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 ... 

The hydrogenases isolated can be broadly classified into two groups [Fe-S]-only, and nickel-[Fe-S]-containing hydrogenases. The first group, the [Fe-S]-only or [Fe] hydrogenases, is characterized by the absence of nickel on the active site, and are rare in sulfate reducing bacteria. They contain [4Fe-4S] clusters and, in addition, a catalytic... 

The Mosshauer data performed on the as-isolated D. vulgaris [Fe] hydrogenase are in agreement with the EPR data, and further support the presence of two ferredoxin-type [4Fe-4S] clusters and that the H-cluster is in the diamagnetic state in the as-purified enzyme (169). 

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.

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

Scheme 65 The calculated mechanism of H-H bond cleavage reaction of the model complex for [Fe] -hydrogenases...

Among several applications, Fe-based hydrogenases play a central role in the stepwise reduction of CO2 to methane. This process is accomplished through various types of Fe-hydrogenases however, in most of these enzymes, the active center is either a binuclear Fe-Fe- or an Ni-Fe-complex. Although the exact... 

Fig. 2 Active center of [FeFe]-hydrogenase 4 (left) and [Fe]-hydrogenase 56 (right), Unk unknown ligand [22]...

A different mechanism for reduction processes by [Fe]-hydrogenase 56 is assumed. The hydride generated by splitting dihydrogen is directly transferred to an electrophilic organic center in methenyltetrahydrocyanopterin. As no electrons need to be transferred this reaction requires only one metal center. Due to its structure the center of [Fe]-hydrogenase 56 does not count to the class of ferrates. 

Brazzolotto X, JK Rubach, J Gaillard, S Gambrelli, M Atta, M Fontecave (2006) The [Fe-Fe]-hydrogenase maturation protein HydF from Thermotoga maritima is a GTPase with an iron-sulfur cluster. J Biol Chem 279, 281 769-774. 

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... 

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