Reaction Mechanism\nfor CO Reduction by Mo-Nitrogenase\nStudied by QM/MM

We have studied the conversion of two molecules of carbon\nmonoxide\nto ethylene catalyzed by nitrogenase. We start from a recent crystal\nstructure showing the binding of two carbon monoxide molecules to\nnitrogenase and employ the combined quantum mechanics and molecular\nmechanics approach. Our results indicate that the reaction is possible\nonly if S2B dissociates as H<sub>2</sub>S (i.e., the charge of the\nFeMo cluster remains the same as in the E<sub>0</sub> state, indicating\nthat the Fe ions are formally reduced two steps when CO binds). Eight\nelectrons and protons are needed for the reaction, and our mechanism\nsuggests that the first four bind alternatively to the two carbon\natoms. The C–C bond formation takes place already after the\nfirst protonation (in the E<sub>3</sub> state). The next two protons\nbind to the same O atom, which then dissociates as water. In the same\nstate (E<sub>8</sub>), the second C–O bond is cleaved, forming\nthe ethylene product. The last two electrons and protons are used\nto form a water molecule that can be exchanged by S2B or by two CO\nmolecules to start a new reaction cycle.