Towards defining mechanisms involved in β-lactam resistance using serial crystallography
Keywords: antimicrobial resistance
beta lactamases
carbapenems
serial crystallography
beta lactams

Antimicrobial resistance is one of the biggest threats to contemporary medicine, with an increasing number of bacteria showing multi-drug resistance. Bacteria have developed mechanisms to survive β- lactam antibiotic treatment, including by the production of β-lactamases (BLs). BLs hydrolyse the β- lactam ring of vital antibiotics, including penicillins and carbapenems, the latter often described as a “last resort” antibiotic. As described in the Introduction, BLs are divided into two mechanistic groups: serine-β-lactamases, which use a nucleophilic serine to hydrolyse β-lactams (SBLs) and metallo-β- lactamases (MBLs), which use one or two zinc ions to aid hydrolysis. My work focussed on understanding the mechanism by which the MBL Verona Integron metallo-β-lactamase 2 (VIM-2) hydrolyses carbapenems using conventional and serial crystallographic methods.
Chapter 2 describes crystallographic, NMR and kinetic studies on VIM-2 and other MBLs and SBLs. Structures of faropenem complexed with VIM-2, the MBL L1 and the SBL KPC-2 (with the University of Bristol) show all three BLs hydrolyse faropenem, with VIM-2 and KPC-2 opening the tetrahydrofuran (THF) side chain. These observations were confirmed by NMR studies, although these also showed that L1 opens the THF ring in solution. Both tautomerisation of the nascent product (enamine to imine) along with epimerisation at the ring junction position were observed. Crystallographic and solution studies with VIM-2 and biapenem indicated the presence of several products, with both imine and enamine complexes being observed at the active site.
The work of Chapters 3 and 4 focused on serial crystallographic studies with penicillin binding protein 5 (PBP5) and VIM-2. The aim with VIM-2 was to perform time resolved crystallography using an X-Ray Free Electron Laser (XFEL) to capture the reaction intermediates during carbapenem hydrolysis. PBP5 was initially used to compare and contrast different sample delivery methods for XFEL analyses as it was easier to produce in larger quantities. Multiple resting state datasets of PBP5 and VIM-2 were collected, both at XFELs and at Diamond Light Source using serial data collection; however, no time resolved structures were captured with VIM-2 and the carbapenem ertapenem. A structure of PBP5 in the format of an acyl-enzyme complex with ertapenem was captured, at 2.9 Å resolution. The XFEL studies indicated that VIM-2 is a good candidate for time resolved studies, however, new crystallisation conditions are needed to visualise structural intermediates.
Chapter 5 describes structural and solution studies with VIM-2 and three bicyclic boronates, one with a thioether side chain which is a potent, broad spectrum inhibitor of SBLs and MBLs, and two with a sulfonamide type side chain.
Overall, the studies performed in this thesis expand our understanding of how VIM-2 hydrolyses carbapenems and penems and of how to potently inhibit a broad range of BLs, including by boronates which are a promising clinically relevant inhibitor class.