Spectroscopic characterization of intermediate steps involved in donor-side-induced photoinhibition of photosystem II.

The reaction center protein D1 in photosystem II shows a high turnover during illumination. The degradation of the D1 protein is preceded by photoinhibition of the electron transport in photosystem II. There are two distinct mechanisms for this: acceptor-side- and donor-side-induced photoinhibition. Here, donor-side-induced photoinhibition was studied in photosystem II membranes after Cl- depletion or washing with tris(hydroxymethyl)aminomethane (Tris) which destroys water oxidation, reversibly or irreversibly, respectively. Photoinhibition after these treatments leads to fast degradation of the D1 protein, and the mechanism behind this was investigated. Illumination of Cl- depleted photosystem II membranes resulted in a rapid and simultaneous inhibition of Cl(-)-reconstitutable oxygen evolution, loss of 2 Mn ions per photosystem II center, increase in the electron transfer between the electron donor diphenylcarbazide and electron acceptor 2,6-dichlorophenolindophenol, and an increase in the EPR signal IIfast from tyrosine-Zox. The destruction of the Mn cluster leads to the loss of oxygen evolution and to an increased accessibility for diphenylcarbazide to donate electrons to Tyr-Zox. The increase in the EPR signal from Tyr-Zox can be explained by slower reduction kinetics of Tyr-Zox due to the Mn release. On a longer photoinhibition time scale, a decrease in the amplitude of Tyr-Zox and inhibition of the electron transport from diphenylcarbazide to 2,6-dichlorophenolindophenol occurred simultaneously in both Cl(-)-depleted and Tris-washed photosystem II membranes. These slower photoinhibition reactions were then studied in detail in Tris-washed photosystem II membranes. Compared to photoinhibition of Tyr-Zox, the EPR signal from tyrosine-Dox decreased much slower. Tyr-Dox was photoinhibited in parallel with the EPRsignals from reduced QA, reduced pheophytin, and an oxidized chlorophyll radical (chlorophyllz). This shows that the acceptor side components and the primary charge separation reaction (P680+ pheophytin-) were operational although Tyr-Z was inactivated. The amount of the D1 protein also declined in parallel with Tyr-Dox, which shows that the D1 protein is not damaged until long after the Mn complex and Tyr-Z have become inactivated. Instead, it is likely that the strongly oxidizing P680+ is responsible for the damage to the D1 protein.