Amino acids, when deaminated, yield a-keto acids that, directly or via additional reactions, feed into the major metabolic pathways (e.g., Krebs Cycle). Pathways for all amino acids will not be discussed in detail here. Emphasis will be placed on reactions involving cofactors that are not discussed elsewhere. Amino acids are grouped into two classes, based on whether or not their carbon skeletons can be converted to glucose:
Glucogenic amino acids: Their carbon skeletons are degraded to pyruvate, or to one of the 4- or 5-carbon intermediates of Krebs Cycle that are precursors for gluconeogenesis. Glucogenic amino acids are the major carbon source for gluconeogenesis when glucose levels are low. They can also be catabolized for energy or converted to glycogen or fatty acids for energy storage.
Ketogenic amino acids: Their carbon skeletons are degraded to acetyl-CoA or acetoacetate. Acetyl CoA, and its precursor acetoacetate, cannot yield net production of oxaloacetate, the precursor for the gluconeogenesis pathway. For every 2-C acetyl residue entering Krebs Cycle, two carbon atoms leave as CO2. (For review, see notes on Krebs Cycle.) Carbon skeletons of ketogenic amino acids can be catabolized for energy in Krebs Cycle, or converted to ketone bodies or fatty acids. They cannot be converted to glucose.
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