Larry B. Anderson

The effect of ethylenediamine chemical modification of plastocyanin on the rate of cytochrome f oxidation and P-700+ reduction

Chemical modification of plastocyanin was carried out using ethylenediamine plus a water-soluble carbodiimide, which has the effect of replacing a negatively charged carboxylate group with a positively charged amino group at pH 6-8. The conditions were adjusted to produce a series of singly and doubly modified forms of plastocyanin. Differences in charge configuration allowed separation of these forms on a Pharmacia fast protein liquid chromatograph using a Mono Q anion exchange column. These forms were used to study the interaction of plastocyanin with its reaction partner cytochrome f. The rate of cytochrome f oxidation was progressively inhibited upon incorporation of increasing numbers of ethylenediamine moieties indicating a positively charged binding site on cytochrome f. However, differential inhibition was obtained for the various singly modified forms allowing mapping of the binding site on plastocyanin. The greatest inhibition was found for forms modified at negatively charged residues Nos. 42-45 and Nos. 59-61 which comprise a negative patch surrounding Tyr-83. In contrast, the form modified at residue No. 68, on the opposite side of the globular plastocyanin molecule, showed the least inhibition. It can be concluded that the binding site for cytochrome f is located in the vicinity of residues Nos. 42-45 and Nos. 59-61. Modification of plastocyanin at residues Nos. 42-45 showed no effect on the rate of P-700+ reduction, suggesting that these residues are not involved in the binding of Photosystem I. However, an increase in the rate of P-700+ reduction was observed for plastocyanins modified at residue No. 68 or Nos. 59-61, which is consistent with the idea that the reaction domain of Photosystem I is negatively charged and Photosystem I binds at the top of the molecule and accepts electrons via His-87 in plastocyanin. These results raise the possibility that plastocyanin can bind both cytochrome f and Photosystem I simultaneously. The effect of ethylenediamine modification on the formal potential of plastocyanin was also examined. The formal potential of control plastocyanin was found to be +372 +/- 5 mV vs. normal hydrogen electrode at pH 7. All modified forms showed a positive shift in formal potential. Singly modified forms showed increases in formal potentials between +8 and +18 mV with the largest increases being observed for plastocyanins modified at residues Nos. 42-45 or Nos. 59-61.

Determination of the redox potential and diffusion coefficient of the protein plastocyanin using optically transparent filar electrodes

Direct and mediated electrolysis of the protein plastocyanin at a gold filar electrode is described. The filar electrode used is of a unique design that allows potentiometric measurements, steady-state voltammetry and absorption spectrophotometry to be performed on a few microliters of solution containing 0.1-1.0 mM protein. As a result, we have determined the formal potential and diffusion coefficient of the blue copper protein, plastocyanin, to be 372 +/- 5 mV vs. normal hydrogen electrode and 8.9 X 10(-7) cm2 X s-1, respectively. The same value of the formal potential is obtained from a steady-state current experiment, an equilibrium spectrophotometric experiment, and a twin-electrode steady-state spectrophotometric experiment. The fact that the diffusion coefficient is measured under conditions of steady-state current, results in significant improvement in signal to background over techniques that monitor a transient current, while the potential is changing.

SPECTRAL AND ELECTROCHEMICAL STUDIES OF THE COPPER(II) COMPLEXES OF TWO MACROCYCLIC LIGANDS

Abstract Copper(II) complexes of the types Cu(CR)(ClO4)2-.H2O, [Cu(CR)X](ClO4).nH2O (where X = Cl, Br, I and n = 1; X = NCS, n = 0), [Cu(CRH)](ClO4)2.H2O and [Cu(CRH)X](ClO4) (where X = Cl, Br, I) have been prepared. The ligands CR and CRH are the related four nitrogen-donor macrocycles 2,12-dimethyl-3,7,11,17-tetraazabicyclo [11.3.1]heptadeca-1(17), 2, 11, 13, 15-pentaene and meso-2, 12-dimethyl-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17), 13,15-triene, respectively. All the compounds have the normal magnetic moments expected for Cu(II) complexes with S = ½. Conductance studies in methanol show that the halo-perchlorate complexes are formally five coordinate. The probable structures of the complexes in both the solid and solution are discussed in terms of their visible, infrared and e.s.r. spectra, as well as on the basis of data obtained from conventional polarography and cyclic voltammetry.

Polarographic Reduction of the Heme Iron in Native Horse-Heart Cytochrome c

Electrochemical relaxation techniques have been applied to the reduction of horse-heart cytochrome c in buffered solutions. It has been shown that protein in low concentration catalyzes the electrolytic reduction of oxygen, some characteristics of the protein-iron reduction are also described. The apparently different effects of cytochrome c and hemin on the reduction of oxygen, point to mechanistic differences in the reduction of the ferriporphyrins themselves.