The nitrogenase mechanism: new roles for the dangler?

Dangler sites protruding from a core metallocluster were introduced into the bioinorganic lexicon in 2000 by R.D. Britt and co-workers in an analysis of the tetramanganese oxygen-evolving cluster in photosystem II. In this perspective, we consider whether analogous dangler sites could participate in the mechanism of dinitrogen reduction by nitrogenase. Two possible roles for dynamic danglers in the active site FeMo cofactor are highlighted that might occur transiently during turnover. The first role for a dangler involves the S2B belt sulfur associated with displacement by carbon monoxide and other ligands, while the second dangler role could involve the entire cluster upon displacement of the His-  $\alpha$  442 side chain to the molybdenum by a free carboxyl group of the homocitrate ligand. To assess whether waters might be able to interact with the cofactor, a survey of small ligands (water and alkali metal ions) contacting [4Fe4S] clusters in synthetic compounds and proteins was conducted. This survey reveals a preference for these sites to pack over the centers of 2Fe2S rhombs. Waters are excluded from the S2B site in the resting state of nitrogenase, suggesting it is unlikely that water molecules coordinate to the FeMo cofactor during catalysis. While alkali metal ions are found to generally influence the properties of catalysts for dinitrogen reduction, no convincing evidence was found that any of the waters near the FeMo cofactor could instead be sodium or potassium ions. Dangler sites, if they exist in the nitrogenase mechanism, are likely formed transiently by localized changes to the resting-state FeMo cofactor structure.