Biotin is a cofactor for carboxylase reactions, where it is a carrier of CO2. Steps of the carboxylation reactions requiring biotin are given below: * Conversion of acetyl CoA into malonyl CoA, catalyzed by acetyl CoA carboxylase. * Synthesis of pyruvate from oxaloacetate, catalyzed by pyruvate carboxylase (Figure 7.3A). * Conversion of propionyl CoA to methylmalonyl CoA, catalyzed by propionyl CoA carboxylase. This involves the pathway of catabolism of propionate to succinate (Figure 7.3B). * It is also involved in the metabolism of branched-chain amino acid, analyzed by 3-methylcrotonyl CoA carboxylase.