Structure-function relationships in the metallo-beta-lactamase of Bacillus cereus 5/B/6

The major route by which pathogenic bacteria become resistant to the chemotherapeutic action of ^lactam antibiotics is through the acquisition of a gene coding for the production of enzymes called p-lactamases (P-lactamhydrolase, E.C. 3.5.2.6) (Livermore, 1991). These enzymes catalyze the hydrolytic inactivation of this type of antibiotic, thereby rendering them ineffective as antimicrobial agents (Figure 1; Livermore, 1991). A wide variety of microorganisms produce these enzymes with gram-positive bacteria producing them at highest concentrations (Citri, 1970). The p-lactamases produced by these microorganisms are divided into three classes (A, B and C) based on their physicochemical properties (Ambler, 1980; Jaurin and Gundstrom, 1981). Class A and C enzymes resemble serine proteases in their use of a catalytic serine residue and the formation of an acyl-enzyme intermediate during substrate turnover (Rahil and Pratt, 1991; Fisher et aL, 1980). Class B enzymes on the other hand are characterized by their absolute requirement for a divalent transition metal ion for catalysis (Ambler, 1980; Jaurin and Gundstrom, 1981). The native enzyme is isolated containing a catalytic zinc atom. The serine active site enzymes are sensitive to those agents which covalently modify serine residues and several inhibitors have been designed for these enzymes which make use of this and/or their catalytic mechanism (Rahil and Pratt, 1991; Gledhill et aL, 1991). The addition of clavulanic acid to the currently available p-lactam antibiotics has extended their usefulness against resistant bacterial strains (Wainwright, 1990). The metalloenzymes however have no known specific inhibitors (although a single report in the literature by Bush et aL claims inhibition of all major structural classes of the P-lactam hydrolyzing enzymes by tazobactam; the data that is contained therein show no significant level of inhibition of the class B enzymes tested (Bush et aL, 1993), other than those agents which will sequester the metal ion from the peptide or denature the peptide. The latter reagents do not offer the specificity which would be required in a pharmaceutically relevant preparation.