Molybdenum and vanadium homocitrates and their homologs — towards the local environments and protonations of FeMo/V-cofactors in nitrogenases

Homocitrate is an organic component in the active site of nitrogenases (N2ase), where their cofactors are in the forms of [Δ-MoFe7S9C(R-Hhomocit)(N-his)(S-cys)], [Δ-VFe7S8C(CO3)(R-Hhomocit/homocit)(N-his)(S-cys)], and [Δ-FeFe7S9C(R-Hhomocit/homocit)(N-his)(S-cys)] (FeMo/V/Fe-cos) respectively, which are based on high-resolution crystallography, XES (XES = X-ray Emission Spectroscopy), ESEEM (ESEEM = Electron Spin Echo Envelope Modulation), and cryogenic electron microscopy. The protonated form of FeMo-co is set up on IR (Infra-red), VCD (Vibrational Circular Dichroism) spectroscopies and the comparisons of model complexes with protein structures in different oxidation states. Homocitrate ligand coordinates with molybdenum through its α-alcoholic and vicinal carboxyl groups, which is served as a possible proton source during the N2 reduction. Here a brief review has been given on the isolations, spectroscopies and structural characterizations of molybdenum and vanadium homocitrates and their homologs, including citrates, malates, tartrates, lactates and glycolates. The structures and configurations of Mo/V-homocitrato complexes are compared with the coordination environments of molybdenum, vanadium and iron atoms in FeMo/V/Fe-cos. Theoretical calculations for the protonation of α-alkoxido group in homocitrate of FeMo-co were also reviewed. A delivery pathway of hydrogen is suggested for the protonation and deprotonation of homocitrate in FeMo/V-cos.