ADC-56, a novel extended-spectrum AmpC (ESAC) β-lactamase, was identified in an Acinetobacter baumannii clinical isolate. ADC-56 possessed an R148Q change compared with its putative progenitor, ADC-30, which enabled it to hydrolyze cefepime. Molecular modeling suggested that R148 interacted with Q267, E272, and I291 through a hydrogen bond network which constrained the H-10 helix. This permitted cefepime to undergo conformational changes in the active site, with the carboxyl interacting with R340, likely allowing for better binding and turnover.
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