The Binding and Reduction of Dinitrogen at an Fe₄ Face of the FeMo Cluster of Nitrogenase

Abstract

The enzyme nitrogenase is comprised of an Fe protein and an FeMo protein, for which crystal structure data are now available.^1,2 The FeMo protein contains the probable site of N₂ binding and reduction; this is an iron molybdenum ( FeMo ) sulfide cluster with the composition [Fe₇MoS₉(S-Cys )(N-His)(O,O′- homocitrate )]. A model is now proposed for the binding of N₂ to the FeMo cluster of the FeMo protein of nitrogenase. The model involves the interactions of the cluster with its protein surrounds, and is supported with local density functional calculations of electronic structure, and geometry optimization. The postulated binding site is an Fe₄ quadrilateral, covered but not enclosed by the side chain of an arginine residue. The N₂ is bound, in various configurations involving twists of the Fe₄ face and twists of the N₂ molecule on the face. These configurations populate a relatively flat energy surface. The two-coordinate sulfur atoms which flank the binding site play an important role, hydrogen bonding to the protein, folding away during N₂ binding, being most responsive to redox changes, and mediating the proton transfer to substrate during reduction. All attributes of the model can function in concert to provide a basic picture of the mechanism for binding, reduction, and protonation of N₂.