Tracing the ‘9(th) Sulfur’ of the Nitrogenase Cofactor via a Semi-Synthetic Approach

The M-cluster is the [(homocitrate)MoFe(7)S(9)C] active site of nitrogenase that is derived from an 8Fe core assembled via coupling and rearrangement of two [Fe(4)S(4)] clusters concomitant with the insertion of an interstitial carbon and a ‘9(th) sulfur’. Combining synthetic [Fe(4)S(4)] clusters with an assembly protein template, here we show that sulfite can give rise to the ‘9(th) sulfur’ that is incorporated in the catalytically important belt region of the cofactor after the radical SAM-dependent carbide insertion and the concurrent 8Fe-core rearrangement have already taken place. Based on the differential reactivity of the formed cluster species, we also propose a new [Fe(8)S(8)C] cluster intermediate, the L*-cluster, that is similar to the [Fe(8)S(9)C] L- cluster but lacks the ‘9(th) S’ from sulfite. This work provides a semi-synthetic tool for protein reconstitution that could be widely applicable for the functional analysis of other FeS systems.