Nitrogenase Is Diagrammed Below

Nitrogenase is diagrammed below. This particular enzyme is shown in the format just before it begins a cycle of nitrogen reduction. This goal is accomplished in four steps. What is the first step?

A) N₂ gas binds to the active site on the FeMo protein.
B) NADPH binds to the active site in order to reduce it prior to N₂ binding.
C) Two e - are transferred to the Fe protein and then to the FeMo protein, which are used to reduce H+ to H₂ prior to N₂ binding.
D) The preexisitng NH4+ is removed prior to N₂ binding.
E) Fe and Mo are bound to the active sites and then released prior to the next cycle.