Theoretical Investigation on the Role of the Central Carbon Atom and Close Protein Environment on the Nitrogen Reduction in Mo Nitrogenase

A theoretical study elucidating the mechanism of N-2 reduction in Mo nitrogenase was carried out using a QM/QM' approach based on density functional theory/semiempirical methods. Resting on the consolidated Lowe-Thorneley catalytic cycle, the identified reaction mechanism corresponds to an alternating pathway where the two nitrogen atoms are alternately reduced. Furthermore, this new mechanism provides a clear mechanistic basis to most of the experimental observations, including the noninnocent role played by the carbon atom located in the center of the MoFe cofactor and by the surrounding amino acids (such as alpha-96(ARG), alpha-195(HIS), and alpha-70(VAL)). It also provides evidence for the presence of H-2 evolution in the global reaction cycle. Our calculations indicate a large flexibility of the cofactor upon hydrogenation and subsequent N-2 chemical binding, with the average Fe-C distance increasing of 0.26 angstrom in going from the E0 to the E4 state, in agreement with experimental evidence. Taken together, these results give new insights into the activity of Mo nitrogenase, clearly considering the most relevant experimental findings.