Synthesis and degradation of fatty acids

Martina Srbová

Fatty acids (FA)

mostly an even number of carbon atoms and linear chain

in esterified form as component of lipids

in unesterified form in plasma binding to albumin

Groups of FA:

according to the number of double bonds

no double bond

one double bond

more double bonds

saturated FA (SAFA)

monounsaturated FA (MUFA)

polyunsaturated FA (PUFA)

according to the chain length

<C6

C6 – C12

C14 – C20

>C20

short-chain FA (SCFA)

medium-chain FA (MCFA)

long-chain FA (LCFA)

very-long-chain FA (VLCFA)

Overview of FA

Triacylglycerols

main storage form of FA

stored mainly in adipose tissue

acyl-CoA and glycerol-3-phosphate synthesis of TAG in liver

TAG transport from the liver to other tissues via VLDL (especially skeletal muscle, adipose tissue)

mainly in the liver, adipose tissue, mammary gland during lactation

localization: cell cytoplasm (up to C16)

endoplasmic reticulum, mitochondrion

enzymes: acetyl-CoA-carboxylase (HCO3- - source of CO2, biotin, ATP)

fatty acid synthase (NADPH + H+, pantothenic acid)

primary substrate: acetyl-CoA

final product: palmitate

(elongation = chain extension)

(always in excess calories)

FA biosynthesis

Acetyl-CoA1.

source: oxidative decarboxylation of pyruvate (the main source of glucose)

NADPH 2.

source: pentose phosphate pathway (the main source)

the conversion of malate to pyruvate (NADP+-dependent malate dehydrogenase - „malic enzyme”)

transport across the inner mitochondrial membrane as citrate

the conversion of isocitrate to α-ketoglutarate (isocitrate dehydrogenase)

degradation of FA, ketones, ketogenic amino acids

Precursors for FA biosynthesis

Precursors for FA biosynthesis

Acetyl-CoA

+HSCoA

OAA - oxaloacetate

Formation of malonyl-CoA catalysed by acetyl-CoA-carboxylase (ACC)

HCO3- + ATP ADP + Pi

enzyme-biotin enzyme-biotin-COO-

enzyme-biotin

acetyl-CoA

malonyl-CoA

+

1 carboxylation of biotin 2 transfer of carboxyl group to acetyl-CoA

formation of malonyl-CoA

enzyme – acetyl-CoA-carboxylase

FA biosynthesis

repeated extension of FA by two carbons in each cycle

on the multienzyme complex – FA synthase

to the chain length C16 (palmitate)

FA biosynthesis

ACP – acyl carrier protein

FA biosynthesis

The course of FA biosynthesis

acetyl-CoA malonyl-CoA

acetyltransacylase

acyl(acetyl)-malonyl-

CoASH CoASH

transacylation

-enzyme complex

malonyltransacylase

acyl(acetyl)-malonyl-enzyme complex 3-ketoacyl-enzyme complex

3-ketoacyl-synthase

CO2

condensation

(acetacetyl-enzyme complex)

FA biosynthesis

The course of FA biosynthesis

3-ketoacyl-enzyme complex(acetoacetyl-enzyme complex)

3-hydroxyacyl-enzyme complex

NADPH + H+ NADP+

3-ketoacyl-reductase

H2O

3-hydroxyacyl- dehydrase

2,3-unsaturated acyl-enzyme complex acyl-enzyme complex

NADPH + H+ NADP+

enoylreductase

first reduction dehydration second reduction

FA biosynthesis

The course of FA biosynthesis

Repetition of the cycle

acyl-enzyme complex(palmitoyl-enzyme complex)

CoASH

malonyl-CoA

FA biosynthesis

The release of palmitate

palmitoyl-enzyme complex

+

palmitate

thioesterase

FA biosynthesis

H2O

The fate of palmitate after FA biosynthesis

palmitate palmitoyl-CoA

acylglycerols

cholesterol esters

acyl-CoA

esterification

elongation

desaturation

acyl-CoA-synthetase

ATP + CoA AMP + PPi

FA biosynthesis

FA elongation

microsomal elongation system1.

in the endoplasmic reticulum

malonyl-CoA – the donor of the C2 units

extension of saturated and unsaturated FA

palmitic acid (C16) FA > C16 elongases (chain elongation)

mitochondrial elongation system2.

in mitochondria

acetyl-CoA – the donor of the C2 unit

NADPH + H+ – the donor of the reducing equivalents

not reverse β-oxidation

FA biosynthesis

fatty acid synthase

FA desaturation

in the endoplasmic reticulum

process requiring O2, NADH, cytochrome b5

FA biosynthesis

enzymes: desaturase, NADH-cyt b5-reductase4 desaturases: double bonds at position 4,5,6,9

linoleic, linolenic – essential FA

stearoyl-CoA + NADH + H+ + O2 oleoyl-CoA + NAD+ + 2H2O

FA biosynthesis - summary

• Formation of malonyl-CoA• Acetyl-CoA-carboxylase

• FA synthesis Palmitic acid

• FA Synthase– cytosol

Saturated fatty acids(>C16) Elongation systems- mitochondria, ER

Unsaturated fatty acids Desaturation system - ER

-

FA degradation

function: major energy source

(especially between meals, at night, in increased demand for energy intake – exercise)

release of FA from triacylglycerols in adipose tissue into the bloodstream

binding of FA to albumin in the bloodstream

transport to tissues

entry of FA into target cells activation to acyl-CoA

transfer of acyl-CoA via carnitine system into mitochondria

β-oxidation

1 2

3

4

5 In the liver , acetyl CoA is converted to ketone bodies

FA degradation

http://che1.lf1.cuni.cz/html/Odbouravani_MK_3sm.pdf

C10 , C12 Branched FAVLCFA

-carbon β-carbon -carbon

-oxidationβ-oxidation-oxidation

mainly in muscles

localization: mitochondrial matrix

peroxisome (VLCFA)

enzymes: acyl CoA synthetase

carnitine palmitoyl transferase I, II; carnitine acylcarnitine translocase

substrate: acyl-CoA

final products: acetyl-CoA

β-oxidation

dehydrogenase (FAD, NAD+), hydratase, thiolase

propionyl-CoA

FA degradation

repeated shortening of FA by two carbons in each cycle

oxidation of acetyl-CoA to CO2 and H2O in the citric acid cycle

generation of 8 molecules of acetyl-CoA from 1 molecule of palmitoyl-CoA

cleavage of two carbon atoms in the form of acetyl-CoA

complete oxidation of FA

PRODUCTION OF LARGE QUANTITY OF ATP

production of NADH, FADH2 reoxidation in the respiratory chain to form ATP

FA degradation

β-oxidation

Activation of FA

fatty acid ATP

pyrophosphate (PPi)

acyl-CoA AMP

acyl-CoA-synthetase

acyl-CoA-synthetase pyrophosphatase

acyl adenylate

fatty acid+ ATP + CoASH acyl-CoA + AMP + PPi

PPi + H2O 2Pi

2Pi

FA degradation

The role of carnitine in the transport of LCFA into mitochondrion

FA transfer across the inner mitochondrial membraneby carnitine and three enzymes:

carnitine palmitoyl transferase I (CPT I)

acyl transfer to carnitine

carnitine acylcarnitine translocase

acylcarnitine transfer across the inner mitochondrial membrane

carnitine palmitoyl transferase II (CPT II)

acyl transfer from acylcarnitine back to CoA in the mitochondrial matrix

FA degradation

Carnitine

FA degradation

3-hydroxy-4-N-trimethylaminobutyrate

Sources:Exogenous: meat, dairy productsEndogenous: synthesis from lysine and methionine

Transported into the cell by specific transporter

Deficiency:Decreased transport of acyl-CoA into mitochondria

lipids accumulationmyocardial damagemuscle weakness

Increased utilization of Glchypoglycemia

Similar symptoms are the genetically determined deficiency carnitinpalmitoyltransferase I or II

acyl-CoA

trans-Δ2-enoyl-CoA

L-β-hydroxyacyl-CoA

β-ketoacyl-CoA

acyl-CoA acetyl-CoA

acyl-CoA-dehydrogenase

enoyl-CoA-hydratase

L-β-hydroxyacyl-CoA-

β-ketoacyl-CoA-thiolase

Steps of cycle:

dehydrogenation

oxidation by FADcreation of unsaturated acid

hydration

addition of water on the β-carbon atomcreation of β-hydroxyacid

dehydrogenation

oxidation by NAD+

creation of β-oxoacid

cleavage at the presence of CoA

formation of acetyl-CoAformation of acyl-CoA (two carbons shorter)

β-oxidation

-dehydrogenase

FA degradation

http://che1.lf1.cuni.cz/html/Odbouravani_MK_3sm.pdf

FA degradation

Oxidation of unsaturated FA

3 rounds of β-oxidation

Normal intermediates of β-oxidation

β-oxidation of oleic acids the most common unsaturated FA in the diet:

degradation of unsaturated FA by β-oxidation to a double bond

conversion of cis-isomer of FA by specific isomerase to trans-isomer

oleic acid, linoleic acid

intramolecular transfer of double bondfrom β- to - β position

Unsaturated FA are cis isomers - aren´t substrate for enoyl-coA hydratase

continuation of β-oxidation

Oxidation of odd-chain FA

propionyl-CoA

methylmalonyl-CoA

succinyl-CoA

HCO3- + ATP

ADP + Pi

propionyl-CoA carboxylase (biotin)

methylmalonyl-CoA mutase (B12)

shortening of FA to C5

formation of acetyl-CoA and propionyl-CoA

carboxylation of propionyl-CoA

intramolecular rearrangement to form succinyl-CoA

entry of succinyl-CoA into the citric acid cycle

stopping of β-oxidation

FA degradation

Peroxisomal oxidation of VLCFA

Very-long-chain FA (VLCFA, > C20)

Differences between β-oxidation in the mitochondrion and peroxisome:

1. step – dehydrogenation by FAD

mitochondrion: electrons from FADH2 are delivered to the respiratory chain where they are transferred to O2 to form H2O and ATP

peroxisome: electrons from FADH2 are delivered to O2 to form H2O2,

which is degraded by catalase to H2O and O2

3. step – dehydrogenation by NAD+

mitochondrion: reoxidation of NADH in the respiratory chain

peroxisome: reoxidation of NADH is not possible, export to the cytosol or the mitochondrion

transport of acyl-CoA into the peroxisome without carnitine

FA degradation

Differences between β-oxidation in the mitochondrion and peroxisome:

4. step – cleavage at the presence of CoA

mitochondrion: metabolization in the citric acid cycle

peroxisome: export to the cytosol, to the mitochondrion (oxidation)

acetyl-CoA

a precursor for the synthesis of cholesterol and bile acids

a precursor for the synthesis of fatty acidsof phospholipids

Peroxisomal oxidation of VLCFA

FA degradation

In peroxisome shortened FA bind to carnitine acylcarnitine

transfer acylcarnitine into mitochondrion β-oxidation

Oxidation carbon

ER liver, kidney

Substrates C10 a C12 FA

Products: dicarboxylic acids

FA degradation

- oxidation

Excreted in the urine

mixed function oxidase

Comparison of FA biosynthesis and FA degradation

in the liver

localization: mitochondrial matrix

substrate: acetyl-CoA

products: acetone

acetoacetate

D-β-hydroxybutyrate

conditions: in excess of acetyl-CoA

function: energy substrates for extrahepatic tissues

Ketone bodies

Ketogenesis

medium strength acids - ketoacidosis

Ketone bodies

Ketogenesis

acetoacetate

spontaneous decarboxylation to acetone

conversion to D-β-hydroxybutyrate by D-β-hydroxybutyrate dehydrogenase

waste product (lung, urine)

energy substrates for extrahepatic tissues

Ketone bodies

Ketogenesis

Utilization of ketone bodies

citric acid cycle

energy source for extrahepatic tissues

(especially heart and skeletal muscle)

in starvation - the main source of energy

energy

production

water-soluble FA equivalents

Ketone bodies

for the brain

lipolysis

FA in plasma

β-oxidation

excess of acetyl-CoA

ketogenesis

increased ketogenesis:

starvation

prolonged exercise

diabetes mellitus

high-fat diet

low-carbohydrate diet

utilization of ketone bodies as an energy source

to spare of glucose and muscle proteins

(skeletal muscle, intestinal mucose, adipocytes, brain, heart etc.)

Ketogenesis

Ketone bodies

http://www.hindawi.com/journals/jobes/2011/482021/fig2/

Marks, A.; Lieberman, M. Marks' basic medical biochemistry: a clinical approach. 3rd edition. Lippincott Williams & Wilkins, 2009.

Meisenberg, G.; Simmons, W. H. Principles of medical biochemistry. 2nd edition. Elsevier, 2006.

Matouš a kol. Základy lékařské chemie a biochemie. Galén, 2010.

Devlin, T. M. Textbook of biochemistry: with clinical correlations. 6th edition. Wiley-Liss, 2006.

Murray et al. Harper's Biochemistry. 25th edition. Appleton & Lange, 2000.

Bibliography and sources