Hydrogenases: Hydrogen‐Activating Enzymes

Many microorganisms, such as methanogenic, acetogenic, nitrogen-fixing, photosynthetic, or sulfate-reducing bacteria, metabolize hydrogen. 2a] Hydrogen activation is mediated by a family of enzymes, termed hydrogenases, which either provide these organisms with reducing power from hydrogen oxidation or act as TMelectron sinks∫, following the reaction: H2 2H 2e . Not surprisingly, hydrogenases are mostly studied with a view to designing chemical or biochemical processes to produce molecular hydrogen more abundantly and cheaply than with platinum catalysts ; molecular hydrogen is an ideally clean fuel. 5] Hydrogenases (cytochrome c3 oxidoreductase, EC 1.18.99.1) are classified into two major families in the present paper on the basis of the metal content of their respective dinuclear catalytic centers, that is nickel ± iron (NiFe) hydrogenases and TMiron only∫ (FeFe) hydrogenases. Some NiFe hydrogenases also contain selenium at their catalytic center in the form of selenocysteine (Table 1). The two hydrogenases families differ functionally from each other in that NiFe hydrogenases tend to be more involved in hydrogen oxidation and FeFe hydrogenases in hydrogen production. Moreover, NiFe hydrogenases are approximately 10 1 ± 10 2 times less active, show 10 times more affinity for hydrogen, and are less sensitive to inhibition by oxygen and carbon monoxide than FeFe hydrogenases (Table 2). A TMmetalfree∫ hydrogenase, found in methanogenic bacteria, catalyzes the reversible reduction of a methenyltetrahydromethanopterin (methenyl-H4MPT) methanogenic cofactor with H2 to form methylene-H4MPTand a proton during methane formation from CO2 and 4H2. The three-dimensional atomic models of four NiFe, one NiFeSe, and, more recently, two FeFe 14] hydrogenases (Table 3) have been elucidated by X-ray crystallography on the basis of gene sequencing and a wealth of biochemical and spectroscopic studies, in some cases coupled with isotopic labeling. 2, 16] These results represent a considerable impetus to research the catalytic mechanism of hydrogenases and the design of organometallic compounds which mimic their structural or functional properties, or both. The aim of this short review is to highlight some recent works and trends in hydrogenase research.