Reduction potentials copper enzymes

Type II copper enzymes generally have more positive reduction potentials, weaker electronic absorption signals, and larger EPR hyperfine coupling constants. They adopt trigonal, square-planar, five-coordinate, or tetragonally distorted octahedral geometries. Usually, type II copper enzymes are involved in catalytic oxidations of substrate molecules and may be found in combination with both Type I and Type III copper centers. Laccase and ascorbate oxidase are typical examples. Information on these enzymes is found in Tables 5.1, 5.2, and 5.3. Superoxide dismutase, discussed in more detail below, contains a lone Type II copper center in each of two subunits of its quaternary structure. 

The NO/NO+ and NO/NO- self-exchange rates are quite slow (42). Therefore, the kinetics of nitric oxide electron transfer reactions are strongly affected by transition metal complexes, particularly by those that are labile and redox active which can serve to promote these reactions. Although iron is the most important metal target for nitric oxide in mammalian biology, other metal centers might also react with NO. For example, both cobalt (in the form of cobalamin) (43,44) and copper (in the form of different types of copper proteins) (45) have been identified as potential NO targets. In addition, a substantial fraction of the bacterial nitrite reductases (which catalyze reduction of NO2 to NO) are copper enzymes (46). The interactions of NO with such metal centers continue to be rich for further exploration. 

Cu(II) is intramolecular. The effect of fluoride on the reduction rate is consistent with both a direct involvement of type 2 Cu(II) in the reduction or an indirect effect mediated via a change in conformation or in redox potential of the type 1 Cu(II). The type 2 copper ion could be the primary electron-accepting site of the laccase molecule, as has been proposed for the reduction of the enzyme by hydroquinone (36), the first-order process observed being therefore the electron transfer from type 2 Cu to type 1 Cu(II). The particaption of type 3 Cu(II) instead of type 2 Cu(II) is not excluded, but no associated change of its absorption band at 330 nm could be observed during the redox cycle described for the 614-nm band. 

Table III). Atomic absorption analysis shows that the copper is still present in the enzyme. These two facts coupled with the 600-nm region intensity increase might be interpreted to suggest that a stereochemical distortion accompanies tryptophan oxidation in galactose oxidase. For example, a saddling of the planar environment would be expected to lower the reduction potential of the Cu(II)-Cu(I) couple. 

C (66). If electron transfer from type 1 to type 3 copper couples the two halves of the enzyme cycle, as proposed for laccase, then this intramolecular redox reaction must be extremely rapid to account for the effects of trace dioxygen on the reduction of the type 1 copper. Consequently, despite the fact that an ambiguous assignment of a type 1 to type 3 transfer is not possible in this example, facile intramolecular electron transfer processes probably ensure a rapid distribution of electrons among the type 1 and type 3 copper centers, at least in some of the enzyme molecules. The equilibrium distribution, and quite conceivably the relative rates of approach to this state, should be influenced by the oxidation-reduction potentials, which, as described earlier in this chapter (Figure 5), favor electron occupancy of the type 3 copper pairs at 10.0°C. 

The His-63Cys mutant seems to have the Cys bound to zinc, thus leaving copper(II) coordinated to three histidines and to solvent. Two water molecules have been proposed to interact with copper. Now water is regularly coordinated. The enzyme has no activity (Table III), although the reduction potential is still in the correct range to function. It has been proposed that a water bound to copper causes a decrease in the activity because the electron transfer has to occur through a water molecule rather than directly 179). 

Electrochemical studies have shown a linear dependence of Cu2Zn2SOD reduction potential on pH between pH 5.0 and about 8.5. The slope of 0.059 V/pH indicates the uptake of one proton by the enzyme on copper reduction (223, 227) (see Section VII). 

Like other chemical fuel cells, EFCs have cathode-receiving oxidant and anodereceiving reductant or fuel. For most EFCs, O2 is the oxidant of choice because it is freely available and has a high reduction potential, thus maximising the voltage output of the cell. The enzymes commonly used for O2 reduction at cathode are blue copper oxidases such as laccase or bilirubin oxidase. Peroxidases containing iron... 

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