The metabolism of propionic acid (propanoate) begins with its conversion to propionyl coenzyme A (propionyl-CoA), which is the usual first step in the metabolism of carboxylic acids. Since propanoate has three carbons, propionyl-CoA cannot directly enter either the beta oxidation nor the citric acid cycles. In most vertebrates, propionyl-CoA is carboxylated to D-methylmalonyl-CoA, which is isomerised to L-methylmalonyl-CoA. A vitamin B12-dependent enzyme, called methylmalonyl CoA mutase catalyzes the rearrangement of L-methylmalonyl-CoA to succinyl-CoA, which is an intermediate of the citric acid cycle. Malonyl-CoA, another product of propanoate metabolism, is also used in transporting alpha-ketoglutarate across the mitochondrial membrane into the mitochondrial matrix. Malonyl-CoA is formed by carboxylating acetyl-CoA using the enzyme acetyl-CoA carboxylase. One molecule of acetyl-CoA joins with a molecule of carbon dioxide, requiring energy rendered from ATP.