ABSTRACT
Amino acid degradation As there is no store for excess amino acids, and as proteins are constantly being turned over, amino acids have to be continually degraded. The a-amino group is removed first and the resulting carbon skeleton is converted into one or more major metabolic intermediates and used as metabolic fuel. The carbon skeletons of the 20 standard amino acids are funneled into only seven molecules: pyruvate, acetyl CoA, acetoacetyl CoA, a-ketoglutarate, succinyl CoA, fumarate and oxaloacetate (Figure 2). Amino acids that are degraded to pyruvate, a-ketoglutarate, succinyl CoA, fumarate and oxaloacetate are termed glucogenic as they can give rise to the net synthesis of glucose. This is because the citric acid cycle intermediates and pyruvate can be converted into phosphoenolpyruvate and then into glucose via gluconeogenesis (Sections J4 and L1). In contrast, amino acids that are degraded to acetyl CoA or acetoacetyl CoA are termed ketogenic because they give rise to ketone bodies (Section K2); the acetyl CoA or acetoacetyl CoA can also be used to synthesize lipids (Section K3). Of the standard set of 20 amino acids, only Leu and Lys are solely ketogenic. Ile, Phe, Trp and Tyr are both ketogenic and glucogenic as some of their carbon atoms end up in acetyl CoA or acetoacetyl CoA, whereas others end up in precursors of glucose. The remaining 14 amino acids are classified as solely glucogenic.
