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The hydrogenases are the enzymes involved in the metabolism of dihydrogen. Through their activity, many microorganisms can use H2 as an energy source (H2 uptake) or use protons as an electron sink (H2 evolution) according to the equation:
H2   2H+ + 2e-
There are three types of hydrogenases:
The Ni-Fe Hydrogenase
The wireframe display in the left screen shows the structure of hydrogenase form the sulfate-reducing bacterium Desulfovibrio Gigas. The enzyme has 800 amino acid residues and weighs 88 kDa.
4S4 center. Commonly Fe4S4 clusters have 4 cysteine ligands while here one cysteine ligand is replaced by a histidine (His 185). This cys - his substitution probably gives the cluster a more positive redox potential. A Fe3S4 cluster is located in the middle while the second 4Fe - 4S cluster is in the proximity of the subunit interface and the active site.
-) and one carbonyl (CO) ligand. The presence of these small ligands is quite surprising and without precedent in biological structures.
The bridging oxygen donor ligand X is lost when the active site is reduced. In the catalytic cycle, dihydrogen penetrates the protein, reaches the active site, and then binds at the iron(II) or nickel(II). Binding in an η2, side-on fashion makes dihydrogen acidic. A proton is transmitted from dihydrogen to the protein surface via a network of hydrogen bonds. The loss of this proton leaves at the active site a hydride that bridges between the iron and nickel (i.e. X is a hydride now). There is the subsequent loss of electrons via the iron-sulfur cluster conduit to cytochrome electron carriers and the loss of a proton. This leaves Fe(II) and Ni(II) and a vacant site to bind another equivalent of dihydrogen.
The Fe-Fe Hydrogenase
2NHCH2S)Fe(CO)(CN) where the azadithiolate ligand is represented in the crystal structure as a 1,3-propaneditholate ligand (the resolution is not good enough to distinguish between nitrogen and carbon.
If you would like to learn more about the hydrogenase and its model compounds see the following:
The development of the hydrogenase model complexes should provide important insights into the chemistry of this enzyme. However, the study of these synthetic analogues is not of purely scientific importance. A better understanding of hydrogenases might lead to better ways to produce dihydrogen for the "Hydrogen Economy", i.e. the utilization of dihydrogen as a clean energy source.
A Structural Model For the NiFe Active site
 The complex can be formulated as [(C6H4S2)Ni (µ -'S'3)Fe(CO)(PMe3)2] where 'S'3 stands for the bis(2-mercaptophenyl)sulfide ligand. Like in the active site of the hydrogenase, the nickel atom is coordinated by four sulfur atoms two of which are bridging to the iron atom. However, the geometry around the Ni center is square planar rather than tetrahedral. The iron atom has a carbonyl and two PMe3 ligands. The geometry around Fe is octahedral. Successful substitution of PMe3 with cyano ligands has not been reported yet. This complex, despite its similarities with the hydrogenase's active site, does not react with the dihydrogen.
A Functional Model For the NiFe Active site
 report the synthesis, structure and reactivity of the bimetallic RuHNi complex shown at left. The complex can be formulated as [(C6Me6)Ru(µ-H)NiL]NO3 where L stands for an S-N-N-S ligand N,N'-dimethyl-N,N'-bis(2-mercaptoethyl)-1,3-propanediamine. This complex has a hydride ligand and two thiolate ligands bridging between ruthenium(II) and nickel(II). Ruthenium is in the same group as iron. The active Ni-C state of hydrogenase has similar ligands bridging between Ni(III) and Fe(II). Like the Ni-C state, the Ru(H)Ni complex is formed by reaction of hydrogen gas with a bimetallic complex. It is a rare example of a paramagnetic transition metal hydride complex.
A Model For the FeFe Active site
FeII(µ-H)(SCH2CH2CH2S)FeII(CO)2(CN) with a hydride bridging between the two iron(II) ions. This complex is a catalyst for the electrochemical reduction of protons  .