title: Differential scanning calorimetry of chloroplast membranes: identification of an endothermic transition associated with the water-splitting complex of photosystem II.
abstract: The structure of spinach thylakoid membranes has been investigated by sensitive differential scanning calorimetry. Six endotherms are observed between 20 and 85 degrees C, corresponding to order--disorder transitions of different structural domains within the thylakoid membrane. In a medium of relatively high ionic strength, endothermic transitions occur at 42, 54, 65, 72, 79, and 84 degrees C, with the 65 degrees C transition being particularly prominent. At a lower ionic strength, transitions are centered at 44, 61, 66, 70, 78, and 83 degrees C. The 42--44 degrees C endothermic transition (the A transition) can be correlated with the modification of three electron-transport components or properties associated with photosystem II: (i) release of manganese from the membrane, (ii) the loss of O2 evolution with water as a donor, and (iii) a decrease in the redox potential of the hydroquinone-reducible cytochrome b-559. Both the A transition and the ability to evolve O2 are irreversibly lost after heating to 49 degrees C and also after exposure to trypsin, suggesting the involvement of protein in this transition. The interpretation of these observations is that one effect of the A transition involves the thermal disruption of a protein component on the donor side of photosystem II.
mesh_terms: D002152:Calorimetry, Differential Scanning; D002736:Chloroplasts / Q000187:drug effects / Q000648:ultrastructure*; D007425:Intracellular Membranes / Q000187:drug effects / Q000648:ultrastructure*; D010788:Photosynthesis; D010944:Plants / Q000648:ultrastructure; D013696:Temperature; D014357:Trypsin / Q000494:pharmacology
chemical_list: D014357:Trypsin