Model, enzyme hydrogenase

Nickel is found in thiolate/sulflde environment in the [NiFe]-hydrogenases and in CODH/ACS.33 In addition, either a mononuclear Ni-thiolate site or a dinuclear cysteine-S bridged structure are assumed plausible for the new class of Ni-containing superoxide dismutases, NiSOD (A).34 [NiFe]-hydrogenase catalyzes the two-electron redox chemistry of dihydrogen. Several crystal structures of [NiFe]-hydrogenases have demonstrated that the active site of the enzyme consists of a heterodinuclear Ni—Fe unit bound to thiolate sulfurs of cysteine residues with a Ni—Fe distance below 3 A (4) 35-39 This heterodinuclear active site has been the target of extensive model studies, which are summarized in Section 6.3.4.12.5. 

Thiolate-bridged dinickel complexes and, in particular, heterobimetallic Ni/Fe complexes have attracted much interest as model systems for the hydrogenase enzymes. A review covering this... 

Interpretation of the electron density maps showed that the large subunit could not be modelled beyond His536 (Fig. 6.10), that is fifteen amino acids short of the 551 residues predicted by the nucleotide sequence (Table 6.2). At about the same time, the cleavage of this fifteen-residue stretch, which is performed by a specific protease, was reported to be an obligatory step for the maturation of the enzyme (Menon et al. 1993). It is also of interest to note that in all [NiFe] hydrogenase crystal structures this buried C-terminal histidine is ligated to a metal atom which is either a magnesium or an iron (see above). 

Recently, the structure and spectral properties of a well-known diiron Fe model complex ( ji-SCH2CH2CH2S)Fe2(CO)6 were reinvestigated by Darensbourg and coworkers (Lyon et al. 1999). Also the Fe2(CO)4(CN)2 derivative was prepared and investigated. The v(CO) and v(CN) bands of the latter complex best fitted those found for the reduced D. vulgaris [Fe] hydrogenase and hence the possibility of an Fe -Fe pair in the reduced enzyme was suggested by the authors. 

In conclusion, the presented dinuclear iron structure is the first example of a bio-mimetic iron compound, which can be regarded as a first generation model for the class of [Fe]-only hydrogenases. The complex incorporates both relevant carbon monoxide ligands, as well as three bridging thiolato ligands, which could be possibly present in the active site of these enzymes. 

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