Serine metabolic fate

As shown in Figure 10.9, the methyl donor is S-adenosyl methionine, which is demethylated to S-adenosyl homocysteine. After removal of the adenosyl group, homocysteine may undergo one of two metabolic fates remethylation to methionine or condensation with serine to form cystathionine, foUowed by cleavage to yield cysteine - the transulfuration pathway (Section 9.5.5). Cystathionine synthetase has a relatively low Tni compared with normal intra-ceUular concentrations of homocysteine. It functions at a relatively constant rate, and under normal conditions, most homocysteine wUl be remethylated to methionine. 

Figure 9-3. Fates of the carbon skeletons upon metabolism of the amino acids. Points of entry at various steps of the tricarboxylic acid (TCA) cycle, glycolysis and gluconeogenesis are shown for the carbons skeletons of the amino acids. Note the multiple fates of the glucogenic amino acids glycine (Gly), serine (Ser), and threonine (Thr) as well as the combined glucogenic and ketogenic amino acids phenylalanine (Phe), tryptophan (Trp), and tyrosine (Tyr). Ala, alanine Cys, cysteine lie, isoleucine Leu, leucine Lys, lysine Asn, asparagine Asp, aspartate Arg, arginine His, histidine Glu, glutamate Gin, glutamine Pro, proline Val, valine Met, methionine.

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