Catalytic Mechanism of Metallo beta-Lactamases: Insights from Calculations and Experiments

Abstract

b-Lactam antibiotics are the most widespread antibiotics on the market nowadays. They have exerted great evolutionary pressure on bacteria, triggering sophisticated resistance mechanisms. Among them, the most widespread is the expression of blactamases. This is a family of hydrolases that uses different protein scaffolds and catalytic architectures to inactivate b-lactam drugs (Scheme 21.1) [1]. b-Lactamases from classes A, C and D are serine hydrolases, whereas class B blactamases are characterized by the presence of Zn ions bound to their active sites [2]. Metallo b-lactamases (MbLs), despite not being as ubiquitous as serine b-lactamases, represent the largest group of carbapenemases, that is, they hydrolyze a very important class of antibiotics, the carbapenems. MbLs are increasingly spreading among pathogenic bacteria in the clinical setting and are resistant to all current clinical inhibitors on the market [3–6]. Thus, understanding their function at the molecular level is of paramount importance for designing effective drugs (either MbL inhibitors or antibiotics refractory to hydrolysis by MbLs). MbLs are classified by sequence homology in three subclasses: B1, B2 and B3 [4–7]. The B1 subclass includes several chromosomally-encoded enzymes, such as the ones from Bacillus cereus (BcII) [8–13], Bacteroides fragilis (CcrA) [14–16], Elizabethkingia meningoseptica (BlaB) [17, 18], as well as the transferable VIM, IMP, SPM and GIM-type enzymes [19–25]. Subclass B2 includes the CphA and ImiS lactamases from Aeromonas species, and Sfh-I from Serratia fonticola [26–28]. Subclass B3, along with the extensively characterized enzyme L1 from Stenotrophomonas maltophilia [29–32], includes enzymes from environmental bacteria, such as CAU-1 from Caulobacter crescentus [33], and THIN-B from Janthinobacterium lividum [34], and from opportunistic pathogens like FEZ-1 from Legionella gormanii [35–37], and GOB from E. meningoseptica [38]. B1 and B3 enzymes display a broad substrate spectrum, being able to actively hydrolyze b-lactams of all three b-lactam families (e.g., penicillins, cephalosporins and carbapenems), whilst B2 MbLs are selective carbapenemases.