Fig. 7.

Modeling the tetrahedral intermediate formed by the attack of the nucleophile Ser75 of EstB on the carbonyl functions of the ester (a) and β-lactam (b) of 7-ACA. In generating these complexes, the constraint was imposed that the oxygen atom of the tetrahedral intermediate should remain within hydrogen-bonding distance from the putative oxyanion hole. The figure illustrates that the tetrahedral intermediate for ester cleavage (a) fits the active site well, whereas the corresponding intermediate for hydrolysis of the β-lactam (b) leads to steric clashes between protein and substrate. Figure produced with Sybyl, Vers. 6.4 (Tripos).