Biochemistry and Molecular Biology - Belyasova N.A. 2002

Metabolism. Energy-requiring processes
Metabolism of nitrogen-containing compounds
Fate of amino acid carbon skeletons

The strategy for breaking down the carbon skeletons generated during AMINO ACID DEAMINATION is to produce 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 standard Amino Acids are ultimately converted into seven distinct degradation products: Pyruvate, acetyl-CoA, acetoacetyl-CoA, a-ketoglutarate, succinyl-CoA, fumarate, and oxaloacetate.

Five of these seven metabolites (pyruvate, a-ketoglutarate, succinyl-CoA, fumarate, and oxaloacetate) can serve as substrates for Gluconeogenesis; consequently, amino acids whose degradation yields these products are termed glucogenic. All standard amino acids are glucogenic, with the exception of leucine and Lysine. It is worth noting that all the aforementioned amino acid degradation products can be oxidized in the TCA cycle as its 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 termed ketogenic. These include leucine and lysine.

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