Modeling the resting state of oxalate oxidase and oxalate decarboxylase enzymes

In view of the. biological and commercial interest in models for Oxalate Decarboxylases (OxDC) and Oxalate Oxidases (OxOx), we have synthesized and characterized three new Mn-II complexes (1-3) employing N3O-donor aminocarboxylate ligands (TCMA, 1,4,7-triazacyclononane-N-acetic acid; K(i)Pr(2)TCMA, potassium 1,4-diisopropyl-1,4,7-triazacyclononane-N-acetate; and KBPZG, potassium N,N-bis(3,5-dimethylpyrazolyl methyl)glycinate). These complexes were characterized by several techniques including X-ray crystallographic analysis, X-band electron paramagnetic resonance (EPR), electrospray ionization mass spectrometry (ESI-MS), and cyclic voltammetry. The crystal structures of 1 and 3 revealed that both form infinite polymeric chains of Mn-II complexes linked by the pendant carboxylate arms of the TCMA(-) and the BPZG(-) ligands in a syn-antipattern. Complex 2 crystallizes as a mononuclear Mn-II cation, six-coordinate in a distorted octahedral geometry. Although complexes 1 and 3 crystallize as polymeric chains, all compounds present the same N3O-donor set atoms around the metal center as observed in the crystallographically characterized OxDC and OxOx. Moreover, complex 2 also contains two water molecules coordinated to the Mn center as observed in the active site of OxDC and OxOx. ESI-MS spectrometry, combined with EPR, were useful techniques to establish that complexes 1-3 are present as mononuclear Mn-II species in solution. Finally, complexes 1-3 are able to model the resting state active sites, with special attention focused on complex 2 which provides the first exact first coordination sphere ligand structural model for the resting states of both OxDC and OxOx.