Chlororespiratory Pathways and Their Physiological Significance
There is now overwhelming evidence that the thylakoid membrane of green plants contains, in addition to the photosynthetic electron-transfer complexes, a set of respiratory complexes that are capable of reducing and oxidising the plastoquinone pool. These “chlororespiratory” enzymes include the Ndh complex, which is related to complex I found in mitochondria and eubacteria, and the plastid terminal oxidase (PTOX), which is a distant member of the family of alternative oxidases found in mitochondria. In addition the molecular basis of other chlororespiratory activities, including the long sought after ferredoxin:plastoquinone reductase (FQR), implicated in cyclic electron flow around photosystem one, are now being uncovered using a combination of genetics, biochemistry and bioinformatics. Here we assess the possible components of the various chlororespiratory pathways and discuss their potential physiological importance. The emerging picture suggests that the main role of chlororespiratory enzymes, at least in mature chloroplasts, is not actually to participate in a classical respiratory chain to drive ATP synthesis. Instead, chlororespiratory activities play important auxiliary roles in various aspects of photosynthesis including cyclic electron flow around photosystem one, carotenoid biosynthesis and photoprotection. The detection of chlororespiratory enzymes in non-photosynthetic plastids suggests that their activities are not restricted to the chloroplast.
KeywordsCyclic Electron Flow Cyclic Electron Transport Plastoquinone Pool Dark Reduction Cyclic Electron Transfer
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