We have now seen how the 20 different amino acids, after loss of their nitrogen atoms, are degraded by dehydrogenation, decarboxylation, and other reactions to yield portions of their carbon backbones in the form of five central metabolites that can enter the citric acid cycle. Here they are completely oxidized to carbon dioxide and water. During electron transfer, ATP is generated by oxidative phosphorylation, and in this way amino acids contribute to the total energy supply of the organism.
Metabolism of propanoate begins with its conversion to propionyl coenzyme A (propionyl-CoA), the usual first step in the metabolism of carboxylic acids. Since propanoic acid has three carbons, propionyl-CoA can directly enter neither 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 B 12 -dependent enzyme catalyzes rearrangement of L-methylmalonyl-CoA to succinyl-CoA, which is an intermediate of the citric acid cycle and can be readily incorporated there.