Structural Insight into the Mechanism of Inhibitor Resistance in CTX-M-199, a CTX-M-64 Variant Carrying the ST Substitution.

The smart design of β-lactamase inhibitors allowed us to combat extended-spectrum β-lactamase (ESBL)-producing organisms for many years without developing resistance to these inhibitors. However, novel resistant variants have emerged recently, and notable examples are the CTX-M-190 and CTX-M-199 variants, which carried a ST amino acid substitution and exhibited resistance to inhibitors such as sulbactam and tazobactam. Using mass spectrometric and crystallographic approaches, this study depicted the mechanisms of inhibitor resistance. Our data showed that CTX-M-64 (ST) did not cause any conformational change or exert any effect on its ability to hydrolyze β-lactam substrates. However, binding of sulbactam, but not clavulanic acid, to the active site of CTX-M-64 (ST) led to the conformational changes in such active site, which comprised the key residues involved in substrate catalysis, namely, Thr, Lys, Lys, Asn, and Asn. This conformational change weakened the binding of the sulbactam -enamine intermediate (TSL) to the active site and rendered the formation of the inhibitor-enzyme complex, which features a covalent acrylic acid (AKR)-T bond, inefficient, thereby resulting in inhibitor resistance in CTX-M-64 (ST). Understanding the mechanisms of inhibitor resistance provided structural insight for the future development of new inhibitors against inhibitor-resistant β-lactamases.