The nature of the electron transfer reaction between reduced plastocyanin and P700 oxidized by flash illumination was studied in P700-enriched Triton subchloroplast fraction 1 particles. An addition of monovalent salts to the suspension at neutral pH increased the reaction rate at low concentrations (>20 mM). Salts of divalent cations showed a similar effect at much lower concentrations (>2 mM), This effect was not dependent on the concentration and the valence of anions. The increase of rate at low salt concentrations was observed at pH's above 5, but below pH 5 the rate was decreased by adding salts. At about pH 5, the rate was not affected by salts. Apart from these salt effects, the optimum pH for the reaction rate was observed between 5.5 and 6.5. The reduction rate depended sigmoidally on the added plastocyanin concentration at pH 6.8 and 4. A Michaelis-Menten type relationship was observed at about pH 5. The half-saturation concentration of plastocyanin became lower as the salt concentration increased at pH 6.8, while it became higher by adding salt at pH 4. The effects of salts on the rate of electron donation from other metalloproteins and artificial electron donors to P700 were also studied. It is concluded, from the analysis with the Gouy-Chapman theory, that the net charges on the electron donors and the membrane surfaces mainly determine the response of the P700 reduction rate to salt addition. The salt addition changes mainly the local concentration or accessibility of electron donors to P700.
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Tamura et al. (1981) studied this question.