Extended-spectrum AmpC cephalosporinase in Acinetobacter baumannii: ADC-56 confers resistance to cefepime

Abstract

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.

Fig. 1.

(a) ADC-30 versus ADC-56 β-lactamase. In the molecular model, R148 interacts with Q267, E272, and I291 (H-10 helix) through hydrogen bond networking. These important interactions likely constrain the H-10 helix and part of the R2 loop. By changing R148 to Q (or any other amino acid, except K, which restores partial hydrogen bonds), the interactions are disrupted and the helix gains more flexibility. (b) Cefepime modeled in the active site of ADC-30. We show, after minimization, that the carbonyl is pushed out of the oxyanion hole and creating hydrogen bonds with K67 and Y150. In addition, the C-4 carboxyl makes hydrogen bonds with T317. Based upon this model, cefepime is less readily hydrolyzed. (c) Connolly representation of panel b. (d) Cefepime modeled in the active site of ADC-56. The carbonyl of cefepime is positioned out of the oxyanion hole and makes hydrogen bonds with K67 and Y150. The carboxyl is making hydrogen bonds with R340. In a comparison of the two models, cefepime is in a different conformation in the active site. The substitution R148Q disrupts the hydrogen bonds with Q267, E272, and I291 and may allow for better binding and turnover. (e) The Connolly representation of panel d presents a better image of the binding modes of cefepime with ADC-56 β-lactamase.