Carboxylation of Acetyl CoA The first step in the fatty acid synthesis is the carboxylation of acetyl CoA to form malonyl CoA. The acetyl CoA carboxylase is not a pa of the multienzyme complex. But it is the rate-limiting enzyme. Biotin, a member of B complex vitamins, is necessary for this reaction (step 1 in Fig. 11.15). The enzyme is allosterically regulated, the major effectors being citrate (positive) and palmitoyl CoA (negative). The reaction is similar to carboxylation of pyruvate to form oxaloacetate. The elongation of the fatty acid occurs by addition of 2 carbon atoms at a time. But the 2-carbon units are added as 3-carbon, malonyl units. The whole reaction sequence occurs while the intermediates are bound to ACP (acyl carrier protein).Ref: DM Vasudevan - Textbook of Biochemistry, 8th edition, page no: 137