VAL-237 FOR ALA SUBSTITUTION IN THE TEM-2 BETA-LACTAMASE DRAMATICALLYALTERS THE CATALYTIC EFFICIENCIES TOWARDS CARBENICILLIN AND TICARCILLIN

The mutant 554 of TEM-2 beta-lactamase was selected for a decrease in the resistance to carbenicillin of an Escherichia coli K12 carrier. Th e amino acid sequence of the mutant beta-lactamase was determined by m anual Edman degradation analysis of proteolytic peptides. A single sub stitution Val for Ala was localized at position 237. The mutant exhibi ted only 2% of the catalytic efficiency of the wild-type enzyme toward s carbenicillin and ticarcillin, whereas it retained 30-60% of the hyd rolytic activity towards other penicillin and cephalosporin substrates , Carfecillin, the phenyl ester of the side-chain carboxyl group of ca rbenicillin, was hydrolysed as a good substrate. This suggests that th e behaviour of the mutant enzyme towards carbenicillin may result from ionic rather than steric constraints. A molecular model of the Val-23 7 TEM-2 mutant suggests possible electrostatic interaction between Glu -171 and the carboxylic group of the side chain of carbenicillin.