Regulation of proton-to-electron stoichiometry in photosynthetic electron transport: physiological function in photoprotection |
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Authors: | T Shikanai Y Munekage K Kimura |
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Institution: | (1) Graduate School of Biological Sciences, Nara Institute of Science and Technology, Takayama 8916-5, Ikoma 630-0101, Japan Tel. +81-743-72-5487; Fax +81-743-72-5489 e-mail: shikanai@bs.aist-nara.ac.jp, JP |
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Abstract: | The primary stable products of photosynthetic electron flow are NADPH and ATP. Stoichiometry of their production depends
on the ratio of protons pumped across the thylakoid membrane to electrons passed through the electron transport pathway (H+/e− ratio). Flexible requirements of the ATP/NADPH ratio by various assimilatory reactions in chloroplasts must be fulfilled
by the H+/e− ratio during the electron flow. In addition to the well-known role of ΔpH during ATP synthesis, ΔpH also functions as a trigger
of the down-regulation of photosystem II (PSII) photochemistry. Excessive light energy is safely dissipated as heat by this
regulatory process to suppress the generation of toxic reactive oxygen species. Thus, regulation of the H+/e− ratio may function in the photoprotection, as well as in the regulation of the ATP/NADPH production ratio. It has long been
the consensus that the H+/e− ratio can be controlled by regulating the proton-transporting Q-cycle in the cytochrome b
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f complex and by the cyclic electron flow around photosystem I (PSI). Despite the possible physiological importance and the
long history of interest, the molecular identity of Q-cycle regulation and the cyclic electron flow around PSI have been remained
unclear. The recent improvements in research tools, including the genetic approach using chlorophyll fluorescence imaging
and establishment of the chloroplast transformation technique, are providing new insights into classical topics. In this review,
we focus on regulation of the H+/e− ratio especially from the view of photosynthetic regulation.
Received: August 2, 2001 / Accepted: October 1, 2001 |
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Keywords: | Cyclic electron flow around PSI Cytochrome b6f complex NDH Photosynthesis Q-cycle Thermal dissipation |
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