Methionine is Synthesized from Cysteine

Methionine and cysteine are the two sulfur-containing amino acids found in proteins. Methionine is synthesized in plastids from cysteine. In the first step of the pathway, cysteine and O-phosphohomoserine react to form cystathionine via the enzyme cystathionine-γ-synthase (Web Figure 13.3.A). Cystathionine is cleaved into homocysteine, pyruvate, and ammonia by cystathionine-β-lysase. Finally, methionine synthase methylates homocysteine to form methionine (Lea 1997).

Web Figure 13.3.A   The modification of cysteine that yields the other sulfur-containing amino acid, methionine. For the complete pathway, see textbook Figure 13.15.