engleski

Emerging concept of alternative electron partitioning in photosynthesis

In vascular plants, photosynthetic electron transport (PET) chain produces reductive power that is utilised by diverse acceptors involved in both chloroplast and cellular metabolism. In the last steps of this process, transfer of energy-conserving electrons beyond photosystem I (PSI) is performed by a small iron-sulphur protein ferredoxin (Fd). Fd acts simultaneously as a bottleneck and as a hub which distributes high-energy electrons to a multitude of enzymes. The dominant pathway in chloroplasts is, however, the one that produces NADPH, by the activity of ferredoxin-NADP+-oxidoreductase (FNR). In order to function efficiently Fd/FNR redox chemistry has to be constantly poised. The question remains how is the partitioning of electrons between the various energy-conserving and -dissipating pathways achieved. Thylakoid rhodanase-like protein TROL has been shown to act as bona fide membrane attachment point for the FNR, although other membrane associations of FNR have been described. We have proposed a scheme for the dynamic FNR recruitment to TROL. We posit that TROL-FNR interaction presents the branching point between electron-conserving and electron-dissipating pathways. Without the TROL, light-dependent O2.- generation is reduced, while the generation of other ROS is enhanced. We propose that other Fd-dependent pathways downstream of PSI and different from the linear electron transfer become dominant by the dynamic detachment of FNR form TROL, thus suggesting a novel mechanism of photosynthesis regulation. Alternatively, efficient scavenging of O2.- by FNR-dependent pathways can be envisaged.

photosystem I; redox regulation and signaling; regulation of electron flow

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