Ni-Fe hydrogenase

Spectroscopic studies have been instrumental in elucidating the catalytic mechanism of Ni-Fe hydrogenases. A great deal of controversy concerning this mechanism arises from the fact that, as the as the X-ray crystallographic analysis has shown, there are at least three potential redox-active species at the enzyme s active site the thiolate ligands (75) and the Fe (65) and Ni (9) ions. 

A somewhat more complete version of it is presented in Fig. 8. The model is attractive because it is consistent with many of the observations concerning the active site of Ni-Fe hydrogenases ... 

Fig. 6. Representative EPR spectra displayed by trinuclear and tetranucleEir iron-sulfur centers, (a) and (b) [3Fe-4S] + center in the NarH subunit of Escherichia coli nitrate reductase and the Ni-Fe hydrogenase fromD. gigas, respectively, (c) [4Fe-4S] + center in D. desulfuricans Norway ferredoxin I. (d) [4Fe-4S] center in Thiobacillus ferrooxidans ferredoxin. Experimental conditions temperature, 15 K microwave frequency, 9.330 GHz microwave power, (a) 100 mW, (b) 0.04 mW, (c) smd (d) 0.5 mW modulation amplitude (a), (c), (d) 0.5 mT, (b) 0.1 mT.

Biomimetic chemistry of nickel was extensively reviewed.1847,1848 Elaborate complexes have been developed in order to model structural and spectroscopic properties as well as the catalytic function of the biological sites. Biomimetic systems for urease are described in Section 6.3.4.12.7, and model systems for [Ni,Fe]-hydrogenases are collected in Section 6.3.4.12.5. 

The interest in low-valent Ni complexes in S-rich environments has been stimulated by the presence of Ni in [Ni,Fe] hydrogenase and CODH. While thiolate ligation usually favors higher oxidation states, thioethers stabilize Ni1 and Ni°. In most cases, however, Ni1 ions of an NiS4 chromophore are unstable with respect to disproportionation. The cyclic voltam-mogram of square planar (983) with homoleptic thioether coordination exhibits a quasi-reversible wave at —0.42V (vs. NHE), which on the basis of the rhombic EPR spectrum (gi 2.27, g2 2.11, and g3 2.03) of the chemically reduced species (Na/Hg) is assigned to metal-centered reduction. 8... 

Note that some Ni/Fe hydrogenases have both CO and CN- as metal ligands. (Methane which may well have been present is very difficult to activate.)... 

There are several types of hydrogenases, all of which have a metal-containing active site. In fact, they are classified as Fe-only hydrogenases, Ni-Fe hydrogenases and Ni-Fe-Se hydrogenases. 

Figure 51 The chain of metal fragments forming the active site of the Ni-Fe hydrogenase o/Desulfovibrio gigas. (a) Overall view (b) structural details of the NiS2Fe centre...

De Gioia, L., Fantucci, P., Guigliarelli, B. and Bertrand, P. (1999a) Ni-Fe hydrogenases A density functional theory study of active site models. Inorg. Chem., 38, 2658-62. 

Fontecilla-Camps, J. C. (1996) The active site of Ni-Fe hydrogenases Model chemistry and crystallographic results. J. Biol. Inorg. Chem., 1, 91-8. 

Kruger, H.-J. and Holm, R. H. (1990) Stabilization of trivalent nickel in tetragonal NiS4N2 and NiNg environments Synhesis, structures, redox potentials and observations related to [Ni-Fe]-Hydrogenases./. Am. Chem. Soc., 112, 2955-63. 

Montet Y, Amara P, Volbeda A, et al. 1997. Gas access to the active site of Ni-Fe hydrogenases probed by X-ray crystallography and molecular dynamics. Nat Struct Biol 4 523-6. 

Enzymological and redox potentiometric studies by EPR have indicated that the catalytic sites of the oxygen-stable [Ni-Fe] hydrogenases exhibit at least six enzy-mologically distinct states [75-78], These are schematically represented in Figure 3, which depicts a speculative model for the hydrogenase redox cycle, as discussed later. 

Fan H-J, Hall MB (2001) Recent theoretical predictions of the active site for the observed forms in the catalytic cycle of Ni-Fe hydrogenase. J. Biol. Inorg. Chem. 6 467 173... 

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