KINETICS OF STATIC AND DIFFUSIVE ELECTRON-TRANSFER BETWEEN ZINC-SUBSTITUTED CYTOCHROME-C AND PLASTOCYANIN - INDICATIONS OF NONELECTROSTATIC INTERACTIONS BETWEEN HIGHLY CHARGED METALLOPROTEINS

Cupriplastocyanin, pc(II), quenches the triplet state of zinc cytochrome c, 3Zn(cyt), by electron transfer as shown in Scheme I. All the experiments are done at pH 7.0. Nonredox modes of quenching are ruled out by detection of the cation radical Zn(cyt)+ and by experiments in which redox quenching is precluded. At the ionic strengths of 0.100 M and higher, the electron-transfer reaction occurs bimolecularly, via the encounter complex 3Zn(cyt)/pc(II); k(f) = (2.8 +/- 0.6) x 10(5) s-1, and the equilibrium constant K(a) depends on ionic strength. At the ionic strengths of 10 mM and lower, the reaction can be made to occur predominantly unimolecularly, within the preformed complex 3Zn(cyt)/pc(II); k(F) = (2.5 +/- 0.4) x 10(5) s-1 within 3Zn(cyt)/pc(II), and k(B) = (1.1 +/- 0.5) x 10(6) s-1 within Zn(cyt)+/pc(I). The rate constant k(f) is independent of ionic strength (in the range from 10 to 100 mM) and so is the rate constant k(F) (below 20 mM). The equality of k(f) and k(F) shows either that the encounter complex and the preformed complex have structures with equal electronic couplings and activation energies for electron transfer or that both complexes can reach such structures by fast rearrangement before the electron-transfer step. The estimated association constant K(a) for zinc cytochrome c and cupriplastocyanin at zero ionic strength is (2 +/- 1) x 10(7) M-1. Cytochrome c interacts similarly with various anionic metalloproteins, and replacement of iron with zinc does not noticeably alter these docking interactions. As the ionic strength increases, the efficiency of charge separation in the bimolecular reaction first increases and then decreases. At high ionic strength, even these charged proteins perhaps attract each other by hydrophobic or other nonelectrostatic forces.