Ramifications of C-centering rather than N-centering of the active site FeMo-co of the enzyme nitrogenase

Abstract

Nitrogenase catalyses the hydrogenation of N₂ to NH₃, and of CO to hydrocarbons. The active site is FeMo-co, an Fe₇MoS₉ cluster with an atom (X^c) at the centre of the inner trigonal prism of six Fe atoms. Calculations extending over almost a decade yielded consensus that this atom was nitrogen. The first strong experimental data, reported very recently, indicate that the central atom is carbon. This paper evaluates differences between C-centered and N-centered FeMo-co, and addresses the questions: (a) does the finding of C^c diminish the validity of the many previous theoretical simulations (with N^c) of the reactivity and reactions of FeMo-co? and (b) does the published 21-step mechanism for N₂ + 6H → 2NH₃ need major revision? Accordingly, this paper first reports comparative (C^c/N^c) calculations of the electronic structure of FeMo-co, describing the ground and low-lying electronic states, and the distribution of electron spin density and of partial charge. The differences are clear, but minor. Then, reaction profiles and structures of intermediates and transition states are reported for the C-centered and N-centered versions of the four key types of reaction step involved in the overall mechanism: (i) binding of N₂, (ii) conformational preparation of H on FeMo-co, (iii) H transfers to N, and (iv) N–N breaking. Again the differences are small, and the calculated activation energies for the previous complete mechanism appear to be essentially transferable to C-centered FeMo-co.