Substitution of Met-69 by Ala or Gly in TEM-1 β-lactamase confer an increased susceptibility to clavulanic acid and other inhibitors

Abstract

In some inhibitor-resistant TEM-derived β-lactamases, Met-69 is substituted by Leu, Ile or Val. Residue 69 is located in a region of strong structural constraints, at the beginning of H2 α-helix, and in the vicinity of B3 and B4 β-strands. Analysis of the three-dimensional structure of TEM-1 β-lactamase suggests that alteration of the substrate-binding site can be produced by changes of the size of residue 69 side chain. Met-69 was substituted by alanine or glycine in TEM-Bs β-lactamase (a TEM-1-related enzyme) using site-directed mutagenesis. The minimum inhibitory concentrations of the mutants compared with the wild-type revealed an increased susceptibility to β-lactamase inhibitor–β-lactam combinations and to first-generation cephalosporins. Comparing the Met69Ala and Met69Gly β-lactamases with TEM-Bs, K m constants of the mutants showed an increased affinity for most β-lactams but the k cat for most substrates did not change substantially. Mutants also demonstrated lower IC 50 for the three inhibitors (clavulanic acid, tazobactam and sulbactam). The two substitutions of the residue 69 by alanine and glycine had a noticeable effect on K m values of TEM-Bs β-lactamase, and on affinity for β-lactamase inhibitors.