Electron transfer bound copper complex

In this copper-amine complex, electron transfer from oxygen to copper would give a phenoxy radical which, since the principal reaction of free phenoxy radicals appears to be C-C coupling, in this case must remain bound to the catalyst. Coupling of two of these radicals would then give dimer. 

The other way to study the "conductivity of protein molecules towards electron tunneling is to investigate the quenching of luminescence of electron-excited simple molecules by redox sites of proteins [95,96]. Experiments of this sort on reduced blue copper proteins have involved electron-excited Ru(II)(bpy)3, Cr(III)(phen)3, and Co(III)(phen)3 as oxidants. The kinetics of these reactions exhibit saturation at protein concentrations of 10 3 M, suggesting that, at high protein concentrations, the excited reagent is bound to reduced protein in an electron transfer precursor complex. Extensive data have been obtained for the reaction of reduced bean plastocyanin Pl(Cu(I)) with Cr(III)(phen)3. To analyze quenching experimental data, a mechanistic model that includes both 1 1 and 2 1 [Pl(Cu(I))/ Cr(III)(phen)3] complexes was considered [96]... 

With a few exceptions, cupredoxins are freely diffusible proteins. They accept and donate a single electron to their redox partners during which process the protein-bound copper oscillates between Cu(II) and Cu(I). The cupredoxin and its redox partners form a transient complex that will dissociate upon a successful electron transfer act. Therefore, the protein-protein interactions between a diffusible cupredoxin and its redox partner may not be as specific as one might expect. Indeed, the binding... 

Davidson, V. L., Graichen, M. E., and Jones, L. H., 1993, Binding constants for a physiologic electron-transfer protein complex between methylamine dehydrogenase and amicyanin. Effects of ionic strength and bound copper on binding, Biochim. Biophys. Acta 1144 3 9n 45. 

This topic has been reviewed by Ingledew (55). The major components of the respiratory chain for T. ferrooxidans are a cytochrome oxidase of the Ci type, cytochromes c, and the blue copper protein rusticyanin. Initial electron transfer from Fe(II) to a cellular component takes place at the outer surface of the plasma membrane in the periplasmic space. The rate of electron transfer from Fe(II) to rusticyanin is too slow for rusticyanin to serve as the initial electron acceptor. Several proposals have been made for the primary site of iron oxidation. Ingledew (56) has suggested that the Fe(II) is oxidized by Fe(III) boimd to the cell wall the electron then moves rapidly through the polynuclear Fe(III) complex to rusticyanin or an alternative electron acceptor. Other proposals for the initial electron acceptor include a three-iron-sulfur cluster present in a membrane-bound Fe(II) oxidoreductase (39, 88), a 63,000 molecular weight Fe(II)-oxidizing enzyme isolated from T. ferrooxidans (40), and an acid-stable cytochrome c present in crude extracts of T. ferrooxidans (14). 

Fig. 55. Reaction scheme of superoxide dismutation following the bovine enzyme numbering. The first O2 molecule binds to Cu(II) and is stabilized by the H bond to Arg-141. A second superoxide molecule then approaches the active site and, by an outer-sphere electron transfer via the Cu-bound first O2 molecule, reduces the copper to Cud) 44) (step III). Alternatively, O2" directly reduces superoxide to peroxide 336) (step IV), leaving as dioxygen. Note that the Cu(I)-superoxide and Cu(II)-peroxide complexes are resonant forms of the same molecular arrangement. The newly formed peroxide is protonated by Arg-141 and leaves as HO2. Arg-141 receives a proton from the solvent, restoring the active enz5Tne (I). These reaction proposals do not require the breaking and reforming of the Cu-His-61 bridge.

The enhanced fluorescence of the ethidium-DNA complex can be quenched partially by the addition of a second ligand. Two mechanisms have been proposed to account for quenching, namely displacement of ethidium bromide by melting of the helix or electron transfer from external quencher molecules. The most efficient quenching occurs of course when both the quencher and ethidium bromide molecules are bound to DNA. This nondisplacement quenching is correlated with DNA-enhanced electron transfer, either from excited ethidium to an acceptor (methylviologen, copper (II) counterions) or from an donor to an excited ethidium acceptor. The DNA double helix works as a well-defined matrix for an organized electron transfer. It enhances its yield by a factor K. (DNA) K, (H,0), which is often in the order of 5 x 10. ... 

Some of these activated species like HO Cu -hydroperoxo, or Cu -hydroxo have been also proposed in the case of the oxidations of the DNA nucleobases (55). Various mechanisms like HO addition on a double-bound, hydrogen abstraction on the methyl groups or electron transfer induce nucleobases oxidations and copper complexes are oxidant enough to perform them, but, in the presence of excess of reductants, such as in the conditions often used during DNA oxidation by copper complexes, oxidized nucleobases (base radicals and radical cations) may be reduced back to undamaged species. Thus the ability of copper complexes to oxidize nucleobases could be underestimated. 

Complex IV, or cytochrome c oxidase, was the first of the mitochondrial electron transport complexes to have its molecular stmcture and the internal path of electron transfer revealed by X-ray crystallography. The catalytic core of the complex consists of two subunits. Subunit II contains a binuclear copper center (Cua) that is directly responsible for the oxidation of cytochrome c. From there electrons are passed to haem a and then to the adjacent binuclear center that consists of haem 03 and another copper ion (Cub), which are all held within subunit I (Fig. 13.1.4). Oxygen is bound and reduced between Cub and the iron of haem 03, and access paths for protons from the inside of the membrane and for oxygen from within the membrane have been defined from several crystal stmctures available for bovine and bacterial enzymes. In addition to the protons taken up for the reduction of oxygen, translocation of further protons across the membrane is coupled to electron transfer by a mechanism that is not yet understood (reviewed in Refs. [71, 72]). 

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