Oxidoreduction reactions involving the electrostatic and the covalent complex of cytochrome c and plastocyanin: importance of the protein rearrangement for the intracomplex electron-transfer reaction

Abstract

Horse heart cytochrome c and French bean plastocyanin are cross-linked one-to-one by a carbodiimide [Geren, L. M., Stonehuerner, J., Davis, D. J., & Millett, F. (1983) Biochim. Biophys. Acta 724, 62] in the same general orientation in which they associate electrostatically [King, G. C., Binstead, R. A., & Wright, P. E. (1985) Biochim. Biophys. Acta 806, 262]. The reduction potentials of the Fe and Cu atoms in the covalent diprotein complex are respectively 245 and 385 mV vs NHE; the EPR spectra of the two metals are not perturbed by cross-linking. Four isomers of the covalent diprotein complex, which probably differ slightly from one another in the manner of cross-linking, are separated efficiently by cation-exchange chromatography. Stopped-flow spectrophotometric experiments with the covalent diprotein complex show that the presence of plastocyanin somewhat inhibits oxidation of ferrocytochrome c by [Fe(CN)6]3- and somewhat promotes oxidation of this protein by [Fe(C5H5)2]+. These changes in reactivity are explained in terms of electrostatic and steric effects. Pulse-radiolysis experiments with the electrostatic diprotein complex yield association constants of greater than or equal to 5 X 10(6) and 1 X 10(5) M-1 at ionic strengths of 1 and 40 mM, respectively, and the rate constant of 1.05 X 10(3) s-1, regardless of the ionic strength, for the intracomplex electron-transfer reaction. Analogous pulse-radiolysis experiments with each of the four isomers of the covalent diprotein complex, at ionic strengths of both 2 and 200 mM, show an absence of the intracomplex electron-transfer reaction. A rearrangement of the proteins for this reaction seems to be possible (or unnecessary) in the electrostatic complex but impossible in the covalent complex.

Proton NMR comparison of noncovalent and covalently cross-linked complexes of cytochrome c peroxidase with horse, tuna, and yeast ferricytochromes c

“…In order to directly compare cross-linking results for physiological and nonphysiological CcP complexes, we have carried out identical cross-linking procedures in tandem on the CcP/yeast iso-1 ferricytochrome c pair and the CcP/horse ferricytochrome c pair. In view of other protein cross-linking work using EDC (Mauk & Mauk, 1989;Peery & Kostic, 1989;Zhou & Kostic, 1991; Matthews & Brittain, 1986), we have been careful to use appropriate controls so that valid comparisons can be made for cross-linking results with CcP and physiological and nonphysiological partners. The specific controls are described in subsequent sections.…”

The complex formed between plastocyanin and cytochrome c

“…A model of this ternary complex involving relative motion of Cr(NH&+, cytochrome c and plastocyanin would then explain why, on the NMR time scale, no plastocyanin residues are masked from the inorganic probe on complexation with cytochrome c. The proposal that plastocyanin and cytochrome c move relative to one another within the encounter complex, perhaps by making and breaking ion-pair bonds [41], is consistent with recent experimental and theoretical studies of protein-protein complexes which emphasise the dynamic nature of the interactions involved. Examples include electrostatic analysis of the cytochrome c/cytochrome b5 interaction [41], calculations of Brownian dynamics [42], and comparison of the covalent and electrostatic complexes formed by plastocyanin and cytochrome c [43]. In the last of these studies, it was found that, when cross-linked, the proteins are 'locked' in unproductive orientations (i.e.…”

Electrostatic effects on electron-transfer kinetics in the cytochrome f-plastocyanin complex

“…Studies of the cytf-PC reaction in vitro have suggested a substantial role for electrostatic interactions between the reactants (Morand et al, 1989;He et al, 1991;Modi et al, 1992a,b;Kostic, 1992, 1993;Meyer et al, 1993;Gross, 1993;Wagner et al, 1996;Kannt et al, 1996). Electrostatic effects were also observed in reactions of PC with different cytochromes (e.g., Peery and Kostic, 1989;Bagby et al, 1990b;Nordling et al, 1990;Roberts et al, 1991; Peery et al, 1991;Meyer et al, 1993;Qin and Kostic', 1996) and with photosystem I (Haehnel et al, 1994;Farah et al, 1995;Sigfridsson et al, 1996;Drepper et al, 1996). The structural data show the existence of two patches of negatively charged residues on the PC surface, viz., Asp-42 and -44, and, for the second patch, Glu-59 and -60 and Asp-61 (Guss and Freeman, 1983; domains.…”