A Two Amino Acid Duplication, L167E168, in the X-Loop Drastically Decreases Carbapenemase Activity of KPC-53, a Natural Class A β-Lactamase

KPC-53 enzyme is a natural KPC variant which showed a duplication of L167E168 residues in the X-loop structure. The blaKPC-53 gene was cloned both into pBC-SK and pET-24a vectors, and the recombinant plasmids were transferred by transformation in Escherichia coli competent cells to evaluate the antimicrobial susceptibility and to produce the enzyme. Compared to KPC-3, the KPC-53 was less stable and showed a dramatic reduction of kcat and kcat/Km versus several β-lactams, in particular carbapenems. Indeed, a 2,000-fold reduction was observed in the kcat values of KPC-53 for imipenem and meropenem. Concerning inhibitors, KPC-53 was susceptible to tazobactam and clavulanic acid but maintained resistance to avibactam. The molecular modeling indicates that the L167E168 duplication in KPC-53 modifies the interactions between residues involved in the catalytic pocket, changing the flexibility of the X-loop, which is directly coupled with the catalytic properties of the KPC enzymes.