) of pigeon and pheasant are listed below.

(a) Discuss the relative importance of glycogen metab-olism and fat metabolism in generating ATP in the pectoralmuscles of these birds.

(b) Compare oxygen consumption in the two birds.(c) Judging from the data in the table, which bird is the

long-distance flyer? Justify your answer.(d) Why were these particular enzymes selected for

comparison? Would the activities of triose phosphate iso-merase and malate dehydrogenase be equally good bases forcomparison? Explain.

8. Effect of Carnitine Deficiency An individualdeveloped a condition characterized by progressive

muscular weakness and aching muscle cramps. The symp-toms were aggravated by fasting, exercise, and a high-fat diet.The homogenate of a skeletal muscle specimen from the patient oxidized added oleate more slowly than did controlhomogenates, consisting of muscle specimens from healthyindividuals. When carnitine was added to the patient’s mus-cle homogenate, the rate of oleate oxidation equaled that inthe control homogenates. The patient was diagnosed as hav-ing a carnitine deficiency.

(a) Why did added carnitine increase the rate of oleateoxidation in the patient’s muscle homogenate?

(b) Why were the patient’s symptoms aggravated byfasting, exercise, and a high-fat diet?

(c) Suggest two possible reasons for the deficiency ofmuscle carnitine in this individual.

9. Fatty Acids as a Source of Water Contrary to legend,camels do not store water in their humps, which actually con-sist of large fat deposits. How can these fat deposits serve asa source of water? Calculate the amount of water (in liters)that a camel can produce from 1.0 kg of fat. Assume for sim-plicity that the fat consists entirely of tripalmitoylglycerol.

10. Petroleum as a Microbial Food Source Some mi-croorganisms of the genera Nocardia and Pseudomonas cangrow in an environment where hydrocarbons are the only foodsource. These bacteria oxidize straight-chain aliphatic hy-drocarbons, such as octane, to their corresponding carboxylicacids:

CH3(CH2)6CH3 � NAD� � O2

CH3(CH2)6COOH � NADH � H�

How could these bacteria be used to clean up oil spills? What

zy

Vmax

(�mol substrate/min/g tissue)

Enzyme Pigeon Pheasant

Hexokinase 3.0 2.3Glycogen phosphorylase 18.0 120.0Phosphofructokinase-1 24.0 143.0Citrate synthase 100.0 15.0Triacylglycerol lipase 0.07 0.01

OR PC

O

O O

O

Adenine

H HH H

OH

CH2

OH

O�

R COO� � ATP � CoA

CoA � AMP � PPiR C

O

Chapter 17 Problems 655

would be some of the limiting factors to the efficiency of thisprocess?

11. Metabolism of a Straight-Chain Phenylated Fatty

Acid A crystalline metabolite was isolated from the urineof a rabbit that had been fed a straight-chain fatty acid con-taining a terminal phenyl group:

A 302 mg sample of the metabolite in aqueous solution wascompletely neutralized by 22.2 mL of 0.100 M NaOH.

(a) What is the probable molecular weight and structureof the metabolite?

(b) Did the straight-chain fatty acid contain an even oran odd number of methylene (OCH2O) groups (i.e., is n evenor odd)? Explain.

12. Fatty Acid Oxidation in Uncontrolled Dia-

betes When the acetyl-CoA produced during � ox-idation in the liver exceeds the capacity of the citric acidcycle, the excess acetyl-CoA forms ketone bodies—acetone,acetoacetate, and D-�-hydroxybutyrate. This occurs insevere, uncontrolled diabetes: because the tissues cannot useglucose, they oxidize large amounts of fatty acids instead.Although acetyl-CoA is not toxic, the mitochondrion mustdivert the acetyl-CoA to ketone bodies. What problem wouldarise if acetyl-CoA were not converted to ketone bodies? Howdoes the diversion to ketone bodies solve the problem?

13. Consequences of a High-Fat Diet with No Carbo-

hydrates Suppose you had to subsist on a diet of whaleblubber and seal blubber, with little or no carbohydrate.

(a) What would be the effect of carbohydrate depriva-tion on the utilization of fats for energy?

(b) If your diet were totally devoid of carbohydrate,would it be better to consume odd- or even-numbered fattyacids? Explain.

14. Metabolic Consequences of Ingesting �-Fluoro-

oleate The shrub Dichapetalum toxicarium, native toSierra Leone, produces �-fluorooleate, which is highly toxicto warm-blooded animals.

This substance has been used as an arrow poison, and pow-dered fruit from the plant is sometimes used as a rat poison(hence the plant’s common name, ratsbane). Why is this sub-stance so toxic? (Hint: Review Chapter 16, Problem 13.)

15. Role of FAD as Electron Acceptor Acyl-CoA dehy-drogenase uses enzyme-bound FAD as a prosthetic group todehydrogenate the � and � carbons of fatty acyl–CoA. Whatis the advantage of using FAD as an electron acceptor ratherthan NAD�? Explain in terms of the standard reduction po-tentials for the Enz-FAD/FADH2 (E�� � �0.219 V) andNAD�/NADH (E�� � �0.320 V) half-reactions.

16. � Oxidation of Arachidic Acid How many turns ofthe fatty acid oxidation cycle are required for complete oxi-dation of arachidic acid (see Table 10–1) to acetyl-CoA?

17. Fate of Labeled Propionate If [3-14C]propionate(14C in the methyl group) is added to a liver homogenate,14C-labeled oxaloacetate is rapidly produced. Draw a flowchart for the pathway by which propionate is transformedto oxaloacetate, and indicate the location of the 14C in oxaloacetate.

18. Sources of H2O Produced in � Oxidation The com-plete oxidation of palmitoyl-CoA to carbon dioxide and wa-ter is represented by the overall equation

Palmitoyl-CoA � 23O2 � 108Pi � 108ADP OnCoA � 16CO2 � 108ATP � 23H2O

Water is also produced in the reaction

ADP � Pi On ATP � H2O

but is not included as a product in the overall equation. Why?

19. Biological Importance of Cobalt In cattle, deer,sheep, and other ruminant animals, large amounts of pro-pionate are produced in the rumen through the bacterialfermentation of ingested plant matter. Propionate is theprincipal source of glucose for these animals, via the routepropionate n oxaloacetate n glucose. In some areas ofthe world, notably Australia, ruminant animals sometimesshow symptoms of anemia with concomitant loss of appetiteand retarded growth, resulting from an inability to trans-form propionate to oxaloacetate. This condition is due to acobalt deficiency caused by very low cobalt levels in thesoil and thus in plant matter. Explain.

20. Fat Loss during Hibernation Bears expend about25 � 106 J/day during periods of hibernation, which may lastas long as seven months. The energy required to sustain lifeis obtained from fatty acid oxidation. How much weight loss(in kilograms) has occurred after seven months? How mightketosis be minimized during hibernation? (Assume the oxi-dation of fat yields 38 kJ/g.)

(CH2)7 (CH2)7

�-FluorooleateF CH2 C C

H H

COO�

COO�(CH2)nCH2