Redox potentials, cupredoxins

Stellacyanins are different from other phytocyanins and cupredoxins by the nature of the axial ligand coordinated to the copper the axial ligand is a conserved glutamine instead of methionine. They also exhibit lower reduction potentials (180-280 mV vs. NHE) than other cupredoxins, and undergo redox reactions with small inorganic complexes or at electrodes at unusually fast rates.The stellacyanin subfamily includes stellacyanins from lacquer tree Rhus Vernicifera and cucumber Cucumis sativus, mavicyanin from green zucchini, and umecyanin from horseradish root. 

The reduction potentials of Cua centers are influenced by the same factors that contribute to those of mononuclear cupredoxins discussed above, including hydrophobic encapsulation and axial ligand interactions from a weak methionine and a backbone carbonyl oxygen. However, the presence of two covalent Scys ligands would significantly stabilize the oxidized center and further lower the reduction potentials. This effect is believed to be countered by the presence of a Cu—Cu bond in the oxidized Cua the presence of a second copper ion in close proximity would tend to increase the effective nuclear charge felt by the redox active electron and thus would increase the reduction potentials of the Cua centers. ... 

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