A thermodynamic study by laser-flash photolysis of plastocyanin and cytochrome c6 oxidation by photosystem I from the green alga Monoraphidium braunii.

Plastocyanin and cytochrome c6 from the green alga Monoraphidium braunii reduce the photo-oxidized algal photosystem I (PSI) reaction center chlorophyll (P700) with similar kinetics, as expected from their functional equivalence. The observed P700+ reduction rate constants show a non-linear dependence on metalloprotein concentration, which indicates a (minimal) two-step kinetic mechanism involving complex formation prior to electron transfer. The dependence of the observed rate constants on NaCl concentration suggests that the electrostatic interaction forces between the negatively charged donor proteins and PSI are repulsive at neutral pH and relatively low ionic strength (I), although attractive dipole-dipole interactions may play a role at higher ionic strengths. Activation parameters for P700+ reduction by cytochrome c6 and plastocyanin have been determined by studying the temperature dependence of the respective rate constants at varying ionic strength and pH. Changes in NaCl concentration and pH induce significant changes in the activation free energy of the overall reaction, even though the corresponding values for activation enthalpy and entropy undergo changes in opposite directions. Such a compensation effect between enthalpy and entropy is observed with both cytochrome c6 and plastocyanin. Protein concentration dependencies of the observed rate constants at different temperatures has allowed an estimate of the free energy change during complex association, as well as the activation parameters for electron transfer, according to a two-step kinetic model.