a) Structural comparison of the global fold of the transpeptidase domain of a PBP enzyme (PBP2a – PDB 3ZG0) and SBLs from classes A (KPC-2 – PDB 20V5), C (PDC-3 – PDB 4GZB) and D (OXA-10 – PDB 1K55), and a class B enzyme, i.e., an MBL (NDM-1 – PDB 4EYL). The Ω-loop region in SBLs are highlighted in yellow. b) Active sites of class A, C and D SBLs, with key catalytic residues displayed as sticks.