Mechanistic basis for inhibition of the extended-spectrum β-lactamase GES-1 by enmetazobactam and tazobactam.

β-Lactamase-catalysed hydrolysis is the primary form of β-lactam antibiotic resistance in Gram-negative bacteria. The penicillanic acid sulfone (PAS) enmetazobactam is thought to inhibit extended-spectrum β-lactamases (ESBLs) by fragmentation of an initial acyl-enzyme to form an active-site lysinoalanine cross link. We investigate interactions of enmetazobactam and its congener tazobactam with GES-1, an ESBL with structural features of carbapenem-hydrolysing β-lactamases. Crystal structures show different breakdown products of the two inhibitors covalently bound to the catalytic Ser70, assigned using quantum mechanics/molecular mechanics (QM/MM) calculations. We find no evidence for lysinoalanine formation, with mass spectrometry indicating active enzyme regeneration, behaviour previously observed for carbapenem-hydrolysing enzymes, but not ESBLs. This work establishes that PAS inhibitors interact with diverse β-lactamases by differing mechanisms, which should inform development of future compounds.