Promiscuous metallo-β-lactamases: MIM-1 and MIM-2 may play an essential role in quorum sensing networks.

MIM-1 and MIM-2 are two recently identified metallo-β-lactamases (MBLs) from Novosphingobium pentaromativorans and Simiduia agarivorans, respectively. Since these organisms are non-pathogenic we speculated that the biological role(s) of MIM-1 and MIM-2 may not be related to their MBL activity. Although both sequence comparison and homology modeling indicate that these proteins are homologous to well-known MBLs such as AIM-1, the sequence analysis also indicated that MIM-1 and MIM-2 share similarities with N-acyl homoserine lactonases (AHLases) and glyoxalase II (GLX-II). Steady-state kinetic assays using a series of lactone substrates confirm that MIM-1 and MIM-2 are efficient lactonases, with catalytic efficiencies resembling those of well-known AHLases. Interestingly, unlike their MBL activity the AHLase activity of MIM-1 and MIM-2 is not dependent on the metal ion composition with Zn(II), Co(II), Cu(II), Mn(II) and Ca(II) all being able to reconstitute catalytic activity (with Co(II) being the most efficient). However, these enzymes do not turn over S-lactoylglutathione, a substrate characteristic for GLX-II activity. Since lactonase activity is linked to the process of quorum sensing the bifunctional activity of "non-pathogenic" MBLs such as MIM-1 and MIM-2 may provide insight into one possible evolutionary pathway for the emergence of antibiotic resistance.