Reaction Mechanism for CO Reduction by Mo-Nitrogenase Studied by QM/MM

We have studied the conversion of two molecules of carbon
monoxide
to ethylene catalyzed by nitrogenase. We start from a recent crystal
structure showing the binding of two carbon monoxide molecules to
nitrogenase and employ the combined quantum mechanics and molecular
mechanics approach. Our results indicate that the reaction is possible
only if S2B dissociates as H2S (i.e., the charge of the
FeMo cluster remains the same as in the E0 state, indicating
that the Fe ions are formally reduced two steps when CO binds). Eight
electrons and protons are needed for the reaction, and our mechanism
suggests that the first four bind alternatively to the two carbon
atoms. The C–C bond formation takes place already after the
first protonation (in the E3 state). The next two protons
bind to the same O atom, which then dissociates as water. In the same
state (E8), the second C–O bond is cleaved, forming
the ethylene product. The last two electrons and protons are used
to form a water molecule that can be exchanged by S2B or by two CO
molecules to start a new reaction cycle.