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Abstract

Abstract The activation of molecular hydrogen is of interest both from a chemical and biological viewpoint. The covalent bond of H2 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 H2 binding. ? 2007 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 7: 37?46; 2007: Published online in Wiley InterScience (www.interscience.wiley.com) DOI 10.1002/tcr.20111