Iron-containing enzymes hydrogenase

Many microbes contain a hydrogenase that lacks nickel. The best studied of these proteins is the hydrogenase from C. pasteurianum (Adams, 1990). The active site of this protein has been called the H cluster. Two structures of this protein recently appeared, a 1.6 structure of the het-erodimeric Desulfovibrio desuljuricans enzyme (Nicolet et al., 1999) and the 1.8 structure of the Cpl enzyme from Clostridium pasteurianum (Peters et al., 1998). The H cluster at the active site contains 6Fe as proposed earlier (Adams et al., 1989). There are two subcomponents a typical [4Fe64S] cubane bridged by a cysteine residue to an active site binuclear Fe center (Figure 4b). The unusual nature of the iron site in the NiFe hydrogenase is... 

Iron Sulfur Compounds. Many molecular compounds (18—20) are known in which iron is tetrahedraHy coordinated by a combination of thiolate and sulfide donors. Of the 10 or more stmcturaHy characterized classes of Fe—S compounds, the four shown in Figure 1 are known to occur in proteins. The mononuclear iron site REPLACE occurs in the one-iron bacterial electron-transfer protein mbredoxin. The [2Fe—2S] (10) and [4Fe—4S] (12) cubane stmctures are found in the 2-, 4-, and 8-iron ferredoxins, which are also electron-transfer proteins. The [3Fe—4S] voided cubane stmcture (11) has been found in some ferredoxins and in the inactive form of aconitase, the enzyme which catalyzes the stereospecific hydration—rehydration of citrate to isocitrate in the Krebs cycle. In addition, enzymes are known that contain either other types of iron sulfur clusters or iron sulfur clusters that include other metals. Examples include nitrogenase, which reduces N2 to NH at a MoFe Sg homocitrate cluster carbon monoxide dehydrogenase, which assembles acetyl-coenzyme A (acetyl-CoA) at a FeNiS site and hydrogenases, which catalyze the reversible reduction of protons to hydrogen gas. 

The 3D crystal structure of Dsm. baculatum [NiSeFe] hydrogenase has been solved 185), and it was indicated that the enzyme contains three [4Fe-4S] centers. A cysteine (replacing a proline usually found near the [3Fe-4S] core) provides an extra ligand, enabling the acceptance of a fourth iron site at this cluster. 

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