FIG. 2.

Acyl serine transferase-catalyzed reaction on R₁—CO—X—R₂ carbonyl donors. (A) Overall reaction. (B) Role of a water molecule as proton transmitter. X is O, S, or NH (in penicillin, the CO—N bond is endocyclic). HY is an acceptor, i.e., water or an amino compound. The carbonyl of the donor and that of the acyl enzyme are polarized by NH groups of the enzyme polypeptide backbone. TS, transition state; ∗, scissile bond of the carbonyl donor (step 1) and scissile bond of the acyl enzyme (step 2). The shaded area symbolizes the active-site environment.