One of the resistance mechanisms of superbugs is to hydrolyze antibiotics by producing metallo-β-lactamases (MβLs). To verify how MβLs evolved to increase in activity in response to various β-lactam antibiotics, the mutation and evolution of CphA from Aeromonas hydrophila (Zn-dependent MβL) was investigated in a medium with a continuous biapenem (BIA) concentration gradient. The results showed that a single-base mutation M1 and two frameshift mutations M3 and M4 were observed. Furthermore, a nonsense mutation M2 was observed. Compared with wild-type (WT), the minimum inhibitory concentrations (MICs) of the M3 and M4 increased by more than 128 times, and the catalytic efficiency of BIA by the M3 and M4 increased by 752% and 376% respectively. In the mutants, the carbon skeleton migration caused by the outward motion of the loop3 near the entrance of the binding pocket increased the cavity volume of the binding pocket and was more conducive to the entry and expulsion of BIA and its hydrolytic product in the binding pocket. The conformational change effect originated from mutations is transmitted to the binding pocket through the interactions between the side chain amino acid residues of the C-terminal and those of the loop3, thus affecting the binding and hydrolysis capability of the mutants to BIA in the binding pocket. All these indicated that during the repeated drug-endurance and -resistance, the CphA completed its mutation and conformational change and evolved to the mutants with a more delicate structure and stronger hydrolysis ability by a genetic mutation.
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