NiFe hydrogenase

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 [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... 

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... 

Although the mechanistic insights for the model complex of [FeFe] hydrogenase became clearer little by little, those for the [NiFe] hydrogenase are still challenging topics. 

Heterodinuclear Ni-Fe complexes, which are not stabilized by the phosphine and NO ligands, were synthesized by Tatsumi and coworkers as [NiFe] hydrogenase mimics [208-210]. Several examples are shown in Fig. 8. However, the catalytic activities of these complexes are not ascertained. 

Fig. 9 Electrochemical H2 generation catalyzed by complex 27 as a model for the [NiFe] hydrogenase...

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