tail groups (C2 substituents) seem to be dynamic and able to adopt multiple conformations, which suggests they do not form strong, specific interactions with the enzyme active site. The generalized β-lactam hydrolysis mechanism for SBLs consists of acylation followed by deacylation (Scheme 1). Both acylation and deacylation reactions include the formation of a shortlived tetrahedral intermediate (TI) through a nucleophilic attack; the respective TI species collapses to yield either a covalent AC structure (after acylation), or the final hydrolyzed product (after deacylation). In both reactions, the nucleophile (conserved serine (Ser70) in acylation and a water molecule (deacylating water, DW) in deacylation) is activated via proton abstraction by a general base. For OXA enzymes, this general base is a carboxylated lysine residue (Lys73). Notably, Lys73 needs to be carboxylated for optimal activity; this carboxylation is reversible and pH dependent, i.e. more carboxylation is observed at higher pH values. At lower pH values, protonation of the Lys73:Nζ would lead to decarboxylation. Based on pH dependence studies of the reaction between OXA-10 and penicillin or nitrocefin, the pKa of the carboxylated Lys73 is expected to be ~5.8-6.2. For carbapenems, the pyrroline ring can undergo Δ2 → Δ1 tautomerization in the AC state, the Δ1 tautomer also having two stereoisomers (R and S). For class A SBLs, the Δ2 tautomer has been suggested to be the catalytically competent form, whereas the Δ1 form would essentially inhibit the enzyme. For OXA-48 enzymes, all three tautomers have Scheme 1. Top: Structures of meropenem and imipenem (with atoms numbered), the 6α-hydroxyethyl group is highlighted in red. Bottom: Deacylation mechanism in OXA-48 with a carbapenem substrate (Δ2 tautomer). Starting from the acylenzyme, the antibiotic is deacylated via tetrahedral intermediate formation (1 → 2), which collapses to yield the hydrolysed antibiotic (3).