Hydrogen activation NiFe -hydrogenase

For hydrogen oxidation bi-component metal doped systems deposited on Raney nickel for AFC Mo and W carbides for AFC prepared by method of precipitation from a gas phase Radicals of following composition -OH, -OSO3H, -COOH, -OPO(OH)3 for PEMFC Some organic catalysts like biologically active [NiFe]-hydrogenase, pyropolymers, etc. ... 

The hydrogen-consumption activity of the [NiFe] hydrogenase of Allochromatium vinosum in different redox states... 

As explained in Section 7.4 the [NiFe] hydrogenase of Allochromatium vinosum can exist in varions states, three of which are enzymically active. Activity is usually measured by following hydrogen consnmption at 30°C with benzyl viologen (E o = —359 mV) as electron acceptor. If performed with enzyme in the ready state (see Fig. 5.6 for an overview of all states), it only takes several minutes until full activity is... 

Figure SJ Activity of the various states of the [NiFe] hydrogenase from A. vinosum as determined with a Pt electrode at 30°C.The reaction was performed in SOmM Tris/HCI (pH 8.0) in a volume of 2 ml. Oxygen was scavenged by adding glucose (90 mM) and glucose oxidase (2.5 mg/ml). Hydrogen peroxide was removed by catalase. When the system was anaerobic, an aliquot of H2-saturated water was added, and a little later enzyme (S-IOnM) was injected. Benzyl viologen (4.2mM) was used as electron acceptor.

A frequently asked question concerns the actual functional state of the crystallised protein. EPR analyses have shown, for example, that the enzyme present in aerobically prepared crystals of [NiFe] hydrogenase from D. gigas is mainly in its inactive, unready (Ni-A) state. However, it can be activated after prolonged incubation under hydrogen in the presence of methyl viologen (NiviHe et al. 1987). 

We do not know exactly where the hydrogen binds at the active site. We would not expect it to be detectable by X-ray diffraction, even at 0.1 nm resolution. EPR (Van der Zwaan et al. 1985), ENDOR (Fan et al. 1991b) and electron spin-echo envelope modulation (ESEEM) (Chapman et al. 1988) spectroscopy have detected hyperfine interactions with exchangeable hydrous in the NiC state of the [NiFe] hydrogenase, but have not so far located the hydron. It could bind to one or both metal ions, either as a hydride or H2 complex. Transition-metal chemistry provides many examples of hydrides and H2 complexes (see, for example. Bender et al. 1997). These are mostly with higher-mass elements such as osmium or ruthenium, but iron can form them too. In order to stabilize the compounds, carbonyl and phosphine ligands are commonly used (Section 6). 

Pershad, H. R., Duff, J. L., Heering, H. A., Duin, E. C., Albracht, S. P. and Armstrong, F. A. (1999) Catalytic electron transport in Chromatium vinosum [NiFe]-hydrogenase Application of voltammetry in detecting redox-active centers and establishing that hydrogen oxidation is very fast even at potentials close to the reversible H+/H2 value. Biochemistry, 38, 8992-9. 

Figure 3.2. Overview of the three states of the active standard [NiFe]-hydrogenase from A. vinosum.The wavelengths indicate the infrared frequencies for the two CN groups and the CO group, respectively. The reactions with hydrogen are fast (thick arrows) or extremely slow (dotted arrow). Protons are not shown, a, active C, C state L, light-induced state R, reduced S, EPR silent , the active site in this state is a S = V2 system (detectable by EPR) 4Fe, [4Ee4S] cluster.

As ascribed, the EPR spectrum with g = 2.10 can be low-spin Fec(III). When the isolated enzyme is reductively titrated this signal disappears at a potential Emj -0.3 V [65]. This would seem to indicate that the putative Fec(III) form is not relevant, at least not to hydrogen-production activity. The cubane is a one-electron acceptor as it can shuttle between the 2+ and 1 + oxidation states. Therefore, if the active center were to take up a total of two electrons, then the oxidation state of the Fec would, as least formally, shuttle between II and I. Recently, a redox transition in Fe hydrogenase with an Em below the H2/H+ potential has been observed in direct electrochemistry [89]. This superreduced state has not been studied by spectroscopy. It might well correspond to the formal Fec(I) state. For NiFe hydrogenases Fec(I) has recently been proposed as a key intermediate in the catalytic cycle [90] (cf. Chapter 9). 

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