Question

Can anyone explain the structure and complex of Fatty Acid synthase I? I know the purpose of the enzyme, I just need a good explanation of how it works and moves the growing chain around the complex.

Answer 1

The first committed and irreversible step in fatty acid synthesis is the ATP dependent carboxylation of acetyl - CoA to form malonyl CoA .This reaction is catalyzed by acetyl - coA carboxlyase which has biotin as a prosthetic group .this enzyme is an allosteric in nature. Palmitoyl CoA , the final product of fatty acid synthesis acts as a negative modulator whereas citrate acts as an allosteric activator of this enzyme . Regulatory effects of citrate and palmitoyl coA are dependent on the phosphorylation state of the enzyme . Phosphorylation of the regulatory sites decreases the affinity of the enzyme for citrate and in this case high levels of citrate are required for activation. The next steps in the biosynthesis pathway are the formation of acetyl - ACP and malonyl - ACP by the enzymes acetyl transacylase and malonyl transacylase respectively. The activated intermediate of fatty acid biosynthesis are bound to a sulfhydryl group of a phosphopantetheine molecules. In prokaryotes, phosphopantetheine group is covalently bound to a small protein consists of a single polypeptide chain called Acyl carrier protein [ ACP ] . In eukaryotes ,it covalently bound to the ACP domain of multifunctional polypeptide termed fatty acid synthase.The last four enzymatic reactions of fatty acid biosynthesis constitute the elongation step.It includes condensation , reduction , dehydration , and reduction. Repetition of the elongation step , increses chain length by two carbon atoms.