Differential scanning calorimetry (DSC) has been used to investigate the macroscopic structure of photosystem II (PS II). Five endothermic transitions, A1A2, B, C, and D, are observed in the 30–70 °C temperature range and are partially assigned on the basis of heat inactivation experiments, relative peak areas, and the effect of MgCl2on the DSC trace. We suggest that peaks C and D correspond to the denaturation of the light-harvesting chlorophyll alb proteins and peak B to the denaturation of components critical to the electron-transport chain. In a DSC study of thylakoid membranes [Cramer, W. A., Whitmarsh, J., & Low, P. S. (1981) Biochemistry 20, 157–162], the lowest temperature shoulder was assigned to the denaturation of the oxygen-evolving complex (OEC), By correlating the temperature of heat inactivation with the temperatures of the DSC peaks of PS II in a range of detergent concentrations (causing shifts in the peak positions), we assign peak A2 to the functional denaturation of the OEC. We have used peak A2 as a new probe of the OEC and have found this peak to be sensitive to the oxidation state of cytochrome b559. Oxidation of cytochrome b559with 1 mM ferricyanide, which has no effect on oxygen evolution activity, causes peak A2 to disappear, probably by making it too broad to observe. In contrast, inhibitory treatments such as the removal of the 17-, 23-, and 33-kilodalton polypeptides and/or Mn by treatments with 2 M NaCl or 0.8 M tris(hydroxymethyl)aminomethane, all of which also lead to the oxidation of cytochrome b559, cause no further changes in the DSC trace. Oxidation of cytochrome b559apparently decreases the cooperativity of the denaturation of component(s) critical to oxygen evolution activity, which indicates that cytochrome b559plays a significant role in the macroscopic structure of the OEC.
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