BIOLOGICAL ELECTRON-TRANSFER

Many oxidoreductases are constructed from (a) local sites of strongly coupled substrate-redox cofactor partners participating in exchange of electron pairs, (b) electron pair/single electron transducing redox c enters, and (c) nonadiabatic, long-distance, single-electron tunneling between weakly coupled redox centers. The latter is the subject of an expanding experimental program that seeks to manipulate, test, and ap ply the parameters of theory. New results from the photosynthetic reac tion center protein confirm that the electronic-tunneling medium appea rs relatively homogeneous, with any variances evident having no impact on function, and that control of intraprotein rates and directional s pecificity rests on a combination of distance, free energy, and reorga nization energy. Interprotein electron transfer between cytochrome c a nd the reaction center and in lactate dehydrogenase, a typical oxidore ductase from yeast, are examined. Rates of interprotein electron trans fer appear to follow intraprotein guidelines with the added essential provision of binding forces to bring the cofactors of the reacting pro teins into proximity.