Photoinhibition And The Damage Repair Cycle Of Photosystem Ii

Excessive illumination of photosystem II in oxygenic photosynthetic organisms, such as cyanobacteria, algae and higher plants, causes photoinhibition and blocks electron transport in photosystem II. In this event, the reaction centre D1 protein is damaged primarily by reactive oxygen species or other endogenous radicals produced by the photochemical reaction, and degradation and/ or irreversible aggregation of the damaged proteins occur subsequently. When the D1 protein is photo damaged under moderate high light or weak light, the damaged protein is proteolysed and replaced by a newly synthesized copy. The photoinhibition under these conditions is reversible and photo system II activity recovers rapidly in the dark. In contrast, irreversible aggregation of the D1 protein, which is caused by excessive illumination, prevents proper D1 turnover. Once the aggregated products accumulate in photosystem II complexes their removal by proteases is hampered. The photoinhibition observed under these conditions is irreversible. In higher plant chloroplasts, illumination with high light leads to dynamic changes in the structure of the thylakoids at the molecular and membrane levels. It has become clear recently that these structural changes are necessary so that photosystem II can endure the effects of light stress effectively.