Production of malonyl-CoA is the initial and rate limiting step in fatty acid synthesis.Acetyl-CoA needs to be conveed to activated form, which will serve as the donor of carbon units to growing fatty acid chain.Malonyl-CoA) a 3- carbon compound is such activated form.It is produced by carboxylation of acetyl-CoA, a reaction catalyzed by acetyl-CoA caroxylase.Acetyl-CoA carboxylase requires biotin as a cofactor.Acetyl-CoA carboxylase is a multienzyme protein containing variable number of identical subunits, each containing biotin carboxylase, biotin carboxyl carrier protein and transcarboxylase.The reaction also requires HCO3- (as a source of CO) and ATPThe reaction takes place in two steps:(i) Carboxylation of biotin involving HCO3 and ATP(ii) transfer of the carboxyl group to acetyl-CoA to form malonyl-CoA.