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Energy conductance from thylakoid complexes to stromal reducing equivalents

Working in synchrony, photosynthetic charge separation, electron transfer, and redox reactions generate proton motif force necessary for the synthesis of ATP and funneling of electrons toward stromal reducing equivalent NADPH. The last step of electron transfer from ferredoxin to NADP+ is catalyzed by ubiquitous flavin adenine dinucleotide (FAD)-binding enzyme, ferredoxin-NADP+ oxidoreductase (FNR). Apart from this most notable activity, FNR has been implicated in various other thylakoid energy transduction pathways ranging from cyclic electron flow around photosystem I (PSI) to regulation and management of oxidative stress. Different supramolecular complexes of FNR have so far been described, but until recently, specific FNR interacting partners have eluded detection. Two proteins, TROL (thylakoid rhodanese-like) and Tic62 (62 kDa component of the translocon at the inner envelope of chloroplasts) have been characterized and shown to form dynamic complexes with FNR. Being multi-pass membrane protein, TROL qualifies for the long-sought membrane anchor of FNR. Both TROL and Tic62 can also be found at chloroplast inner envelope, revisiting the notion of electron transfer chain specific for this compartment. Additionally, Tic62 in complex with FNR can be found in chloroplast stroma, implicating its role in formation and stabilization of FNR dimmers. Tethering of FNR to Tic62/TROL is accomplished via a conserved Ser/Pro-rich motif which forms type II alpha-helix and presumably mediates the interaction in pH-dependent manner. Inactivation of TROL leads to changes in efficiency of electron transfer and induction of non-photochemical quenching. TROL-deficient plants have changed nuclear gene expression with up-regulation of NADPH-dependent malic enzyme, which can form NADPH in an alternative pathway. Thus, NADPH synthesis, mediated by FNR-TROL interaction, may be the source element in metabolic retrograde signal-transduction pathway linking light reactions with nuclear gene expression. This review will focus on recent developments in the field of FNR/TROL-Tic62 mediated electron transfer and will explore future perspectives in the research of these important elements of photosynthetic energy transduction.

oxygenic photosynthesis ; TROL protein ; ferredoxin NADP+ oxidoreductase ; electron partitioning

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