"Mind the Gap": Raman Evidence for Rapid Inactivation of CTX-M-9 β-Lactamase Using Mechanism-Based Inhibitors that Bridge the Active Site.

CTX-M β-lactamases are one of the fastest growing extended-spectrum β-lactamase (ESBL) families found in Escherichia coli rendering this organism extremely difficult to treat with β-lactam antibiotics. Although they are grouped in class A β-lactamases, the CTX-M family possesses low sequence identity with other enzymes. In addition, they have high hydrolytic activity against oxyimino-cephalosporins, despite having smaller active sites compared to other ESBLs in class A. Similar to most class A enzymes, most of the CTX-M β-lactamases can be inhibited by the clinical inhibitors (clavulanic acid, sulbactam, and tazobactam), but the prevalence of inhibitor resistance is an emerging clinical threat. Thus, the mechanistic details of inhibition pathways are needed for new inhibitor development. Here, we use Raman microscopy to study the CTX-M-9 inactivation reaction with the three commercially available inhibitors and compare these findings to the analysis of the S130G variant. Characterization of the reactions in CTX-M-9 single crystals and solution show the formation of a unique cross-linked species, probably involving Ser70 and Ser130, with subsequent hydrolysis leading to an acrylate species linked to Ser130. In solution, a major population of this species is seen at 25 ms after mixing. Support for this finding comes from the CTX-M-9 S130G variant that reacts with clavulanic acid, sulbactam, and tazobactam in solution, but lacks the characteristic spectroscopic signature for the Ser130-linked species. Understanding the mechanism of inactivation of this clinically important ESBL-type class A lactamase permits us to approach the challenge of inhibitor resistance using knowledge of the bridging species in the inactivation pathway.