seminar 8 - warszawski uniwersytet medyczny · amide linkage fatty acid phosphocholine ester bond ....

Seminar 8

Fatty acid metabolism


• Why are fatty acids important to cells?

• fuel molecules

• stored as triacylglycerols

• building blocks

• phospholipids

• glycolipids

• precursors of hormones and other messengers

• used to target proteins to membrane sites


• Why do triacylglycerols store large amounts of energy?

• fatty acid portion is highly reduced

• nonpolar molecules are stored in anhydrous form

• Where are triacylglycerols stored?

• adipocytes


• What is needed for triacylglycerol breakdown?

• bile salts

• made in liver, stored in gall bladder

• glycocholate

• lipases

• from pancreas

• hydrolyze ester bond

Triglyceride hydrolysis

• fatty acids and monoacylglcerols are absorbed across plasma membrane of intestinal epithelial cells

Lipid transport system

• chylomicrons are particles consisting of triacylglycerols and proteins (apolipoproteins)

Fatty acids as an energy source

• How are fatty acids made available to peripheral tissues as an energy source?

• hormones trigger lipolysis in adipose tissue

• epinephrine, glucagon, ACTH

• insulin inhibits lipolysis

• released fatty acids are insoluble in plasma

• must be attached to serum albumin for transport

Fatty acids as an energy source

• What happens to the glycerol released?

• It is converted to glyceraldehyde-3-phosphate

• glycolysis

• gluconeogenesis

Fatty acid degradation

Beta-oxidation of fatty acids

• the first step is an activation of fatty acids


• carnitine transports activated fatty acids into mitochondria matrix in fatty acid oxidation

Carnitine shuttle


• what is the reaction sequence for the oxidation of fatty acids?

1. first step is an oxidation (acyl CoA dehydrogenase)


2. second step is a hydration (enoyl CoA hydratase) this enzyme is stereospecific only L isomer is formed


3. third step is a second oxidation (L-3-hydroxyacyl CoA dehydrogenase)

Bata-oxidation of fatty acids

4. last step is cleavage of 3-ketoacyl CoA by thiol group of CoA acyl CoA shortened by 2 carbons acetyl CoA formed


• what are the products of fatty acid degradation?

for a C16 fatty acid:

• 8 acetyl CoA

• 7 FADH2

• 7 NADH + 7 H+

how much energy does this generate?

8 x 10 ATP = 80

7 x 1.5 ATP = 10.5

7 x 2.5 ATP = 17.5

Total = 108 ATP – 2 ATP (activation)

= 106 ATP

Beta-oxidation of unsaturated fatty acids

• unsaturated fatty acids require additional steps for degradation

isomerization shifts position and configuration of a double bond

reduction needed to remove double bond in wrong position

Beta-oxidation of odd-chain fatty acids

• how is the oxidation of odd-chain fatty acids different from even-chain ones?

• in final round of degradation products are acetyl CoA and proprionyl CoA (3C)

• proprionyl CoA is converted to succinyl CoA


• how is the oxidation of odd-chain fatty acids different from even-chain ones?

• in final round of degradation products are acetyl CoA and proprionyl CoA (3C)

• proprionyl CoA is converted to succinyl CoA

Step 1 Step 2 Step 3


Step 1

• proprionyl CoA is carboxylated to give D-methylmalonyl CoA (proprionyl CoA carboxylase) it uses biotin


Step 2

• D-methylmalonyl CoA is racemized to L form (methylmalonyl CoA mutase) it uses a derivative of vitamin B12


Step 3

• 5-deoxyadenosyl free radical removes a H atom to aid in rearrangement of L-methylmalonyl CoA to succinyl CoA


• where, in addition to the mitochondria does fatty acid oxidation take place?

• peroxisomes

• how is this different from oxidation?

• in first step electrons are transferred to O2

Ketone bodies

• what are ketone bodies?

• acetoacetate

• -hydroxybutyrate

• acetone

• under what conditions are they formed?

• when fats are rapidly broken down

Synthesis of ketone bodies

1. formation of acetoacetyl CoA (thiolase)


2. third molecule of acetyl CoA condenses with the acetoacetyl CoA, forming 3-hydroxy-3-methylglutaryl CoA (HMG CoA synthase) present only in the liver


3. HMG CoA is cleaved to yield acetoacetate and one molecule of acetyl CoA (HMG CoA lyase) present only in the liver


4. Acetoacetate can be reduced to beta-hydroxybutyrate (-hydroxybutyrate dehydrogenase)

4. Acetoacetate spontaneously decarboxylates to yield acetone (acetoacetate decarboxylase) (it cannot be metabolised any further and excreted through lungs)

Ketone bodies

• how can ketone bodies be used?

• major fuel source for heart muscle and kidney cortex

• during starvation or diabetes may be used by brain

• high levels of acetoacetate decreases lipolysis

Ketone body synthesis in the liver and use in peripheral tissues

Ketone bodies

• what is one important difference between plants and animals with respect to fatty acid metabolism?

• animals cannot use fatty acids to make glucose

• acetyl CoA cannot be converted to oxaloacetate („bypass 3” in gluconeogenesis)

Fatty acid synthesis

Transport of acetyl CoA to the cytosol

To synthesise palmitate you need: • 8 acetyl CoA • 14 NADPH • 7 ATP

Sources of NADPH for fatty acid synthesis

• Oxaloacetate must re-enter mitochondria

What is a main source of NADPH for lipogenesis?


• what is the first committed step in fatty acid synthesis?

• formation of malonyl CoA (acetyl CoA carboxylase) it uses biotin


• intermediates in fatty acid synthesis are linked to an acyl carrier protein (ACP)

Syntaza kwasów tłuszczowych

Stage 2 – formation of acetyl ACP and malonyl ACP

Stage 3 – condensation of acetyl ACP and malonyl ACP (+ decarboxylation)

Stage 4 – reduction of ketone group (C3) to form a methylene grup

Cycles 0. C2 + HCO3

- = C3

1. C2 + C3 = C4 – CO2

2. C4 + C3 = C6 – CO2

3. C6 + C3 = C8 – CO2

…

7. C14 + C3 = C16 – CO2

Stechiometry

Acetyl CoA + 7 malonyl CoA + 14 NADPH + 20 H+ →

Palmitate + 7 CO2 + 14 NADP+ + 8 CoA + 6 H2O

Control of fatty acid metabolism

• Acetyl CoA carboxylase (ACC) regulates this metabolism

• ACC is regulated by:

– glucagon i adrenaline (-)

– insulin (+)

– AMP (AMP-activated protein kinase) = phosphorylated ACC is inactive

– regulation of phosphatase

– concenration of citrate (allosterc control) = restores partially the activity of phosphorylated (inactive) ACC

Long-term control

• Adaptative control

• Based on a change of the rate of synthesis and degradation of enzymes in:

– -oxidation

– Synthesis of fatty acids

– Lipolysis (lipases)

Elongation and the synthesis of

unsturated fatty acids

Elongation and the synthesis of unsaturated bonds

• enzymes are on the cytosolic surface of ER

• process can investigated in microsomal systems (fragmentaded ER)

• to make unsaturated bonts additional enzymes are needed:

– cytochrome b5 reductase – cytochrome b5

– desaturase

e-

FAD

reductasa cyt b5 cytochrom b5 desaturase

Fe3+→Fe2+

hemowe Fe3+→Fe2+

niehemowe

e- e-

stearoil CoA NADH+H+

O2

oleil CoA NAD+

2H2O

• in mammals unsaturted fatty acids come from: – palmitoleic acid (16:1)

– oleic acid (18:1)

– linoleic acid (18:2)

– linolenic acid (18:3)

lack of enzymes to make double bonds in position futher than C9

Phospholipids

• Lecithin, was discovered in 1870 by the German

biochemist Ernst Hoppe- Selyer

• Strecker characterized choline in 1861

• In 1884, Thudichum JLW described sphingosine,

sphingomyelin, cerebrosides, cephalin and

lecithin in brain tissue

Phospholipids

• These are derivatives of phosphatidic acid, which is the

simplest phospholipid.

• Phosphatidic acid is made up of one glycerol to which two

fatty acid residues are esterified to carbon atoms 1 and 2

The 3rd hydroxyl group is esterified to a phosphoric acid

L-Phosphatidic acid

Phospholipids

• Phospholipids in general are amphipathic,

particularly Lecithin

• They have both hydrophobic and hydrophilic

portion in their molecule

Phospholipids

Phospholipids form the bilayer

Amphipathic nature

Phospholipids form micelles and liposomes

Amphipathic nature

• The glycerol along with the phosphoric acid and

choline constitute the polar „head” of a

phospholipid molecule, whereas the hydrocarbon

chains of the fatty acids represent the nonpolar

„tail”

Amphipathic nature

Micellar Formation

• When phospholipids are distributed in water, their

hydrophobic parts keep away from water, forming

molecular aggregates called micelle.

Phospholipids form micelles and liposomes

Liposomes

• A lipid bilayer will close on itself under appropriate conditions

to form liposomes. Unilamellar or multilamellar liposomes

may be formed. They may be prepared by sonication of

mixtures of phospholipids and cholesterol.

Liposomes

• Liposomes are microscopic spherical vesicles

• When mixed in water under special conditions,

the phospholipids arrange themselves to form a

bilayer membrane which encloses some of the

water in a phospholipid sphere

Liposomes

• Drugs, proteins, enzymes, genes, etc. may be

encapsulated by the liposomes which could act as

carriers for these substances to target organs

• Liposome-entrapped drugs exhibit superior

pharmacological properties than those observed

with conventional formulations

Liposomes

• Liposomes have important applications in:

• cancer chemotherapy

• antimicrobial therapy

• gene therapy

• vaccines and diagnostic imaging

Aquasomes

• They are one of the most recently developed delivery systems that are making a niche as the peptide/protein carriers

• These are nano particulate carrier systems with three layered self-assembled structures

Aquasomes

• They comprise the central solid nanocrystalline core coated

with polyhydroxy oligomers onto which biochemically

active molecules are adsorbed

• The solid core provides the structural stability

• The carbohydrate coating stabilizes the biochemically

active molecules

• As the conformational integrity of bioactive molecules is

maintained, aquasomes are being proposed as a carrier

system for delivery of peptide based pharmaceuticals

Membrane lipids

Phospholipids

• Derivatives of glycerol OR sphingosine

glycerol sphingosine

Fosfoglicerydy

• phosphatides (diacylglycerol 3-phosphate): – Hydroxylic groups C1 i C2 are estriried with fatty acid

– Hydroxylic groups C1 i C2 are estriried with phosphoric acid

• Not common in membranes

-

Fosfoglycerides in membranes

• Derivatives of phosphatides (phosphate is eterificated)

serine etanolamine choline

glycerol inozytol

Sphingolipids

• Phospholipids which are derivatives of sphingosine

• Amine group is connected to fatty acid (amide linkage)

• Hydroxyl group is esrificated with phosphocholine

sphingosine

sphingomyeline

sphingosine

Fatty acid Amide linkage

phosphocholine

Ester bond

Glycolipids

• Aminal glycolipods are derivatives of sphingosine

• Amine group is connected to fatty acid (amide)

• Hydroxyl group is connected to sugar

sphingosine

Fatty acid

sugar

ceramide

Cerebroside

• The simpliest glycolipid

sphingosine

Fatty acid

Amide linkage Galactose (or glucose)

Ester bond

Gangliosides

• Cantain branched chain up to 7 sugar residues

N-acetylneuraminic acid

Biomembranes • The molecules align themselves to form

monolayers with the polar heads pointing in one

direction and the nonpolar tails in the opposite

direction.

Phospholipids form the bilayer

Phosphatidylcholine or Lecithin

• This is a nitrogen containing phospholipid

• The word lecithin is derived from the Greek word,

lekithos = egg yolk

• It contains glycerol


• The alpha and beta positions are esterified with

fatty acids

• Usually, the fatty acid attached to the

betacarbon, is a PUFA molecule


Lecithin R1 and R2 are fatty acids Red rectangle – glycerol group The blue rectangle is choline which shows polar or hydrophilic property


• The phosphoric acid is added to the third position, to form

hosphatidic acid. The phosphate group is esterified to the

quaternary nitrogen containing group – Choline

Action of Phospholipases

• phospholipases are enzymes that hydrolyze

phospholipids

• different phospholipases are involved in the

hydrolysis of specific bonds in lecithin


• Phospholipase A2 acts on an intact lecithin molecule

hydrolyzing the fatty acid esterified to the beta (second)

carbon atom

• The products are Lysolecithin and fatty acid

• Lysolecithin is a detergent and hemolytic agent

• The enzyme is present in the venom of viper snakes

• The hemolysis and consequent renal failure seen in viper

poisoning could be thus explained


• The products formed in each case may be summarized

as follows:

Phospholipase A2

Lecithin Lysolecithin + fatty acid

Phospholipase A1

Lecithin 1 Acyl glycerophosphorylcholine + fatty acid

Phospholipase C

Lecithin 1,2 diacylglycerol + Phosphoryl choline

Phospholipase D

Lecithin Phosphatidic acid + choline

Lung Surfactants

• Normal lung function depends on a constant

supply of lung surfactants

• It is produced by epithelial cells

• It decreases surface tension of the aqueous

layer of lung and prevents collapse of lung

alveoli

Lung Surfactants

• Constituents of surfactants are dipalmitoyl lecithin,

phosphatidyl glycerol, cholesterol and surfactant

proteins A, B and C

• During fetal life, the lung synthesizes sphingomyelin

before 28th week of gestation

• But as fetus matures, more lecithin is synthesized

Lung Surfactants

• The lecithin-sphingomyelin (LS) ratio of amniotic

fluid is an index of fetal maturity

• A ratio of 2 indicates full lung maturity

• Low surfactant level can lead to respiratory

distress syndrome (RDS), which is a common

cause of neonatal morbidity

Respiratory Distress Syndrome (RDS)

• It is due to a defect in the biosynthesis of

dipalmitoyl lecithin (DPL), the main pulmonary

surfactant

• Premature infants have a higher incidence of

RDS because the immature lungs do not

synthesize enough DPL

Phosphatidylethanolamine or Cephalin

• Cephalin differs from lecithin in that the ethanolamine is

present instead of choline

• Cephalin is also found in biomembranes and possesses

amphipathic properties

Cephalin (Phosphatidylethanolamine)

Phosphatidylinositol

• Here, phosphatidic acid is esterified to inositol

• Phosphatidyl inositol bisphosphate or PIP2 is present in

biomembranes

• This compound plays a vital role in the mediation of hormone

action on biomembranes and acts as a second messenger

Phosphatidylinositol

Plasmalogens

Plasmalogens

• these are phospholipids

• presence of a vinyl ether linkage at the 1st C position and an ester linkage at the 2-nd C position

Ethanolamine plasmalogen

Plasmalogens

• The phosphoric acid is attached to choline or

ethanolamine.

Ethanolamine plasmalogen

Phosphatidylglycerol

• It is formed by esterification of phosphatidic

acid to glycerol

• When two molecules of phosphatidic acid are

linked with a molecule of glycerol,

diphosphatidylglycerol or cardiolipin is formed

Phosphatidylglycerol

• It is the major lipid of mitochondrial membrane

(Commercially, it is extracted from myocardium)

• Decreased cardiolipin level leads to

mitochondrial dysfunction, and is accounted for

• heart failure

• hypothyroidism

• some types of myopathies

Sphingolipids

• The sphingosine containing lipids may be of 3

types

• phosphosphingosides

• glycosphingolipids

• sulfatides

Phosphosphingosides

• They contain phosphoric acid group

• A common phosphosphingoside present abundantly in

membranes, especially of the nervous system, is

sphingomyelin (it contains choline)

Sphingomyelin

Sphingomyelins

• Sphingomyelins are the only sphingolipid that

contain phosphate and have no sugar moiety

• They are found in large quantities in nervous

system

Sphingomyelins

• Different sphingomyelins may be formed

depending on the fatty acid attached.

• Common fatty acids found are

• lignoceric (24 C),

• nervonic (24 C, one double bond) and

• cervonic (22 C, 6 double bonds) acids

Sphingomyelins

• Because of its amphipathic nature sphingomyelin can act

as an emulsifying agent and detergent

• The relative proportion of lecithin and sphingomyelin is

important in biological fluids like bile, amniotic fluid, etc.

• Sphingosine combined with fatty acid is called ceramide,

which is a component of glycosphingolipids

Clinical relevance of antiphospholipid antibody

Non-phosphorylated Lipids

• Glycosphingolipids (Glycolipids)

• They are seen widely in nervous tissues. This

group of lipids do not contain phosphoric acid;

instead they contain carbohydrates and ceramide

• Ceramide + Glucose → Glucocerebroside

• Ceramide + Galactose → Galactocerebroside

Globosides (Ceramide Oligosaccharides)

• They contain two or more hexoses or

hexosamines, attached to a ceramide molecule.

• Ceramide + Galactose + Glucose → Lactosyl

ceramide

• Lactosyl ceramide is a component of erythrocyte

membrane

Gangliosides

• They are formed when ceramide oligosaccharides

have at least one molecule of NANA (N-acetyl

neuraminic acid) (sialic acid) attached to them

• Ceramide—Glucose—galactose—NANA;

• This is designated as GM3 (ganglioside M3)

Gangliosides

• Gangliosides contribute to stability of

paranodal junctions and ion channel clusters

in myelinated nerve fibers.

• Autoantibodies to GM1 disrupt lipid rafts,

paranodal or nodal structures, and ion

channel clusters in peripheral motor nerves.

Sulfolipids or Sulfatides

• These are formed when sulfate groups are

attached to ceramide oligosaccharides

• All these complex lipids are important

components of membranes of nervous tissue

Sulfolipids or Sulfatides

• Failure of degradation of these compounds

results in accumulation of these complex

lipids in CNS

• This group of inborn errors is known as lipid

storage diseases

seminar 8 - warszawski uniwersytet medyczny · amide linkage fatty acid phosphocholine ester bond ....

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