Nickel hydrogenases redox states

Davidson, G., Choudhury, S. B., Gu, Z., Bose, K., Roseboom, W., Albracht, S. P. and Maroney, M. J. (2000) Structural examination of the nickel site in chromatium vinosum hydrogenase Redox state oscillations and structural changes accompanying reductive activation and CO binding. Biochemistry, 39, 7468-79. 

Amara, P., Volbeda, A., Fontecilla-Camps, J. C., Field, M. J., 1999, A Hybrid Density Functional Theory/Mo-lecular Mechanics Study of Nickel-Iron Hydrogenase Investigation of the Active Site Redox States , J. Am. Chem. Soc., 121, 4468. 

Amara P, Volbeda A, Fontecilla-Camps JC, Field MJ (1999) A hybrid density functional theory/molecular mechanics study of nickel-iron hydrogenase investigation of the active site redox states J. Am. Chem. Soc. 121 4468—1477... 

Albracht SPJ, van derZwaan JW, Fontijn RD, Slater EC (1986) On the possible redox states of nickel and the iron-sulphur cluster in hydrogenase from Chromatium vinosum. In Xavier AV (ed) Frontiers in bioinorganic Chemistry. VCH, Weinheim, pp 11-19... 

Coremans, J. M. C. C., Van der Zwaan, J. W. and Albracht, S. P. J. (1989) Redox behaviour of nickel in hydrogenase from Methanobacterium thermoautotrophicum (strain Marburg). Correlation between the nickel valence state and enzyme activity. Biochim. Biopbys. Acta, 997, 256-67. 

The three-pulse electron spin-echo envelope modulation (ESEEM) technique is particularly sensitive for detecting hyperfine couplings to nuclei with a weak nuclear moment, such as 14N. It has been used to probe the coordination state of nickel in two hydrogenases from M. tkermoautotrophicum, strain AH (56). One of these enzymes contains FAD and catalyzes the reduction of F420 (7,8-dimethyl-8-hydroxy-5-deazaflavin), while the other contains no FAD and has so far only been shown to reduce artificial redox agents such as methyl viologen. 

Fig. 10. Hypothetical reaction cycle for D. gigas hydrogenase, based on the EPR and redox properties of the nickel (Table II). Only the nickel center and one [4Fe-4S] cluster are shown. Step 1 enzyme, in the activated conformation and Ni(II) oxidation state, causes heterolytic cleavage of H2 to produce a Ni(II) hydride and a proton which might be associated with a ligand to the nickel or another base in the vicinity of the metal site. Step 2 intramolecular electron transfer to the iron-sulfur cluster produces a protonated Ni(I) site (giving the Ni-C signal). An alternative formulation of this species would be Ni(III) - H2. Step 3 reoxidation of the iron-sulfur cluster and release of a proton. Step 4 reoxidation of Ni and release of the other proton.

The binuclear nickel-thiolate macrocyclic complex (101) displays noteworthy redox behavior, in which one-electron oxidation yields a NP F NP product with significant delocalization of the unpaired electron density onto the bridging thiolate ligands and not onto the second nickel ion. The charge delocalization consequently lies between the two redox extremes of nickel (III)-thiolate and nickel (II)-thiyl radical, thus mimicking the Ni-C state in [NiFe]hydrogenase (see Section 8). 

Studies on the role of nickel in biological processes have received much attention because of the involvement of nickel in the active site of many hydrogenases [93]. The most noteworthy characteristics of the nickel sites in the hydrogenases are the existence of stable tervalent nickel and the low Ni(III/II) redox potential ( -150 to -400 mV vs. NHE) [94]. The redox potential of the Ni(III/TI) couple in the enzyme is much lower than the potentials of commonly observed Ni(III/II) couples in synthetic complexes [95]. Different approaches have been made to increase the stability of Ni(III) and thus lower the redox potential of the Ni(IIFII) redox couple in synthetic complexes [96]. Studies on the stabilization of the Ni(III) state in synthetic complexes could afford information regarding the stabilization of Ni(III) in the enzymes. The nickel redox centers of the enzymes are considered to be the binding site for the catalytic cycle in the biological process [93]. 

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