Transulfuration pathways

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. 

The sulfur atom of methionine becomes the sulfur atom of cysteine. The sulfate generated in cysteine catabolism is excreted or used in several biosynthetic or catabolic pathways. The transulfuration and methylation pathways are intimately related. 

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