THE MECHANISM OF CATALYSIS AND THE INHIBITION OF BETA-LACTAMASES

Formation of a tetrahedral intermediate by nucleophilic attack on the beta-lactam carbonyl carbon of penicillins generates a lone pair on th e beta-lactam nitrogen which is syn to the incoming nucleophile, in co ntrast to the normal anti arrangement found in peptides. Ring opening of the beta-lactam requires protonation of the beta-lactam nitrogen by a general acid catalyst. The general acid/base catalyst in beta-lacta mases is probably a glutamate and a tyrosine residue in class A and C enzymes, respectively. Phosphonamidates inactivate class C beta-lactam ases by phosphonylation of the active site serine, the rate of which i s enhanced by a factor of at least 10(6). The enzyme's catalytic machi nery used for hydrolysis is also used for phosphonylation. The rate en hancement may be greater than 10(9) if the mechanism occurs by an inhi bitor assisted reaction involving intramolecular general acid catalysi s, Class B metallo-beta-lactamases are inhibited by thiol derivatives with K-i as low as 10 mu M. The mechanism of hydrolysis of the metallo -beta-lactamase involves a dianionic tetrahedral intermediate stabilis ed by zinc(II).