Introduction:

Early radioisotope incorporation experiments demonstrated that all of the carbons in fatty acids are derived from acetate. In vitro incubations with acetyl-CoA were unsuccessful in generating fatty acids. Salih Wakil discovered that malonate was required for active synthesis. How does this happen?

The key reaction is the formation of malonyl-CoA from acetyl-CoA. Carboxylation of acetate is catalyzed by acetyl-CoA carboxylase, which requires the cofactor biotin. The carbon from bicarbonate does not appear in the product of the condensation reaction (see below).

The 77 amino acid residues of the acyl carrier protein replace the ADP found in coenzyme A. The long arm of the remaining portion of this structure permits the substrate to swings to the various enzyme sites the catalyze the sequence of reactions below:

Reactions:

The reactions of a cycle of fatty acid synthesis can be summarized as:

a. a condensation reaction between acetyl-CoA and malonyl-CoA

b. reduction of the beta keto function to an alcohol

c. dehydration of the beta hydroxyl group (produces a double bond)

d. reduction of the double bond

The net reaction for the synthesis of palmitate (C-16) is

Acetyl-CoA + 7 malonyl-CoA + 14 NADPH + 7 H+ ----> palmitate + 7 CO2

+ 8 CoA + 6 H2O

The 14 moles of NADPH required for this synthesis is the equivalent of 42 moles of ATP!

Acetate Shuttle Reactions:

Acetate is formed in the mitochondrion via the pyruvate dehydrogenase reaction. Transfer of this acetate from the mitochondrion to the cytoplasm is required for fatty acid synthesis. One important reaction in this sequence, catalyzed by citrate lyase, is

citrate (from the mitochondrion) + ATP + CoA -------> Ac-CoA + ADP + Pi + OAA

Regulation

Fatty acid biosynthesis is regulated at the level of acetyl-CoA carboxylase reaction:

Ac-CoA + HCO3- + ATP ----> malonyl-CoA + ADP + Pi

High levels of malonate also inhibit the carrier protein that transports citrate from the mitochondrion to the cytoplasm (see above).

Comparison of Biosynthesis with Oxidation:

a. in cytosol vs. in mitochondrion

b. uses acyl carrier protein vs. CoA

c. multienzyume complex vs. a series of enzymes

d. addition of acetate to activated malonate vs. thiolysis to acetate

e. reductant is NADPH vs. NAD+ & FAD as oxidants

f. stops at the level of palmitate (C-16) vs. stops at acetate (C2) and C3