text: The Fe-only hydrogenases are particularly common enzymes for synthetic organometallic chemists to mimic.  This interest is motivated by the inclusion of high field ligands like cyano and CO (metal carbonyl) in the first coordination sphere of the pertinent di-iron cluster.  Free cyano and carbonyl ligands are toxic to many biological systems. So, their inclusion in this system suggests they play pivotal roles.  These high field ligands may ensure the iron centers at the active site remain in a low spin state throughout the catalytic cycle.  In addition, there is bridging dithiolate between the two iron centers.  This dithiolate has a three atom backbone in which the identity of the central atom is still undetermined; it models crystallographically as a CH, NH or O group.  There is reason to believe that this central atom is an amine which functions as a Lewis base.  This amine combined with Lewis acidic iron centers makes the enzyme a bifunctional catalyst which can split hydrogen between a proton acceptor and a hydride acceptor or produce hydrogen from a proton and hydride.
  Since none of the ligands on the iron centers are part of the enzyme's amino acid backbone, they can not be investigated through site-directed mutagenesis, but enzyme mimicry is a feasible approach.