Biochemistry and Molecular Biology - Belyasova N.A. 2002

Metabolism: Processes Requiring Energy Input
Metabolism of Nitrogen-Containing Compounds
Fate of Amino Acid Carbon Skeletons

The strategy for the degradation of carbon skeletons formed during AMINO ACID DEAMINATION is to generate Key Intermediates within The Cell that are suitable both for The Biosynthesis of cellular compounds and for further Oxidation coupled with energy storage. Through numerous complex transformations, the carbon skeletons of the 20 Proteinogenic Amino Acids are ultimately converted into seven different degradation products: Pyruvate, acetyl-CoA, acetoacetyl-CoA, α-ketoglutarate, succinyl-CoA, fumarate, and oxaloacetate.

Five of the seven listed metabolites (pyruvate, α-ketoglutarate, succinyl-CoA, fumarate, and oxaloacetate) can serve as substrates for Gluconeogenesis; therefore, amino acids that degrade to yield these products are referred to as glucogenic. All proteinogenic Amino acids are glucogenic, with the exception of leucine and Lysine. It should be noted that all the listed amino acid degradation products can be oxidized in the TCA cycle as intermediates, or (like pyruvate) can enter the cycle after being converted into acetyl-CoA or oxaloacetate.

The remaining breakdown products of carbon skeletons of amino acids—acetyl-CoA and acetoacetyl-CoA—cannot be converted into glucose in animal organisms. They are used for the synthesis of Ketone Bodies (acetoacetate, acetone, 3-hydroxybutyrate), Fatty acids, and Isoprenoids. Amino acids whose degradation leads to the Formation of Acetyl-CoA and acetoacetyl-CoA are called ketogenic. These include leucine and lysine.

Four amino acids (phenylalanine, Tyrosine, Tryptophan, and isoleucine) are both glycogenic and ketogenic, since their degradation yields substances capable of being converted into both glucose and ketone bodies.