Rate limiting enzyme in fatty acid synthesis -

Ans. is 'c' i.e., Acetyl CoA carboxylase o Production of malonyl-CoA is the initial and rate limiting step in fatty acid synthesis.o Acetyl-CoA needs to be converted to activated form, which will serve as the donor of carbon units to growing fatty acid chain.o Malonyl-CoA) a 3- carbon compound Is such activated form.o It is produced by carboxylation of acetyl-CoA, a reaction catalyzed by acetyl-CoA caroxylase.o Acety l-CoA carboxylase requires biotin as a cofactor.o Acetyl-CoA carboxylase is a multienzyme protein containing variable number of identical subunits, each containing biotin carboxylase, biotin carboxyl carrier protein and transcarboxylase.o The reaction also requires HCO- (as a source of CO) and A TPo The reaction takes place in two steps: (i) Carboxylation of biotin involving HCO. and ATP, and (ii) transfer of the carboxyl group to acetyl-CoA to form malonyl-CoA.o This is a typical ATP-dependant carboxylation reaction, with biotin (bound to enzyme) serving as a carrier of carboxyl group.o Since malonyl-CoA is not used in any other metabolic pathway, this reaction is the commited step of fatty acid synthesis i.e. once malonyl-CoA is formed, the cycle will proceed towards fatty acid synthesis only.