Hydrogenases NiFe hydrogenase

The hydrogenases, [NiFe] hydrogenase, [NiFeSe] hydrogenase and [Fe] hydrogenase, have been studied. [NiFe] hydrogenase has Ni and Fe atoms at its active site, which play a central role in the catalytic reaction. It is found in anaerobic... 

The hydrogenases are classified according to their composition. Three distinct classes of metal-containing hydrogenases have been identified [3] Fe-only hydrogenases NiFe-hydrogenases and NiFeSe-hydrogenases. 

The first reported (1995) structure of a hydrogenase, [NiFe]-hydrogenase from Desulfovibrio gigas, is a heterodimer in which a large subunit contains the... 

NiFe-hydrogenase Bacteria H2 2H + 2e- [FesS4] 2[Fe4S4p- NiFe center -70 59... 

Fig. 1. Proposed electron transport pathway in D. gigas NiFe-hydrogenase. Selected distances are given in angstroms. Modified with permission from Ref. (157).

The spatial arrangement of the Fe-S clusters in D. gigas NiFe-hydrogenase (see Fig. 1) suggests an active role for the [Fe3S4] ° cluster in mediating electron transfer from the NiFe active site to the... 

The multifrequency EPR and Mdssbauer properties of the [FesSJ in C. vinosum NiFe-hydrogenase are particularly interesting since they provide evidence of magnetic interactions with nearby paramagnetic species (151, 154, 155). The magnetically isolated form exhibits a well-resolved, almost axial EPR signal, g = 2.018, 2.016, 2.002, indicative of minimal conformational heterogeneity. However, a com-... 

EPR spectra and g values for the various states of the hydrogenase from Thiocapsa roseopersicina 64) are depicted in Fig. 4. These spectra are representative of those of the other NiFe hydrogenases. 

The biologically uncommon Ni center associated with FeS clusters is a powerful and unique catalytic unity. In this chapter we have reviewed the structural and mechanistic aspects of three NiFeS centers the active site of hydrogenase and Clusters A and C of CODH/ACS. In the former, the association of a Ni center with the most unusual FeCOCN2 unit is a fascinating one. Model chemists, spectroscopists, and crystallographers have joined efforts to try and elucidate the reaction mechanism. Although a consensus is being slowly reached, the exact roles of the different active site components have not yet been fully established. Ni appears to be the catalytic center proper, whereas the unusual Fe center may be specially suited to bind a by-... 

The [NiFe] hydrogenase from D. gigas has been used as a prototype of the [NiFe] hydrogenases. The enzyme is a heterodimer (62 and 26 kDa subunits) and contains four redox active centers one nickel site, one [3Fe-4S], and two [4Fe-4S] clusters, as proven by electron paramagnetic resonance (EPR) and Mosshauer spectroscopic studies (174). The enzyme has been isolated with different isotopic enrichments [6 Ni (I = I), = Ni (I = 0), Fe (I = 0), and Fe (I = )] and studied after reaction with H and D. Isotopic substitutions are valuable tools for spectroscopic assignments and catalytic studies (165, 166, 175). 

Most of the as-isolated [NiFe] hydrogenases are inactive, and the nickel center exhibits an intense rhombic EPR signal termed Ni-A (g = 2.31, 2.26, and 2.02) with variable amounts of a second nickel species, named Ni-B (g = 2.33, 2.16, and 2.02), with slightly different... 

Fig. 8. The structure of D. gigas hydrogenase showing the novel heterodinuclear [NiFe] site, the three iron-sulfur clusters, and the tracing of the polypeptide chain...

In addition, the [NiFe] hydrogenase from D. fructosovorans is very similar to D. gigas hydrogenase, and its structure has been solved 185). In order to understand the role of the [3Fe-4S] cluster, a Pro-432Cys mutant was produced. In this mutant the conversion of a [3Fe-4S] into a [4Fe-4S] center was proven by EPR and X-ray crystallography. 

A crystallographic analysis of xenon binding to [NiFe] hydrogenase, together with a molecular dynamic simulation study of xenon and dihydrogen diffusion in the enzyme interior, suggests the existence of hydrophobic channels connecting the molecular surface with the active site 184). 

We will use here the main results obtained for two complex and distinct situations the structural and spectroscopic information gathered for D. gigas [NiFe] hydrogenase and AOR, in order to discuss relevant aspects related to magnetic interaction between the redox centers, intramolecular electron transfer, and, finally, interaction with other redox partners in direct relation with intermolecular electron transfer and processing of substrates to products. 

The [NiFe] hydrogenase metal sites were completely defined by spectroscopy and X-ray diffraction studies, as discussed before. The... 

Fig. 6 The models for the active site of [FeFe]-, [NiFe]-, and [Fe]-hydrogenases...

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