With few exceptions, the first step in amino acid degradation is removal of the -amino group. This modification, usually a transamination, can also be used to generate glutamate from -ketoglutarate via the glutamate dehydrogenase reaction. The products of these reactions include deamination of the amino acid to the corresponding keto acid plus ammonia. L-amino acid oxidase also catalyzes a similar reaction, yielding a hydrogen peroxide intermediate, as shown here. Kidney and liver cells are also rich in a D-amino acid oxidase, which has an unknown function, because D isomers of amino acids are rare except for in bacterial cell walls.

After the nitrogen group has been removed from the molecules, the carbon backbone can be metabolized in a variety of ways (Figure 20.12). Amino acids whose skeletons generate pyruvate or oxaloacetate are efficiently converted to carbohydrates via gluconeogenesis. Amino acids leading to acetyl-CoA or acetoacetyl-CoA contribute towards ketogenesis. The terms glucogenic and ketogenic are used to classify amino acids as generators of carbohydrates (see here) or ketone bodies, respectively.