Fatty acid synthesis

Glycerol-P Glycerol

Triacylglycerol

Fatty acyl CoA Fatty acid

Malonyl CoA

Acetyl CoA

Glucose

Pyruvate

TCA cycle

Fed state

Glycerol-P Glycerol

Triacylglycerol

Fatty acyl CoA Fatty acid

Malonyl CoA

Acetyl CoA

Glucose

Pyruvate

TCA cycle

Starved state

gluconeogenesis

Fatty acid biosynthesis

A three carbon intermediate, malonyl-CoA, initiates fatty acid synthesis

• FA biosynthesis and breakdown occur by different pathways and take place in different parts of the cell.

• Biosynthesis requires malonyl-CoA

Glycerol-P

Triacylglycerol

Fatty acyl CoA

Malonyl CoA

Acetyl CoA

Glucose

Pyruvate

TCA cycle

The carboxylation of acetyl-CoA yields malonyl-CoA

CH3-C-S-CoA

=O

HCO3-

-OOC-CH2-C-S-CoA

=O

Acetyl-CoA

Malonyl-CoA

Acetyl CoA carboxylase

Assembly of a long chain fatty acid

• Once malonyl-CoA is synthesized, long carbon FA chains may be assembled in a repeating four-step sequence.

• With each passage through the cycle the fatty acyl chain is extended by two carbons.

• When the chain reaches 16 carbons, the product palmitate (16:0) leaves the cycle.

The first round of FA biosynthesis

• To initiate FA biosynthesis, malonyl and acetyl groups are activated on to the enzyme fatty acid synthase.

H

H

Malony-CoA

Acetyl-CoA

+

Step 1.

• Condensation of an activated acyl group and two carbons derived from malonyl-CoA

Step 2.

• The -keto group is reduced to an alcohol by NADPH

Step 3.

• The elimination of water creates a double bond.

• The double bond is reduced to form the corresponding saturated fatty acyl group.

Step 4.

Repetition of these four steps leads to fatty acid synthesis

• When reaches 16 carbons, the product leaves the cycle.

• All the reactions in the synthetic process are catalyzed by a multi-enzyme complex, fatty acid synthase.

A more detailed look at fatty acid synthase

Fatty acyl synthase contains six enzymatic activities

• Each segment of the disk represents one of the six enzymatic activities of the complex.

• At the center is the ACP – acyl carrier protein - with its phosphopantetheine arm ending in –SH.

The function of the prosthetic group of the ACP

• Serve as a flexible arm, tethering the growing fatty acyl chain to the surface of the synthase complex

• Carrying the reaction intermediates from one enzyme active site to the next.

Activation of acetyl and malonyl groups

• Before Steps 1-4, the two thiol groups on the enzyme complex must be charged with the correct acyl groups.

• The acetyl group from acetyl-CoA is transferred to the Cys-SH group of the -ketoacyl ACP synthase.

• This reaction is catalyzed by acetyl-CoA transacetylase.

The activation of the acetyl group

The activation of the malonyl group

• Transfer of the malonyl group to the –SH group of the ACP is catalyzed by malonyl-CoA ACP transferase.

• The charged acetyl and malonyl groups are now in close proximity to each other

• Condensation of the activated acetyl and malonyl groups to form acetoacetyl-ACP, catalyzed by -ketoacyl-ACP synthase.

Step 1.

Step 2.• Reduction. The acetoacetyl-ACP is reduced to -

hydroxybutyryl-ACP, catalyzed by ketoacyl-ACP reductase (needs NADPH + H+)

Step 3.• Dehydration to yield a double bond in the product, trans-

2-butenoyl-ACP, catalyzed by hydroxyacyl-ACP dehydratase.

Step 4.

• Reduction of the double bond to form butyryl-ACP, catalyzed by enoyl-reductase.

• Another NADPH dependent reaction.

The growing chain is transferred from the acyl carrier protein

• This reaction makes way for the next incoming malonyl group.

• The enzyme involved is acetyl-CoA transacetylase.

Beginning of the second round of the FA synthesis cycle

• The butyryl group is on the Cys-SH group.

• The incoming malonyl group is first attached to ACP.

• In the condensation step, the entire butyryl group is exchanged for the carboxyl group on the malonyl residue.

The result of fatty acyl synthase activity

• Seven cycles of condensation and reduction produce the 16-carbon saturated palmitoyl group, still bound to ACP.

• Chain elongation usually stops at this point, and free palmitate is released from the ACP molecule by hydrolytic activity in the synthase complex.

• Smaller amounts of longer fatty acids such as stearate (18:0) are also formed.

The overall reaction for the synthesis of palmitate from acetyl-CoA can be considered in two parts.

Part 1.

• First, the formation of seven malonyl-CoA molecules:

7Acetyl-CoA + 7CO2 + 7ATP 7malonyl-CoA + 7ADP + 7Pi

Part 2.

• Then the seven cycles of condensation and reduction

Acetyl-CoA + 7malonyl-CoA + 14NADPH + 14H+

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

• The biosynthesis of FAs requires acetyl-CoA and the input of energy in the form of ATP and reducing power of NADPH.

Location of FA synthesis

• FA synthase complex is found exclusively in the cytosol.

• The location segregates synthetic processes from degradative reactions.

In hepatocytes:

the [NADPH]/[NAD+] ratio is very high (~75) in the cytosol, furnishing a strongly reducing environment for the reductive synthesis of fatty acids and other biomolecules.

Fatty acid synthesis requires considerable amounts of NADPH + H+

Acetyl-CoA + 7malonyl-CoA + 14NADPH + 14H+

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

• In hepatocytes and adipocytes, cytosolic NADPH is largely generated by the malic enzyme and by the pentose phosphate pathway.

1. The malic enzyme

• The pyruvate produced in the reaction reenters the mitochondrion.

2. The pentose phosphate pathway

• In hepatocytes and the mammary gland of lactating animals, the NADPH is supplied primarily by the pentose phosphate pathway.

Fatty acid synthesis requires considerable amounts of acetyl-CoA

7Acetyl-CoA + 7CO2 + 7ATP 7malonyl-CoA + 7ADP + 7Pi

• Nearly all acetyl-CoA used in fatty acid synthesis is formed in mitochondria from pyruvate oxidation.

• So acetate must go from the mitochondria to the cytosol

Mitochondria – site of acetate manufacture

Cytosol – site of acetate utilization

Acetate is shuttled out of mitochondria as citrate

• The mitochondrial inner membrane is impermeable to acetyl-CoA

• Intra-mitochondrial acetyl-CoA first reacts with oxaloacetate to form citrate, in the TCA cycle catalyzed by citrate synthase.

• Citrate then passes into the cytosol through the mitochondrial inner membrane on the citrate transporter.

• In the cytosol, citrate is cleaved by citrate lyase regenerating acetyl-CoA.

• The other product --oxaloacetate cannot return to the mitochondrial matrix directly.

• Instead, oxaloacetate is reduced to malate

• Malate returns to the mitochondrial matrix on the malate--ketoglutarate transporter in exchange for citrate.

Regulation of fatty acid synthesis

• When a cell has more energy, the excess is generally converted to FAs and stored as lipids such as triacylglycerol.

• The reaction catalyzed by acetyl-CoA carboxylase is the rate limiting step in the biosynthesis of fatty acids.

The carboxylation of acetyl-CoA yields malonyl-CoA

CH3-C-S-CoA

=O

HCO3-

-OOC-CH2-C-S-CoA=O

Acetyl-CoA

Malonyl-CoA

Regulation of acetyl-CoA carboxylase(1)

• Palmitoyl-CoA acts as a feedback inhibitor of the enzyme, and citrate is an activator.

• When there is an increase in mitochondrial acetyl-CoA and ATP, citrate is transported out of mitochondria,

• Citrate becomes both the precursor of cytosolic acetyl-CoA and a signal for the activation of acetyl-CoA carboxylase.

Regulation of acetyl-CoA carboxylase (2)

Regulation of acetyl-CoA carboxylase (3)

• Additionally, these pathways are regulated at the level of gene expression.

• For example, when animals ingest an excess of certain polyunsaturated fatty acids, the expression of genes encoding a wide range of lipogenic enzymes in the liver is suppressed.

Additional modification to the newly synthesized fatty acid

• Extended to form longer fatty acids

• Converted to monounsaturated and polyunsaturated fatty acids

Fatty acid elongation

• Palmitate in animal cells is the precursor of other long-chained FAs.

• By further additions of acetyl groups, through the action of FA elongation systems present in the smooth endoplasmic reticulum and the mitochondria.

The desaturation of FAs

• Palmitate and stearate serve as precursors of the two most common monosaturated fatty acids of animal cells: palmitoleate (16:19), and oleate (18:19).

• The double bond is introduced by fatty acyl-CoA desaturase in the smooth endoplasmic reticulum.

• Mammalian hepatocytes readily introduce double bonds at the 9 position of FAs but cannot between C-10 and the methyl-terminal end.

• Linoleate, 18:29,12 and linolenate 18:39,12,15 cannot be synthesized by mammals, but plants can synthesize both.

Essential fatty acids

The fate of fatty acids

• Most of the FAs synthesized or ingested by an organism have one of two fates:

i. incorporated into triacylglycerols for the storage of metabolic energy

ii. incorporation into the phospholipid components of membranes.

The formation of phosphatidic acid

• Fatty acyl groups are first activated by formation of fatty acyl-CoA molecules.

• then transferred to ester linkage with L-glycerol 3-phosphate.

Phosphatidic acid may be converted to triacylglycerols or phospholipids

• Triacylglycerols and phosholipids are both synthesized from phosphatidic acid

Lecithin (phosphatidyl choline)

O

O

H2C O C RO

R C O CH

H2C O P O CH2 CH2 N+ CH3

CH3

CH3

O_

Phosphatidic Acid Choline

Partitioning of the fates of fatty acids

• Depends on the needs of the organism:

• During rapid growth, synthesis of new membranes requires membrane phospholipid synthesis

• Organisms that have a plentiful supply of food but are not actively growing shunt most of their fatty acids into storage fats.

Summary of lipid metabolism

• FA biosynthesis requires malonyl-CoA formation• The long carbon chains of FA acids are assembled in

a repeating four-step sequence catalyzed by the multifunctional enzyme fatty acid synthase.

• With each passage through the cycle, the fatty acyl chain is extended by two carbons

• When the chain length reaches 16 carbons, the product (palmitate 16:0) leaves the cycle.

• Cytosolic NADPH is largely generated by the malic enzyme and by the pentose phosphate pathway.

• FA biosynthesis occurs in the cytosol• FA biosynthesis is regulated by the activity

of acetyl-CoA carboxylase• Synthesized FA are either stored as TG or

made into membrane lipids