Statistical analyses of the oxidized P-clusters in MoFe proteins using the bond-valence method: towards their electron transfer in nitrogenases

26 well selected oxidized P-clusters (P²⁺) from the crystallographic data deposited in the Protein Data Bank have been analysed statistically by the bond-valence sum method with weighting schemes for MoFe proteins at different resolutions. Interestingly, the oxidation states of P²⁺ clusters correspond to Fe₂³⁺Fe₆²⁺ with high electron delocalization, showing the same oxidation states as the resting states of P-clusters (P^N) in nitrogenases. The previously uncertain reduction of P²⁺ to P^N clusters by two electrons was assigned as a double protonation of P²⁺, in which decoordination of the serine residue and the peptide chain of cysteine take place, in MoFe proteins. This is further supported by the obviously shorter α-alkoxy C-O bond (average of 1.398 Å) in P²⁺ clusters and longer α-hydroxy C-O bond (average of 1.422 Å) in P^N clusters, while no change is observed in the electronic structures of Fe₈S₇ Fe atoms in P-clusters. Spatially, the calculations show that Fe3 and Fe6, the most oxidized and most reduced Fe atoms, have the shortest distances of 9.329 Å from the homocitrate in the FeMo cofactor and 14.947 Å from the [Fe₄S₄] cluster, respectively, and may well function as important electron-transport sites.