Inhibition of AmpC β-lactamase through a destabilizing interaction in the active site

β-Lactamases hydrolyze β-lactam antibiotics, including penicillins and cephalosporins; these enzymes are the most widespread resistance mechanism to these drugs and pose a growing threat to public health. β-Lactams that contain a bulky 6(7)α substituent, such as imipenem and moxalactam, actually inhibit serine β-lactamases and are widely used for this reason. Although mutant serine β-lactamases have arisen that hydrolyze β-lactamase resistant β-lactams (e.g., ceftazidime) or avoid mechanism-based inhibitors (e.g., clavulanate), mutant serine β-lactamases have not yet arisen in the clinic with imipenemase or moxalactamase activity. Structural and thermodynamic studies suggest that the 6(7)α substituents of these inhibitors form destabilizing contacts within the covalent adduct with the conserved Asn152 in class C β-lactamases (Asn132 in class β-lactamases). This unfavorable interaction may be crucial to inhibition. To test this destabilization hypothesis, we replaced Asn152 with Ala in the class C β-lactamase AmpC from Escherichia coli and examined the mutant enzyme's thermodynamic stability in complex with imipenem and moxalactam. Consistent with the hypothesis, the Asn152 → Ala substitution relieved 0.44 and 1.10 kcal/mol of strain introduced by imipenem and moxalactam, respectively, relative to the wild-type complexes. However, the kinetic efficiency of AmpC N152A was reduced by 6300-fold relative to that of the wild-type enzyme. To further investigate the inhibitor's interaction with the mutant enzyme, the X-ray crystal structure of moxalactam in complex with N152A was determined to a resolution of 1.83 Å. Moxalactam in the mutant complex is significantly displaced from its orientation in the wild-type complex; however, moxalactam does not adopt an orientation that would restore competence for hydrolysis. Although Asn152 forces β-lactams with 6(7)α substituents out of a catalytically competent configuration, making them inhibitors, the residue is essential for orienting β-lactam substrates and cannot simply be replaced with a much smaller residue to restore catalytic activity. Designing β-lactam inhibitors that interact unfavorably with this conserved residue when in the covalent adduct merits further investigation.