Structural evidence for a dynamic metallocofactor during N 2 reduction by Mo-nitrogenase

Delicate dance becomes a ballet The enzyme nitrogenase uses adenosine triphosphate and several unusual iron-sulfur cofactors to pump electrons into typically inert dinitrogen (N2 ), providing protons along the way. Previous work has shown that sulfur atoms in the iron-molybdenum cofactor (FeMoCo) are labile and suggests that replacement of one of the sulfurs by N2 is integral to the mechanism of N2 binding and reduction. Through the elimination of excess reducing agent during preparation, Kanget al. determined structures of Mo-nitrogenase in a resting conformation. Unexpectedly, they found that all three sulfurs at the outer edge of FeMoCo appear to be labile, with one subunit even having two of three sulfurs replaced by light, diatomic ligands. Biochemical and spectroscopic data indicate that the protein is active, holds tightly bound N2 , and is in the expected oxidation state. These results may prompt a reassessment of the possible mechanisms of N2 reduction and the role of dynamic belt ligands in FeMoCo.Science , this issue p.1381