STRONG LIGHT PHOTOINHIBITION OF ELECTRON-TRANSPORT IN PHOTOSYSTEM-II - IMPAIRMENT OF THE FUNCTION OF THE 1ST QUINONE ACCEPTOR, QA

Electron paramagnetic resonance (EPR) spectroscopy has been applied in an investigation on the mechanism for photoinhibition of the electron transport in Photosystem II. The experiments were performed in vitro in thylakoid membranes and preparations of Photosystem-II-enriched membranes. Photoinhibition resulted in inhibition of the oxygen evolution and EPR measurements of the S2 state multiline EPR signal show that its induction by illumination at 198 K was decreased with the same kinetics as the oxygen evolution. Further EPR measurements show that the reduction of QA was inhibited with the same kinetics as the oxygen evolution. The amount of photoreducible pheophytin was estimated from photoaccumulation experiments under reducing conditions and the results show that the primary charge separation reaction was inhibited much slower than the oxygen evolution or the reduction of QA. These results indicate that photoinhibition inhibits the electron transfer between pheophytin and QA probably by impairment of the function of QA. In the inhibited centers the primary charge separation reaction is still operational. It is suggested that the event leading to photoinhibition of the electron transport is the double reduction of QA which then leaves its site. Photoinhibition also results in rapid oxidation of cytochrome b-559 and a change of cytochrome b-559 from its high potential form to its low potential form. The reaction is quantitative and proceeds with the same kinetics as the inhibition of oxygen evolution. The potential shift of cytochrome b-559 suggests that photoinhibition induces early conformational changes in Photosystem II. © 1990.