Photoinhibition of manganese enzymes: insights into the mechanism of photosystem II photoinhibition.

Evidence has recently been presented that photoinhibition of photosystem II (PSII) is triggered by absorption of light by the oxygen-evolving manganese cluster. To get insight into the effects of light on enzymes containing manganese or other transition metal cofactors, the photosensitivities of Mn catalase, Mn superoxide dismutase, the haem (Fe)-containing bovine liver catalase, and CuZn superoxide dismutase were investigated. Glucose oxidase was studied as an example of an enzyme that does not have a metal cofactor.

Action spectrum of photoinhibition in leaves of wild type and npq1-2 and npq4-1 mutants of Arabidopsis thaliana.

Photoinhibition is light-induced inactivation of PSII. Hypotheses about the photoreceptor(s) of photoinhibition include the Chl antenna of PSII, manganese of the oxygen-evolving complex (OEC), uncoupled Chl and iron-sulfur centres. We measured the action spectrum of photoinhibition in vivo from wild-type Arabidopsis thaliana L.

Mathematical modelling of the light response curve of photoinhibition of photosystem II.

The light response curves of the acceptor and donor side mechanisms of photoinhibition of Photosystem II were calculated, using Arabidopsis as a model organism. Acceptor-side photoinhibition was modelled as double reduction of QA, noting that non-photochemical quenching has the same effect on the quantum yield of QA double reduction in closed PSII centres as it has on the quantum yield of electron transport in open centres. The light response curve of acceptor-side photoinhibition in Arabidopsis shows very low efficiency under low intensity light and a relatively constant quantum yield above light saturation of photosynthesis.

Evidence for the role of the oxygen-evolving manganese complex in photoinhibition of Photosystem II.

Photoinhibition of PSII occurs at the same quantum efficiency from very low to very high light, which raises a question about how important is the rate of photosynthetic electron transfer in photoinhibition. We modulated electron transfer rate and light intensity independently of each other in lincomycin-treated pea leaves and in isolated thylakoids, in order to elucidate the specific effects of light and PSII electron transport on photoinhibition. Major changes in the rate of electron transport caused only small changes in the rate of photoinhibition, suggesting the existence of a significant photoinhibitory pathway that contains an electron-transfer-independent phase.