engleski

Electron transfer routes in oxygenic photosynthesis : regulatory mechanisms and new perspectives

Light-driven photosynthetic charge separation supplies power for the synthesis of chemical energy equivalents, ATP and NADPH, which consequently fuel metabolic biochemical reactions required by plant. This fundamental process of energy conversion is exceptionally complex and involves light harvesting, water splitting, proton pumping, and electron partitioning mechanisms which have to be poised in order to efficiently synthesize chemical energy equivalents. Photosynthetic membranes possess enormous physiological versatility, in particular the ability to manage short- and long-term changes in the light environment. Various mechanisms can regulate the flow and partitioning of excitation energy between the two photosystems, and others can convert excess excitation energy into thermal energy. Once photodamage has occurred, a regulated protein turnover can re-establish function, or can acclimatize the photosynthetic machinery to seasonal changes. A large number of regulatory and supporting enzymes are involved in these physiological processes. These include, for instance, protein kinases and phosphatases, chaperones, a substantial number of proteases, and diverse other protein components that are required for the biogenesis of multisubunit complexes. Numerous studies have contributed to the understanding of protein phosphorylation involved in regulation of photosynthesis. Several thylakoid (and chloroplast) kinases, STN7, TAK, and phosphateses PPH1/TAP38 have been identified, and shown to be involved in photosynthetic regulatory pathways. Regulatory signals can be of different origin, but redox-dependent inputs have been implicated as major triggers. Various auxiliary proteins have been linked to these processes, with TLP40 lumenal immunofilin being one of them. Further, our understanding of different photosynthetic electron transport chains has substantially increased. We now know that three major pathways, linear, cyclic and pseudo-cyclic, are necessary for poised and sustained synthesis of ATP and NADPH. Also, alternative routes of electron partitioning have been recognized, raising the possibility of further regulation governed by switchable energy partitioning. An example of such novel mechanism is the dynamic binding and release of ferredoxin NADP+ oxidoreductase (FNR) from the thylakoid rhodanese-like protein TROL. Here, we review the above outlined regulatory mechanisms and aim to suggest direction for further research on these topics.

photosynthesis ; electron partitioning ; energy conductance ; TROL ; FNR

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