Fatty Acid SynthesisFatty Acid Synthesis

Dr. Sooad Al-DaihanDr. Sooad Al-DaihanBiochemistry department Biochemistry department

IntroductionIntroduction

There are three systems for the synthesis of fatty acids

1. De novo synthesis of FAs in cytoplasm

2. Chain elongation in mitochondria

3. Chain elongation in microsomes

In mammals fatty acid synthesis occurs primarily in the cytosol of the liver and adipose tissues .It also occurs in mammary glands during lactation.

Acetyl-CoA is the starting material for FA synthesis. However, most acetyl-CoA in mitochondria(from the breakdown of sugars, some amino acids and other fatty acids).

So, acetyl-CoA must be transferred from the mitochondria to the cytosol

BUT Mitochondria not

permeable to acetyl CoA

De novo synthesis of FAsDe novo synthesis of FAs

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Citrate-malate-pyruvate shuttle provides cytosolic acetyl CoA and reducing equivalents NADPH for fatty acid synthesis.

Acetyl–CoA units are shuttled out of the mitochondrial matrix as citrate.

4 Steps repeating cycle, extension 2C:

Condensation Reduction Dehydration Additional reduction

RULE: Fatty acid synthesis is a stepwise assembly of acetyl CoA unit (mostly as malonyl CoA) ending with palmitate (16 C saturated)

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Formation of Malonyl-coenzyme A Formation of Malonyl-coenzyme A (Activation of acetate)(Activation of acetate)

Is the committed step in fatty acid synthesis (Rate Limiting Reaction)

It takes place in two steps:

1. Carboxylation of biotin (involving ATP) 2. Transfer of the carboxyl to acetyl-CoA to form malonyl-CoA

Reactions are catalyzed by acetyl-CoA carboxylase (multienzyme)

It is a multi-enzyme complex consist of 7 enzymes linked covalently in a single polypeptide chain.

It is a dimer, and each monomer is identical, consisting of one chain (250 kD) containing all seven enzyme activities of fatty acid synthase and an acyl carrier protein (ACP)

ACP contains the vitamin pantothenic acid in the form of 4'-phosphopantetheine (Pant). ACP is the part that carry the acyl groups during fatty acid synthesis

Fatty acid synthase

1- A molecule of acetate is transferred from Acetyl CoA to the –SH group of ACP by acetyl CoA-ACP transacylase acetyl CoA-ACP transacylase (initiation or priming). 2- Next, this 2C fragment is transferred to a cysteine residue in the active site of the condensing enzyme. 3-The now-empty ACP accepts a 3C malonate unit from malonyl CoA, malonyl CoA-ACP transacylase catalyzes this reaction

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4 -Acetyl unit (on the condensing enzyme) condenses with 2 carbon portion of malonyl unit on

ACP forming acetoacetyl-S- ACP with release of CO2 .This reaction is catalyzed by β-ketoacyl –ACP synthase

Active site on the condensing enzyme is free .

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5-The β-ketone is reduced to an alcohol by e- transfer from NADPH .

6 -Dehydration yields a trans double bond.7- Reduction by NADPH yields a saturated chain .

8- Following transfer of the growing fatty acid from Pant to the Condensing Enzyme's cysteine sulfhydryl, the cycle begins again, with another malonyl-CoA. Note: Acetyl residue successively added is derived from the 2C atoms of malonyl CoA with the release of the third C as CO2 EXCEPT the 2 donated by the original acetyl CoA which are found at the methyl group end of the fatty acid.

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Product ReleaseProduct Release When the fatty acid is 16 carbon atoms long, a

Thioesterase domain catalyzes hydrolysis of the thioester linking the fatty acid to phosphopantetheine.

The16-C saturated fatty acid palmitate is the final product of the Fatty Acid Synthase complex (but it may produce short chain FAs)

Further elongation and insertion of double bonds are carried out by other enzyme system.

Palmitate, a 16-C saturated fatty acid, is the final product of the Fatty Acid Synthase reactions.

1- a. How many acetyl-CoA used for initial priming of enzyme? 1

b. How many acetyl-CoA used for synthesis of each malonate? 1

c. How many malonate used (how many reaction cycles) per synthesis of one 16-C palminate? 7

d. Total acetyl-CoA used for priming & for syntheisis of malonate, a + b(c): 8

2- a. How many ~P bonds of ATP used for synthesis of each malonate? 1 b. Total ~P bonds of ATP used for synthesis of one 16-C palmitate,

2a(1c): 7

3- a. How many NADPH used per reaction cycle? 2

b. Total NADPH used per synthesis of one 16-C palmitate, 3a(1c): 14

No. of cycles = (C/2) – 1No. of Malonate molecules = (C/2) – 1No. of Acetyl CoA molecules= [(C/2) – 1] +1No. of NADPH molecules = [(C/2) – 1] x2

Regulation of FA SynthesisRegulation of FA Synthesis

Allosteric regulation

• Acetyl CoA carboxylase, which catalyzes the committed step in fatty acid synthesis, is a key control site.

• End-product fatty acid is a feedback inhibitor (palmitoyl-CoA)

• Activated by citrate, which increases in well-fed state and is an indicator of a plentiful supply of acetyl-CoA

• Inhibited by long-chain acyl-CoA

Glucagon inhibits fatty acid synthesis while increasing lipid breakdown and fatty acid β-oxidation.

Acetyl CoA cayboxylase is inactivated by phosphorylation.

Insulin prevents action of glucagonInhibits lipases/activates acetyl Co A cayboxylase

Regulation of FA SynthesisRegulation of FA Synthesis

Further Processing of C16 Fatty Acids Additional Elongation

In mammalian systems FA elongation can occur either in : •Microsomes •Mitochondria

Chain Elongation in Microsomes

The reactions are similar to that which occurs in the cytosolic FA synthase in that: a) The source of the 2 carbon units is malonyl CoA. b) NADPH is used as reducing power.

In contrast to denovo synthesis of Fatty Acids, the intermediates in the subsequent reactions are CoA esters, indicating that the process is carried out by separate enzymes rather than a complex of FA synthase type. (uses CoA instead of ACP as the acyl carrier)

It is the main site for elongation of existing long chain FAs molecules.

It differs from the microsomal system in that acetyl CoA is the source of the added 2C atoms (instead of malonyl CoA)

NADH and NADPH are sources of reducing agents

This system operate by simple reversal of the pathway of FA oxidation with the exception that, NADPH-linked α,β-unsaturated acyl CoA reductase replaces FAD linked acyl CoA dehydrogenase.

The mitochondrial system serves in the elongation of shorter chain fatty acids to long chin FAs.

Chain Elongation in Mitochondria

Biosynthesis of Unsaturated Fatty Acids

Desaturases introduce double bonds at specific positions in a fatty acid chain.

Mammalian cells are unable to produce double bonds at certain locations, e.g., ∆ 12.

Thus some polyunsaturated fatty acids are dietary essentials, e.g., linoleic acid, 18:2 cis ∆ 9,12 (18 C atoms long, with cis double bonds at carbons 9-10 & 12-13)

Formation of a double bond in a fatty acid involves the following endoplasmic reticulum membrane proteins in mammalian cells: • NADH-cyt b5 Reductase, a flavoprotein with

FAD as prosthetic group. • Cytochrome b5, which may be a separate

protein or a domain at one end of the desaturase.

• Desaturase, with an active site that contains two iron atoms complexed by histidine residues

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The ∆9 desaturase in the endoplasmic reticulum catalyzes the conversion of stearate (18:0) to oleate (18:1 cis ∆ 9) .

the overall reaction is: stearate + NADH + H+ + O2

oleate + NAD+ + 2H2O Synthesis of

polyunsaturated fatty acids involves desaturase and elongase systems

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Differences in the oxidation and synthesis of FAs