Sulfide release and rebinding in the mechanism for nitrogenase

Abstract

Nitrogenases are the only enzymes that activate the strong triple bond in N₂. The mechanism for the activation has been very difficult to determine in spite of decades of work. In previous modeling studies it has been suggested that the mechanism for nitrogen activation starts out by four pre-activation steps (A₀–A₄) before catalysis. That suggestion led to excellent agreement with experimental Elecrtron Paramagnetic Resonance (EPR) observations in the step where N₂ becomes protonated (E₄). An important part of the pre-activation is that a sulfide is released. In the present paper, the details of the pre-activation are modeled, including the release of the sulfide. Several possible transition states for the release have been obtained. An A₄(E₀) state is reached which is very similar to the E₄ state. For completeness, the steps going back from A₄(E₀) to A₀ after catalysis are also modeled, including the insertion of a sulfide.