Dinuclear site reactions with metal complexes

There have been a few reports of first generation coordination complex structural models for the phosphatase enzyme active sites (81,82), whereas there are some examples of ester hydrolysis reactions involving dinuclear metal complexes (83-85). Kim and Wycoff (74) as well as Beese and Steitz (80) have both published somewhat detailed discussions of two-metal ion mechanisms, in connection with enzymes involved in phosphate ester hydrolysis. Compared to fairly simple chemical model systems, the protein active site mechanistic situation is rather more complex, because side-chain residues near the active site are undoubtedly involved in the catalysis, i.e, via acid-base or hydrogenbonding interactions that either facilitate substrate binding, hydroxide nucleophilic attack, or stabilization of transition state(s). Nevertheless, a simple and very likely role of the Lewis-acidic metal ion center is to... 

The Tpx ligands can mimic the coordination environment created by three imidazolyl groups from histidine residues, which is frequently found in the active sites of metalloenzymes. Higher valent bis(ix-oxo) species, [(Tpx)M( i-0)2M(Tpx)] via 0-0 cleavage of [(Tpx)M( x-r 2 r 2-02)M(Tpx)] intermediates, but also peroxo, hydroperoxo, and alkylperoxo species, active species undergoing oxidative C-C cleavage reaction, stable hydrocarbyl complexes, and dinuclear xenophilic complexes, [(Tpx)M-M L71], are all relevant to chemical and biological processes, most of which are associated with transition metal catalytic species. 

The nature of the Ngose reaction is described with respect to electron donation, energy requirement, and reduction characteristics, with particular analysis of the seven classes of substrates reducible by N20se, a complex of a Mo-Fe and Fe protein. Chemical and physical characteristics of Fe protein and crystalline Mo-Fe protein are summarized. The two-site mechanism of electron activation and substrate complexation is further developed. Reduction may occur at a biological dinuclear site of Mo and Fe in which N2 is reduced to NH3 via enzyme-bound diimide and hydrazine. Unsolved problems of electron donors, ATP function, H2 evolution and electron donation, substrate reduction, N20se characteristics and mechanism, and metal roles are tabulated, Potential utilities of N2 fixation research include in-creased protein production and new chemistry of nitrogen. 

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