FeMo cofactor of nitrogenase: energetics and local interactions in the protein environment

A combined broken-symmetry density functional and continuum electrostatics approach has been applied to the iron-molybdenum center (FeMoco) of nitrogenase to evaluate the energetic effects of the local amino acid environment for several spin alignments of FeMoco. The protein environment preferentially stabilizes certain spin coupling patterns. The lowest energy spin alignment pattern in the protein displays calculated properties that match the experimental data better than any of the alternative possibilities. The total interaction energy of the protein with FeMoco has been evaluated and the contribution of each amino acid residue has been broken down into sidechain and backbone components. Arginine, lysine, aspartate and glutamate sidechains exert the largest electrostatic influence on FeMoco; specific residues are highlighted and their interaction with FeMoco discussed in the context of the available X-ray data from Azotobacter vinelandii (Av). Observed data for the M-N (resting state)-4M(OX)(one-electron oxidized state) and M-N-->M-R (one-electron reduced state) or M-I(alternative one-electron reduced state) redox couples are compared with those calculated for Av. The calculated redox potentials are fairly insensitive to the spin state of the oxidized or reduced states and the predicted qualitative trend of a more negative redox potential for the more reduced M-N-->M-R or M-I couple is in accord with the available redox data. These calculations represent a first step towards the development of a microscopic model of electron and proton transfer events at the nitrogenase active site.