Binding of exogenous cyanide reveals new active-site states in [FeFe] hydrogenases.

[FeFe] hydrogenases are highly efficient metalloenyzmes for hydrogen conversion. Their active site cofactor (the H-cluster) is composed of a canonical [4Fe-4S] cluster ([4Fe-4S]) linked to a unique organometallic di-iron subcluster ([2Fe]). In [2Fe] the two Fe ions are coordinated by a bridging 2-azapropane-1,3-dithiolate (ADT) ligand, three CO and two CN ligands, leaving an open coordination site on one Fe where substrates (H and H) as well as inhibitors ( O, CO, HS) may bind.