The MSMEG_1586 of M. smegmatis Is a Penicillin-Interactive Enzyme That Can Potentially Hydrolyse Aztreonam and Cephalosporins

Abstract

Mycobacteria are intrinsically resistant to beta-lactams as they possess several putative penicillin-interactive enzymes (PIEs), some of those are with dual-activity, namely DD-carboxypeptidase and beta-lactamase. Here, with help of molecular approaches, we elucidated the nature of one such putative PIE, MSMEG_1586, in Mycobacterium smegmatis. The in vivo expression of the membrane-bound form of MSMEG_1586 enhanced the beta-lactam resistance of a beta-lactamase deleted host E. coli strain (AM1OC), particularly for aztreonam (eight-fold) and cephalosporins (8-16 fold). To understand the reason for such elevation of resistance, soluble-form of MSMEG_1586 (sMSMEG_1586) was created by removing signal peptides and partially eliminating the amphipathic helix, and finally, expressed and purified. The purified sMSMEG_1586 was active and manifested a strong penicillin-binding affinity as shown by its ability to bind to fluorescent penicillin (Bocillin-FL). Interestingly, the steady-state kinetics apparently confirmed the hydrolytic ability of sMSMEG_1586 towards cefotaxime and aztreonam where hydrolysing aztreonam is a unique and rare behaviour among the beta-lactamases. However, sMSMEG_1586 was devoid of exerting DD-carboxypeptidase like activity. Finally, in silico analysis of MSMEG_1586 revealed a special folding that resembles class C beta-lactamase, except for the absence of a characteristic R2 loop. Overall, MSMEG_1586 could be categorized as a cephalosporinase with the ability to hydrolyse aztreonam.