title: Complementary roles of mutations at positions 69 and 242 in a class A beta-lactamase.
abstract: Analysis of the three-dimensional structure of class A beta-lactamases suggests that deformation of the substrate binding site can be produced by changes in the hydrophobicity of residue 69 behind the beta-sheet and by outward movement of the B3 beta-strand by introduction of a non-glycine residue at position 242 on the B4 beta-strand. By site-directed mutagenesis Met69-IleGly242-Cys, a double mutant, of the OHIO-1 beta-lactamase, was constructed. The minimum inhibitory concentrations (MICs) of the double mutant compared with the wild type and each single mutant revealed an increased susceptibility to beta-lactams. Met69-IleGly242Cys hydrolyzed cephaloridine (Km = 213 microM) but had Km > 500 microM for other beta-lactams tested including cefotaxime, and demonstrated a higher apparent Ki for inhibitors (clavulanate Ki = 500 microM sulbactam = 434 microM, and tazobactam = 70 microM). In a competition experiment with cephaloridine, the apparent Ki values for penicillin and cefotaxime remained low, 21 microM and 0.7 microM, respectively. Since Ile is twice as hydrophobic as Met, the Met69-Ile mutation may result in partial collapse of the oxyanion hole. This would also increase the distance between Arg-244 and the carboxyl of clavulanic acid. The Gly242-Cys mutation opens the lower portion of the active site to bulky R groups of cephalosporins. Although these two mutations result in a catalytically impaired enzyme, they can be used to model the complementary role of two distinct residues, neither of which interacts directly with beta-lactam substrates or inhibitors.
authors.affiliation: Research Service, Department of Veterans Affairs Medical Center, Cleveland, OH.
authors.forename: R A
authors.initials: RA
authors.lastname: Bonomo
authors.forename: C G
authors.initials: CG
authors.lastname: Dawes
authors.forename: J R
authors.initials: JR
authors.lastname: Knox
authors.forename: D M
authors.initials: DM
authors.lastname: Shlaes
mesh_terms: D000900:Anti-Bacterial Agents / Q000737:chemistry; D001483:Base Sequence; D001665:Binding Sites; D004926:Escherichia coli / Q000235:genetics; D007700:Kinetics; D008969:Molecular Sequence Data; D015394:Molecular Structure; D016297:Mutagenesis, Site-Directed; D010641:Phenotype; D010957:Plasmids; D001618:beta-Lactamases / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D047090:beta-Lactams
chemical_list: D000900:Anti-Bacterial Agents; D047090:beta-Lactams; D001618:beta-Lactamases