Can the local enzyme scaffold act as an H-donor for a Co(I) - H bond formation? The curious case of methionine synthase-bound cob(I)alamin

The density functional calculations and analysis of the existing X-ray crystallographic data have been carried out to gain mechanistic insight into the reactivation cycle of methionine synthase (MetH) enzyme. The calculations were carried out on the cobinamide-type model complexes of cob(I)alamin (Co(I)Cbx) testing H2O and PhOH as possible beta-axial ligands. The PhOH motif was used to mimic the tyrosine (Y1139) residue that has been found in the active site of the MetH-bound cob(II)alamin (Co(II)Cbx). The calculations indicate that the beta-axial PhOH ligand forms stronger Co(I) - H bonds than H2O ligand due to its better H-donor capacity. The-calculated redox-tuning of Co(I) - H interactions on the reduction potential of Co(II)/Co(I) couple (60-800 mV vs standard hydrogen electrode (SHE)), irrespective of the beta-axial ligand considered, is significantly higher than the biological redox gap between the reduction potential of Co(II)/Co(I) couple and that of the biological reducing agents (50 mV vs SHE). The analysis of existing crystallographic data for the reactivation conformation of MetH enzyme (1K7Y (@3.0 angstrom); 1K98 (@3.8 angstrom) and 3IVA (@2.7 angstrom)) indicates that the Y1139 residue and the beta-axial H2O ligand in the MetH-bound Co(II)Cbx complex are equidistant from the Co(II) ion (Y1139 - Co(II) = 3.97 angstrom; H2O - Co(II) = 3.96 angstrom). Taking into account that the Y1139-induced Co(I) - H linkages are thermodynamically more stable than the H2O-induced ones, the present calculations suggest that the Y1139 residue may serve as the beta-axial ligand in the reactivation conformation of MetH enzyme. Published by Elsevier Inc.