Electron transfer between protein-bound groups

Figure 6.25 Sequential electron transfer processes between protein-bound groups...

For example, in the electron-transfer system shown in Figure 6.25 the protein-bound groups between which electron transfer occurs are designated A, B and C and the direction of electron transfer is shown as arrows. 

Electron transfer between the iron centre in cytochrome c and a transition metal bound at the periphery of the protein has been studied in cases where Cu" 677 and the Rum(NH3)5 group678 are bound to His-33. In the latter case, the Ruu-cytochrome c(III) form can be generated by pulse radiolysis. The intramolecular transfer of an electron between Ru" and Fe111 can be measured. This presents a useful model for electron transfer from the iron via the heme group and the protein. 

An exciting method for accelerating the electron transfer between redox proteins and amperometric electrodes has been described by Heller and Degani (1987), who modified oxidoreductases with electrontunneling relays. The same mediators as are used in chemically modified electrodes are directly bound to groups of the protein molecule. The... 

In 1988, Gray and co workers reported electron transferrates between redox centers bound in large macromolecules as a function of the distance between the redox centers [8,35]. Electron transfer between ruthenium complexes [Ru(NH3)5Hist], for example, where Hist is a histidine group in a protein, was found to increase by a factor of 10 when the complexes were 2.1 A closer. This corresponds to a = 0.91 in Equation 9.9. 

Mitochondrial cytochrome c is perhaps the most widely studied of all metalloproteins with respect to its electrochemical properties. It is located in the inner-membrane space of mitochondria and transfers electrons between membrane-bound complex III and complex IV. The active site is an iron porphyrin with a redox potential (7) of -1-260 mV vs. NHE. The crystal structures of cytochrome c from tuna have been determined (8, 9) in both oxidation states at atomic resolution. It is found that the heme group is covalently linked to the protein via two thioether bridges, and part of its edge is exposed at the protein surface. Cytochrome c is a very basic protein, with an overall charge of -1-7/-l-8 at neutral pH. Furthermore, many of the excess basic lysine residues are clustered around the mouth of the heme crevice, giving rise to a pronounced charge asymmetry. 

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