Abstract
Polypeptides encoded by plastid ndh genes form a complex (Ndh) which could reduce plastoquinone with NADH. Through a terminal oxidase, reduced plastoquinone would be oxidized in chlororespiration. However, isolated Ndh complex has low activity with plastoquinone and no terminal oxidase has been found in chloroplasts, thus the function of Ndh complex is unknown. Alternatively, thylakoid hydroquinone peroxidase could oxidize reduced plastoquinone with H(2)O(2). By immunoaffinity chromatography, we have purified the plastid Ndh complex of barley (Hordeum vulgare L.) to investigate the electron donor and acceptor specificity. A detergent-containing system was reconstructed with thylakoid Ndh complex and peroxidase which oxidized NADH with H(2)O(2) in a plastoquinone-dependent process. This system and the increases of thylakoid Ndh complex and peroxidase activities under photooxidative stress suggest that the chlororespiratory process consists of the sequence of reactions catalyzed by Ndh complex, peroxidase (acting on reduced plastoquinone), superoxide dismutase, and the non-enzymic one-electron transfer from reduced iron-sulfur protein (FeSP) to O(2). When FeSP is a component of cytochrome b(6).f complex or of the same Ndh complex, O(2) may be reduced with NADH, without requirement of light. Chlororespiration consumes reactive species of oxygen and, eventually, may decrease their production by lowering O(2) concentration in chloroplasts. The common plastoquinone pool with photosynthetic electron transport suggests that chlororespiratory reactions may poise reduced and oxidized forms of the intermediates of cyclic electron transport under highly fluctuating light intensities.
Highlights
Polypeptides encoded by plastid ndh genes form a complex (Ndh) which could reduce plastoquinone with NADH
This system and the increases of thylakoid Ndh complex and peroxidase activities under photooxidative stress suggest that the chlororespiratory process consists of the sequence of reactions catalyzed by Ndh complex, peroxidase, superoxide dismutase, and the non-enzymic one-electron transfer from reduced iron-sulfur protein (FeSP) to O2
The level of the NDH-A polypeptide, encoded by the ndhA gene, increased in thylakoid when barley leaves received high irradiances or were incubated under high oxygen concentrations [8], which suggested that ndh genes and the Ndh complex may be involved in the protection against photooxidative stress
Summary
The plastid complex with activity NADH dehydrogenase and homologous to mitochondrial complex I; FeCN, potassium ferricyanide; FeSP, iron-sulfur protein; HQ, p-dihydroxybenzene (hydroquinone); PQ, plastoquinone; PQH2, reduced plastoquinone; SOD, superoxide dismutase; PS, photosystem; PAGE, polyacrylamide gel electrophoresis. To provide biochemical support for this hypothesis, we have reconstructed, with immunopurified Ndh complex and peroxidase isolated from barley thylakoid, an in vitro system which (in a detergent-containing medium) oxidizes NADH with H2O2 in a PQ-dependent reaction. The properties of this in vitro system and the effects of different treatments of leaves on enzyme activities described suggest that chlororespiration may protect against photooxidative stress and could regulate the proportion of reduced and oxidized forms of the intermediates of cyclic electron transport to optimize cyclic photophosphorylation
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