Cysteine biosynthesis from homoserine (Saccharomyces cerevisiae)

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O-Acetyl-L-homoserine sulfhydrylase participates in an alternative pathway of L-homocysteine synthesis. Cysteine can then be produced by the reverse transsulfuration pathway, which involves the conversion of homocysteine to cysteine with the help of a cystathionine intermediate. This process takes place in two steps: First, cystathionine-synthase (Cys4p) catalyzes a -displacement reaction, whereby the thiol group of homocysteine replaces the hydroxyl group of serine, resulting in the formation of cystathionine. Second, cystathionine-lyase (Cys3p) catalyzes an ,-elimination reaction, which hydrolyzes cystathionine to generate cysteine, -ketobutyrate, and ammonia.