ABSTRACT
X-ray crystallographic structures of Fe-only hydrogenases from D. desulfuricans and Clostridium pasteurianum [1,2], together with spectroscopic data on Fe-only hydrogenase from D. vulgaris [3], show that the H-cluster, the active site at which protons are reduced to dihydrogen, is a conventional {Fe4S4}-
cluster linked by a bridging cysteinyl sulfur to an 'organometallic' {Fe2S3} subsite, Figure 1. At the sub-site a terminal carbon monoxide, a bridging carbon monoxide and a cyanide ligand are bound at each iron atom which also share two bridging sulfur ligands of a 1,3-propanedithiolate or possibly the related di(thiomethyl)amine unit. The Fe-atom distal to the {Fe4S4}-cluster has a coordinated water molecule (or vacancy) in the resting paramagnetic oxidised state of the enzyme, {Hox}. This site is occupied by carbon monoxide in the CO inhibited form of the enzyme {Hox(CO)} and is therefore thought to be where hydride/dihydrogen is bound during turnover.
