The continued evolution of the L2 cephalosporinase in Stenotrophomonas maltophilia: A key driver of beta-lactam resistance

The Stenotrophomonas maltophilia L2 cephalosporinase is one of two beta-lactamases which afford S. maltophilia beta-lactam resistance. With the overuse of beta-lactams, selective pressures have contributed to the evolution of these proteins, generating proteins with an extended spectrum of activity. Variant L2 cephalosporinases have been detected, as has their distribution into two main clades (Clade 1 and 2). Comprehensive analysis of six L2 variants, cloned into pET41a(+) and expressed in Escherichia coli BL21(DE3) cells, revealed that Clade 1 variants exhibited higher ceftazidime resistance compared to Clade 2. Notably, the Sm5341 L2 variant, carrying a Phe72Ile variation, displayed a significantly reduced resistance profile across all substrates tested, suggesting a key role of Phe72 in enzymatic activity. A Ile72Phe substitution in the pET41a(+) based Sm5341_L2 variant resulted in a gain-of-function for this protein, confirming the role of Phe72 in the activity of L2. Furthermore, residue interaction network analysis elucidated a pi-cation interaction between Tyr272 and Arg244, which may potentially be stabilizing the enzyme and its binding site. The presence of Tyr272 in Clade 1 variants correlates with higher ceftazidime affinity, contrasting the Asp272 in Clade 2 variants. Displaying lower Km values and higher kcat/Km ratios, Clade 1 L2 enzymes demonstrated a higher binding efficiency and greater catalytic efficiency for most of substrates assessed. These results indicate that L2 enzymes are continuing to evolve and adapt to a selective environment fuelled by the overuse of beta-lactams. This adaptation may signal the beginning of an evolutionary process yielding variant L2 cephalosporinases with extended substrate profiles.