Effects of pH on kinetics of the structural rearrangement that gates the electron-transfer reaction between zinc cytochrome c and plastocyanin. Analysis of protonation states in a diprotein complex

Electron transfer from zinc cytochrome c to copper(II)plastocyanin in the electrostatically-stabilized complex [Crnogorac MM, Shen C, Young S, Hansson O, Kosti} NM (1996) Biochemistry 35, 16465 74]. We study this rearrangement in four complexes Zncyt/pc(II), which zinc cytochrome c makes with the wild-type form and the single mutants Asp42Asn, Glu59Gln, and Glu60Gln of plastocyanin. The rate constant for the rearrangement, kF, differs for the four forms of plastocyanin but is independent of pH from 5.4 to 9.0 in all four cases. That kF is affected by the single mutations but not by pH changes suggests that the residues Asp 42, Glu59, and Glu60 in the wild-type plastocyanin remain deprotonated (i.e., as anions) within the Zncyt/pc(II) complex throughout the pH range examined. This fact agrees with the notion that loss of salt bridges in the initial (redox-inactive) configuration of the complex is compensated by formation of new salt bridges in the rearranged (redox-active) configuration.