Reactivation by chloride of hill activity in heat- and tris-treated thylakoid membranes from Beta vulgaris

Chloride binding proteins: mechanistic implications for the oxygen-evolving complex of Photosystem II

Chloride plays a key role in activating the photosynethetic oxygen-evolving complex (OEC) of Photosystem II, but the OEC is only one of many enzymes affected by this anion. Some of the mechanistic features of Cl(-) involvement in water-splitting resemble those of other proteins whose structure and chemistry are known in detail. An overview of the similarities and differences between these Cl(-)-binding systems is presented.

The effect of chloride on the thermal inactivation of oxygen evolution

The effect of Cl depletion on the sensitivity of the oxygen-evolving complex of Photosystem II (PS II) to heat treatment was examined by a parallel study of the Hill activity (H2O→2,6-dichlorophenolindophenol), Cl(-) binding (by (35)Cl-NMR) and Mn release (by EPR). The extent of thermal inactivation in spinach thylakoids was found to depend on the degree of Cl(-) depletion in the sample. In partially Cl(-)-depleted thylakoids, mild heating (38°C, 3 min) was found to eliminate inflections in plots of both Hill activity versus [Cl(-)] (at low light intensity) and excess (35)Cl-NMR linewidth versus [Cl(-)] (in the dark). In PS II membranes, the same treatment reduced the differences between the linewidth maxima and minima, particularly in the region of 0.3 mM and 7.0 mM Cl(-), as compared to unheated membranes. These results indicate that mild heating affects the Cl(-)-binding domains within the oxygen-evolving complex, OEC, EPR measurements of the temperature dependence of Mn release from heated thylakoids show that Mn release begins to correlate with the loss of Hill activity only at higher temperatures, where the OEC is already substantially inactivated. We conclude from these studies that the Cl(-)-binding domains of the OEC constitute a principal site of damage by heat treatment.

Protection of photosynthetic O2 evolution against heat inactivation: the role of chloride, pH and coupling status

Heat inactivation of photosynthetic O2 evolution was studied in isolated thylakoids from spinach (Spinacia oleracea) and mangrove (Avicennia marina) leaves. Different temperatures, salt, pH and uncoupler effects were investigated. From these results and others in the literature it was concluced that chloride loss from the membrane and, more specifically, the oxygen-evolving complex of photosystem II, may be the cause of inhibition of oxygen evolution during heat inactivation.

Reversal of heat-induced alterations in photochemical activities in wheat primary leaves

Chloroplasts isolated from elevated temperature treated 8-day-old continuous-white-light-grown wheat primary leaves lost the ability to photo-oxidize water. Also, the ability of ascorbate to donate electrons to photosystem II declined. However, a significant increase in reduced dichlorophenolindophenol-supported photosystem-I-mediated methylviologen photo-reduction activity was observed. The plants stressed at 45°C and 47°C were subsequently grown at 25°C and the partial photochemical activities were measured in chloroplasts isolated from the plants at 24-h intervals. The post stress alterations observed are (1) a significant restoration of water oxidation capacity in 45°C- and partial restoration in 47°C-treated leaves. Ascorbate-supported photochemical activities recovered more or less in similar fashion; (2) reversal of enhanced photosystem I activity in both 45°C- and 47°C-treated leaves. These results suggest that the restoration in water oxidation capacity is possible in 45°C-treated leaves and is limited by the severity of heat stress in 47°C-treated leaves. Restoration of water oxidation capacity vis-à-vis to the reversal of heat-enhanced photosystem I activity also indicates the existence of possible endogenous control for repair of alterations during the post stress.