ELECTRON-TRANSFER REACTIONS OF CYTOCHROME-F WITH FLAVIN SEMIQUINONES AND WITH PLASTOCYANIN - IMPORTANCE OF PROTEIN PROTEIN ELECTROSTATIC INTERACTIONS AND OF DONOR-ACCEPTOR COUPLING

Reduction of turnip ferricytochrome f by flavin semiquinones and oxidation of this ferrocytochrome f by French bean cupriplastocyanin are studied by laser flash photolysis over a wide range of ionic strengths. Second-order rate constants (+/- 15%) at extreme values of ionic strength, all at pH 7.0 and 22-degrees-C, are as follows: with FMN semiquinone at 1.00 and 0.0040 M, 5.0 x 10(8) and 3.9 X 10(8) M-1 s-1; with riboflavin semiquinone at 1.00 and 0.0040 M, 1.7 X 10(8) and 1.9 x 10(8) M-1 s-1; with lumiflavin semiquinone at 1.00 and 0.0045 M, 1.8 X 10(8) and 4.5 x 10(8) M-1 s-1; with cupriplastocyanin at 1.00 and 0.100 M, 1.4 x 10(6) and 2.0 X 10(8) M-1 s-1. These reactions of cytochrome f are governed by the local positive charge of the interaction domain (the exposed heme edge), not by the overall negative charge of the protein. Lumiflavin semiquinone behaves as if it carried a small negative charge, probably because partial localization of the odd electron gives this electroneutral molecule some polarity; local charge seems to be more important than overall charge even for relatively small redox agents. The dependence of the rate constants on ionic strength was fitted to the equation of Watkins; this model recognizes the importance of local charges of the domains through which redox partners interact. There is kinetic evidence that a noncovalent complex between cytochrome f and plastocyanin exists at low ionic strength. The driving force for the intracomplex electron-transfer reaction in either direction is virtually nil, and the rate constant in either direction is 2800 +/- 300 s-1. For at least two reasons, cytochrome f reacts faster than cytochrome c with all four redox partners examined. First, the heme edge is exposed more in cytochrome f than in cytochrome c. Second, cytochrome f seems to attract the partners by a stronger electrostatic force than does cytochrome c. These differences should be taken into account when the latter protein is used as a model for the former. They are especially important in the reaction with plastocyanin, a physiological partner of cytochrome f.