In the biosynthesis of S-adenosylmethionine (SAM; Sec. 11.7B), the amino acid methionine undergoes an enzyme-catalyzed reaction with ATP to give SAM and triphosphate. (This is one of only two known biochemical reactions in which triphosphate serves as a leaving group.)

(a) Give a curved-arrow mechanism for this reaction, assuming it is an S_N2 process.

(b) The enzyme active site of the catalyzing enzyme, SAM synthase, contains two Mg²⁺ ions. What role might these ions have in the catalytic process?

Transcribed Image Text:

HO fofofoxan ATP HO adenine OH + CH3 H NH3 CO₂ methionine adenine SAM synthase (enzyme), H₂C XX HI OH H CO, S-adenosylmethionine (SAM) triphosphate +HO