Fatty acid biosynthesis

Reversal of β-oxidation

Two reactions enable the synthesis pathway:

Acetyl-CoA is "activated" by the addition of a carbon dioxide, and

NADPH is substituted as a more powerful reducing agent than FADH2.

The first step in fatty acid biosynthesis is to activate acetyl-CoA by the addition of a carbon dioxide using Acetyl-CoA carboxylase.):

Acetyl-CoA+CO2 CH3CH2COCoA

Fatty acid synthase complex:

There are two carriers on this complex.

The first is referred to as ACP1 which acts as a holding station for acetyl- or fatty acyl- groups. bound to a cysteinyl sulfhydryl group.

The second, ACP2, binds the growing fatty acyl chain during the condensation and reduction reactions of the cycle. In this case the acyl group is carried on a long phosphopantetheine.

The first reaction, catalyzed by Acetyl-CoA:ACP transacylase, transfers an acetyl group from Coenzyme A to the cysteinyl-S on ACP1.

Next, a malonyl-group is transferred from a Coenzyme A to the pantetheinyl-S of ACP2 by Malonyl-CoA:ACP transacylase.

The carbon dioxide leaves the malonyl group, with the electrons from its bond attacking the acyl group on ACP1. This reaction is catalyzed by Ketoacyl-ACP synthase.

Reversal of β-oxidation

Thus the keto-group is reduced to an alcohol using NADPH (β-ketoacyl-ACP reductase),

followed by the elimination of the alcohol (Enoyl-ACP hydrase) to give the cis-2,3-enoyl group.

The enoyl is then reduced with NADPH substituting for FADH2 (Enoyl-ACP reductase) to give the saturated acyl group.

Finally the acyl group is transferred from the pantotheinyl-S of ACP2 to the cysteinyl-S on ACP1 (ACP-acyltransferase) leaving ACP2 available to pick up the next malonyl moiety.

Elongation of f.a

For longer fatty acids use a fatty acid elongation system localized on the ER.

The same reactions occur as in the Synthesis, but now have individual enzymes.

E.g. Palmitate is first activated to palmitoyl-CoA. The enzymes prefer C-16 or less as substrate; thus the major product is stearoyl-CoA

Following each round of elongation the beta keto group is reduced to the fully saturated carbon chain by the sequential action of a ketoreductase (KR), dehydratase (DH), and enol reductase (ER).

The growing fatty acid chain is carried between these active sites while attached covalently to the phosphopantetheine prosthetic group of an acyl carrier protein (ACP), and is released by the action of a thioesterase (TE) upon reaching a carbon chain length of 16 (palmitidic acid

If the Fatty Acid Synthetase Complex only makes palmitate where do the rest of the fatty acids come from?

i. Palmitate can be shortened by ß-oxidation.

ii. For longer fatty acids there is a fatty acid elongation system localized on the ER. The same reactions occur as in the Synthetase, but now have individual enzymes.

a. Palmitate is first activated to palmitoyl-CoA. The enzymes prefer C-16 or less as substrate; thus the major product is stearoyl-CoA.

b. Also, longer unsaturated fatty acids will also bind (the kinking of the cis double bond makes them effectively shorter), so unsaturated fatty acids of 20, 22, and 24-C's are also made. Thus most longer fatty acids are polyunsaturated

A second system for fatty acid elongation exists in the mitosol, probably for provision of long fatty acids for mitochondrial structure.

This system uses most of the same activities of β-oxidation, but an NADPH dependent Enoyl-CoA reductase replaces the FAD dependent dehydrogenase

Integration and control of fat metabolism

The reactions of fatty acid synthesis all take place in the cytosol, but acetyl-CoA is made in the mitochondria and can't cross the inner membrane. The Pyruvate-Malate Cycle (Citrate-Pyruvate Cycle) is used to take acetyl- groups to the cytosol while simultaneously providing a source of NADPH from NADH, and thus coupling fatty acid synthesis to Glycolysis.

Note that the acetyl-CoA is first joined to oxaloacetate to make citrate which is readily transported out of the mitochondria using a co-transporter.

The citrate is then cleaved to acetyl-CoA and oxaloacetate, a process requiring ATP to make it favourable .

Acetyl-CoA for fatty acid synthesis is now available in the cytosol, but oxaloacetate must be regenerated for the mitosol.

The cytosolic oxaloacetate is now dehydrogenated to give malate and NAD+.

Malate is next oxidized by Malic enzyme to give pyruvate in a reaction which also provides NADPH for use in biosynthesis