FIGURE 1.

Schematic representation of the proposed reaction mechanisms of B2 MβL CphA. Top panel, the Henry-Michaelis complex ES1 assumes direct binding of the substrate to the zinc ion and one water molecule (Wat1) in the active site. Wat1 is deprotonated by a general base (His¹¹⁸ or Asp¹²⁰) to perform the nucleophilic attack (15). Nitrogen protonation can take place by a second water molecule (Wat2), involving also an internal rearrangement in the substrate (Path I) or from the general base (in this case, Asp¹²⁰ is shown as the general base and proton donor) (Path II). Bottom panel, in the Henry-Michaelis complex ES2, the interaction of the substrate with the zinc ion is mediated by a water molecule (Wat2), whereas Wat1 occupies a similar position as in ES1. Wat1 is activated by a general base (His¹¹⁸ in this picture) to perform the nucleophilic attack. The zinc-bound Wat2 acts as a proton donor to the lactam nitrogen.