Redox dependence for photoligation of manganese to the apo-water-oxidizing complex in chloroplasts and photosystem II membranes.

Abstract

Effects of reducing reagents and redox potentials on photoactivation were studied in Mn-depleted chloroplasts and PSII membranes. Exogenous reducing reagents abolished photoactivation in PSII membranes, while they stimulated photoactivation in chloroplasts. To determine how reducing reagents can have such opposing effects in these preparations, we studied how redox potentials affect photoactivation in the range from 0 mV to +500 mV. In chloroplasts, a modest yield of photoactivation was obtained in the redox potential range of +100 and +330 mV at pH 7.5. The yield of photoactivation decreased at redox potentials above +330 mV, and drastically increased below potentials of +100 mV. Nernst plots of the data show that an n = 1 redox component with an Em7.5 of +374 mV, as well as an n = 2 redox component with an Em7.5 of +61 mV, is involved in photoactivation of chloroplasts isolated from dark-grown spruce seedlings. In the case of PSII membranes, photoactivation decreased sharply on either side of +335 mV at pH 5.5. The n = 1 redox components with Em5.5 of +375 and +319 mV may be involved, both of which showed pH dependences of -60 mV/pH unit. DCMU abolished photoactivation in chloroplasts, but did not affect the dependence of photoactivation on oxidation-reduction potentials in PSII membranes. The component with an Em5.5 of +319 mV involved in photoactivation of PSII membranes was also observed in the dependence of Mn solubilization on oxidation-reduction potentials with PSII membranes lacking extrinsic proteins, suggesting that the reduction of Mn with higher valences to Mn(II) by exogenous reducing reagents reversibly occurs in the intermediates or an active center during photoactivation in PSII membranes. Involvement of such redox components in photoactivation in chloroplasts and PSII membranes is discussed.