title: A third type of hydrogenase catalyzing H2 activation.
abstract: The activation of molecular hydrogen is of interest both from a chemical and biological viewpoint. The covalent bond of H(2) is strong (436 kJ mol(-1)). Its cleavage is catalyzed by metals or metal complexes in chemical hydrogenation reactions and by metalloenzymes named hydrogenases in microorganisms. Until recently only two types of hydrogenases are known, the [FeFe[-hydrogenases and [NiFe[-hydrogenases. Both types, which are phylogenetically unrelated, harbor in their active site a dinuclear metal center with intrinsic CO and cyanide ligands and contain iron-sulfur clusters for electron transport as revealed by their crystal structures. Fifteen years ago a third type of phylogenetically unrelated hydrogenase was discovered, which has a mononuclear iron active site and is devoid of iron-sulfur clusters. It was initially referred to as "metal free" hydrogenase, but was later renamed iron-sulfur cluster-free hydrogenase or [Fe[-hydrogenase. In this review, we introduce first the [FeFe[-hydrogenases and [NiFe[-hydrogenases, and then focus on the structure and function of the iron-sulfur cluster-free hydrogenase (Hmd) and show that this enzyme contains an iron-containing cofactor. The low-spin iron is complexed by two intrinsic CO-, one sulfur- and one or two N/O ligands and has one open coordination site, which is proposed to be the location of H(2) binding.
authors.affiliation: Max-Planck-Institut für Terrestrische Mikrobiologie, Karl-von-Frisch-Strasse, D-35043 Marburg, Germany. shima@mpi-marburg.mpg.de
authors.forename: Seigo
authors.initials: S
authors.lastname: Shima
authors.forename: Rudolf K
authors.initials: RK
authors.lastname: Thauer
mesh_terms: D018360:Crystallography, X-Ray; D006859:Hydrogen / Q000378:metabolism*; D006864:Hydrogenase / Q000737:chemistry* / Q000378:metabolism*; D017017:Methanococcus / Q000201:enzymology; D008958:Models, Molecular; D013056:Spectrophotometry, Ultraviolet
chemical_list: D006859:Hydrogen; D006864:Hydrogenase