Energetic coupling of an active site residue in penicillin-binding protein 2 from Neisseria gonorrhoeae with a resistance-associated conformational switch in the β3-β4 loop

Mosaic penA alleles encoding highly mutated variants of penicillin-binding protein 2 (PBP2) are the principal determinants of ceftriaxone resistance in Neisseria gonorrhoeae. Resistance-associated mutations in PBP2 from the ceftriaxone-resistant strain H041 restrict formation of the inward conformation of the {beta}3-{beta}4 loop associated with efficient acylation, but how {beta}-lactam recognition is coupled to this conformational switch is unknown. Because the conserved active-site residue Tyr422 interacts with the R1 substituent of {beta}-lactams, we investigated its role in coupling ligand recognition and acylation activity. Mutation of Tyr422 to Ala lowered acylation rates by up to 120-fold for cefoperazone and piperacillin, whereas acylation rates of ceftriaxone increased 4-fold. Unexpectedly, the crystal structure of the Y422A mutant acylated by ceftriaxone revealed that the {beta}3-{beta}4 loop had adopted the inward, high-activity conformation, despite position 422 being spatially distant from the loop. Transformation experiments showed that cell viability requires a tyrosine at position 422, indicating the residue is essential for transpeptidase function. Together, these findings reveal an energetic coupling between an active-site residue in PBP2 and a conformational switch whose equilibrium is altered by resistance mutations. The previously observed higher activity of {beta}-lactams containing extended R1 groups is consistent with stronger interactions with Tyr422 that favor the conformational switch. Molecular modeling suggests that such groups enhance activity by mimicking the iso-Glu region of the pentapeptide substrate. Overall, we propose that access to the high-activity state of PBP2 where the {beta}3-{beta}4 loop is inward is regulated by interactions between Tyr422 and {beta}-lactam R1 groups, and that resistance mutations function by tilting the balance toward a lower activity state.