Serine, catabolism transamination

The major pathway of serine catabolism probably is by way of its enzymatic dehydration and subsequent spontaneous deamination to yield pyruvic acid (see Fig. 2). An evidence for this is the observation of Lien and Greenberg that alanine is the major amino acid formed from serine-3-C by liver mitochondrial preparations. The alanine could be fomed from the pyruvic acid by transamination. 

Serine, like alanine, is converted into pyruvate by a PLP-dependent pathway, but the two reaction sequences are not the same. Whereas alanine catabolism involves a PLP-dependent transamination, serine catabolism involves a PLP-dependent dehydration to form an intermediate enamine that is then hydrolyzed. 

The carbon skeletons of six amino acids are converted in whole or in part to pyruvate. The pyruvate can then be converted to either acetyl-CoA (a ketone body precursor) or oxaloacetate (a precursor for gluconeogenesis). Thus amino acids catabolized to pyruvate are both ke-togenic and glucogenic. The six are alanine, tryptophan, cysteine, serine, glycine, and threonine (Fig. 18-19). Alanine yields pyruvate directly on transamination with... 

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