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Charge transfer between tryptophan and tyrosine in proteins






Biochimica et Biophysica Acta 705(2): 150-162

Charge transfer between tryptophan and tyrosine in proteins

With numerous proteins, most of which are not involved in oxidation reduction reactions, azide radicals reacted with tryptophan units to give Trp radicals. The Trp. radicals commonly transfer their electron deficiencies to tyrosine with rate constants in the region of 102-104 s-1, producing TyrO. For .beta.-lactoglobulin, which was studied in most detail, the rate of transfer is independent of protein concentration, so the reaction must be intramolecular. Sodium perchlorate and SDS [sodium dodecyl sulfate] affect the rate, probably because of alterations in protein conformation. The activation energy for the process is 45 kJ .cntdot. mol-1, inconsistent with any mechanism involving H-bonds or charge conduction through the chain, but consistent with tunnelling. With other proteins there is evidence of intermolecular transfer as well as intramolecular transfer, and there may be several different intramolecular steps corresponding to different tryptophan-tyrosine pairs. Rates are not always the same as those initiated by photoionization of tryptophan, probably because different tryptophan units are involved. The possibility of transformation means that with enzymes the initial site of attack by free radicals is not necessarily the site responsible for any consequent loss in activity. The oxidation-reduction potentials involved in the transfer are such that the process may be important in the mode of action of enzymes such as high-potential Cu proteins and peroxidases, and perhaps also in the mode of other proteins if the environment is suitable.

Accession: 001053518

DOI: 10.1016/0167-4838(82)90173-x

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