Sulfur Transfer

an enzymatic reaction in which sulfur is transferred from one amino acid, methionine, to another, serine, resulting in the formation of a third amino acid, cysteine.

Sulfure transfer is accomplished in several steps. First, methionine loses a methyl group to form homocysteine, which in turn undergoes a condensation reaction with serine to form cystathionine. Cystathionine is then broken down to give cysteine and homoserine. Thus, methionine and serine are the biochemical precursors of cysteine; methionine supplies the sulfhydryl group (—SH), while serine supplies the carbon skeleton. Cystathionine is an intermediate in the conversion of methionine and does not occur as a component of proteins. In the formation of cysteine by sulfur transfer, vitamin B₆ in the form of pyridoxal phosphate plays an important role as a component of the enzyme system that catalyzes this reaction.

In animals, the sulfur-transfer reaction is practically irreversible. The cysteine that is formed is a component of many biologically active compounds, for example, glutathione; it also participates in the neutralization of a group of toxic compounds that includes skatole, cresol, phenol, and indole. In many bacteria, especially in mutants of Escherichia coli, sulfur transfer can proceed reversibly, that is, the synthesis of methionine from cysteine is possible. Sulfur transfer has not yet been sufficiently studied in higher plants.