Theoretical studies of biological nitrogen fixation. I. Density functional modeling of the Mo-site of the FeMo-cofactor.
The Mo-site and its ligand environment of the FeMo-cofactor (FeMo-co) were studied using the hybrid density functional method B3LYP. The structure and stability of the model complex (S-ligand)3(N-ligand)Mo[(S)-OCH(CH3)C(O)O-] along with its various protonated and reduced/oxidized forms were calculated. Several hypotheses were tested: (i) ligand environment of the Mo-site, (ii) monodentate vs bidentate coordination of the Mo-bound homocitrate ligand, (iii) substrate coordination to the Mo center, and (iv) Mo-His interaction. It was found that the decoordination of one of the homocitrate (lactate in the model) "legs", the bidentate-->monodentate rearrangement, does not occur spontaneously upon either single/double protonation or one-electron reduction. However, it could occur only upon substrate coordination to the Mo-center of the single-protonated forms of the complex. It was shown that one-electron reduction, single-protonation, and substrate coordination facilitate the bidentate<-->monodentate rearrangement of the homocitrate (lactate) ligand of FeMo-co. It was demonstrated that the smallest acceptable model of His ligand in FeMo-co is methylimidazolate (MeIm-). Our studies suggest that the epsilon-N of the FeMo-co-bound His residue is not protonated, and as a consequence the cluster is tightly bound to the protein matrix via a strong Mo-N delta bond.
Cherry L. Emerson Center for Scientific Computation and Department of Chemistry, Emory University, Atlanta, Georgia 30322, USA.
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Szilagyi
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Musaev
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Morokuma
D019344:Lactic Acid / Q000737:chemistry*; D008024:Ligands; D008956:Models, Chemical; D008982:Molybdenum / Q000737:chemistry*; D008983:Molybdoferredoxin / Q000737:chemistry*; D009586:Nitrogen Fixation / Q000502:physiology*; D009591:Nitrogenase / Q000737:chemistry; D010084:Oxidation-Reduction
D008024:Ligands; D008983:Molybdoferredoxin; D019344:Lactic Acid; D008982:Molybdenum; D009591:Nitrogenase