AHM-1: An Inclusion to the Arsenal of β-Lactam Resistance in <i>Clostridioides difficile</i>

This study delves into a newly discovered MBL (metallo-β-lactamase) in <i>Clostridioides difficile</i>, a formidable pathogen known for causing nosocomial infections and exhibiting resistance to antimicrobial agents. The primary objective was to unravel its structure-function relationship. This research establishes the enzyme AHM-1 as a subclass B3-like MBL. Experimental results reveal that the enzyme's active site consists of two Zn²⁺ atoms exhibiting tetrahedral and trigonal bipyramidal coordination, similar to B1 and B3 MBLs. Notably, within its active site, it exhibits a lower binding capacity for other transition metal ions such as Fe²⁺, Mn²⁺, and Ni²⁺ compared to Zn²⁺. The zinc-binding sites of B1 and B3 MBLs contain strictly conserved His116-His118-His196 and Asp120-Cys221/His121-His263. The absence of all the conserved residues except His116, Asp120, and His121 in the Zn-binding site distinctly separates this enzyme from these two MBL subclasses. Conserved zinc binding motifs present in B1 and B3 MBLs are H-X-H-X-D and H-X-H-X-D-H, respectively. The presence of the H-X-D-X-D-H motif in the enzyme, similar to that in B3 enzymes, along with sequence and structural analysis, places this new enzyme closer to the enzymes belonging to the B3 subclass. This study also identifies the likely catalytic residues responsible for its β-lactamase activity, similar to B3 MBLs. In contrast to MBLs, this enzyme displays hydrolytic activity toward aztreonam. It also shows higher catalytic efficiency toward higher generation cephalosporins. This study thus underscores the significance of a novel enzyme with β-lactamase activity in <i>Clostridioides difficile</i>, highlighting its potential implications for clinical treatment due to its disparities from conventional MBLs.