Natural D240G Toho-1 mutant conferring resistance to ceftazidime: biochemical characterization of CTX-M-43

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Abstract
The aim of this article is biochemical and kinetic characterization of CTX-M-43, a natural Asp-240→Gly mutant of CTX-M-44 (ex Toho-1), from a clinical isolate of Acinetobacter baumannii isolated in a Bolivian hospital. Steady-state kinetic parameters (Km and kcat) were determined for a large pattern of substrates. Analysis of inactivators and transient inactivators was performed to determine the efficiency of acylation (k+2/K) and the deacylation constant (k+3). Molecular modelling of Michaelis complex of ceftazidime, cefotaxime and ceftibuten, obtained from molecular mechanics calculations, was carried out. CTX-M-43 showed a general increase in affinity towards all cephalosporins tested, with respect to CTX-M-44. Carbapenems acted as inactivators with a good acylation efficiency for meropenem and ertapenem and significant deacylation constant for imipenem. MICs of imipenem obtained at a higher bacterial inoculum of recombinant Escherichia coli were increased. Kinetic data and molecular modelling of Michaelis complex confirmed that Asp-240→Gly allows a better accommodation of the bulky C7β aminothiazol-oxyimino substituent, resulting in a general increase in the enzyme affinity towards oxyimino cephalosporins. The ascertained potentialities of CTX-M-type enzymes, supported by the kinetic data and the behaviour of the recombinant E. coli at different bacterial inocula towards carbapenems, make a possible evolution of those enzymes towards a carbapenemase activity plausible.

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