NiSOD dismutase

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. 

Fig. 2. Rate constants for dismutation of superoxide in the absence (closed circles) and presence of superoxide dismutases [NiSOD (open circle), E. coli MnSOD (open diamond), bovine CuZnSOD (open square)] as a function of pH.

To diminish these threats, nature has created a family of metalloenzymes, the SODs. They catalyze the dismutation of superoxide to dioxygen and hydrogen peroxide (Eqs. (1) and (2)). They are differentiated by the redox-active metal copper (Cu/Zn SOD), manganese (MnSOD), iron (FeSOD), or nickel (NiSOD) superoxide dismutases and fall into three evolutionary families (Fig. 2) (10). The iron and manganese SODs are structurally similar and are found in prokaryotes and in the matrix of mitochondria (near the electron transport chain), respectively. Nickel containing SODs are known in some prokaryotes, whereas Cu/Zn are present in the cytosols of virtually all eukaryotic cells and have an independent evolutionary history. 

Nid has square planar coordination involving two bridging cysteine thiolates and two main chain N atoms (Fig. 4A). A similar structure has been described recently for the active site of nickel superoxide dismutase (NiSOD) [117,118] (Fig. 4B). In NiSOD, Ni(II) reacts with superoxide and is oxidized to Ni(III)-peroxide. On the other hand, ACS functions in reducing environments and Nij is thought to remain as Ni(II) throughout catalysis. Model chemistry supports this proposal indicating that Nip is much more... 

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