Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:1.6.99.1 (NADPH-diaphorase)
 3,903 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Bundle sheath chloroplasts of NADP-malic enzyme (NADP-ME) type C4 species have a high demand for ATP, while being deficient in linear electron flow and oxidation of water by photosystem II (PSII). To evaluate electron donors to photosystem I (PSI) and possible pathways of cyclic electron flow (CEF1) in isolated bundle sheath strands of maize (Zea mays L.), an NADP-ME species, light-induced redox kinetics of the reaction center chlorophyll of PSI (P700) were followed under aerobic conditions. Donors of electrons to CEF1 are needed to compensate for electrons lost from the cycle. When stromal electron donors to CEF1 are generated during pre-illumination with actinic light (AL), they retard the subsequent rate of oxidation of P700 by far-red light. Ascorbate was more effective than malate in generating stromal electron donors by AL. The generation of stromal donors by ascorbate was inhibited by DCMU, showing ascorbate donates electrons to the oxidizing side of PSII. The inhibitors of NADPH dehydrogenase (NDH), amytal and rotenone, accelerated the oxidation rate of P700 by far-red light after AL, indicating donation of electrons to the intersystem from stromal donors via NDH. These inhibitors, however, did not affect the steady-state level of P700+ under AL, which represents a balance of input and output of electrons in P700. In contrast, antimycin A, the inhibitor of the ferredoxin-plastoquinone reductase-dependent CEF1, substantially lowered the level of P700+ under AL. Thus, the primary pathway of ATP generation by CEF1 may be through ferredoxin-plastoquinone, while function of CEF1 via NDH may be restricted by low levels of ferredoxin-NADP reductase. NDH may contribute to redox poising of CEF1, or function to generate ATP in linear electron flow to O2 via PSI, utilizing NADPH generated from malate by chloroplastic NADP-ME.
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PMID:Analysis of donors of electrons to photosystem I and cyclic electron flow by redox kinetics of P700 in chloroplasts of isolated bundle sheath strands of maize. 1755 45

Cyclic electron flow (CEFI) has been proposed to balance the chloroplast energy budget, but the pathway, mechanism, and physiological role remain unclear. We isolated a new class of mutant in Arabidopsis thaliana, hcef for high CEF1, which shows constitutively elevated CEF1. The first of these, hcef1, was mapped to chloroplast fructose-1,6-bisphosphatase. Crossing hcef1 with pgr5, which is deficient in the antimycin A-sensitive pathway for plastoquinone reduction, resulted in a double mutant that maintained the high CEF1 phenotype, implying that the PGR5-dependent pathway is not involved. By contrast, crossing hcef1 with crr2-2, deficient in thylakoid NADPH dehydrogenase (NDH) complex, results in a double mutant that is highly light sensitive and lacks elevated CEF1, suggesting that NDH plays a direct role in catalyzing or regulating CEF1. Additionally, the NdhI component of the NDH complex was highly expressed in hcef1, whereas other photosynthetic complexes, as well as PGR5, decreased. We propose that (1) NDH is specifically upregulated in hcef1, allowing for increased CEF1; (2) the hcef1 mutation imposes an elevated ATP demand that may trigger CEF1; and (3) alternative mechanisms for augmenting ATP cannot compensate for the loss of CEF1 through NDH.
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PMID:An Arabidopsis mutant with high cyclic electron flow around photosystem I (hcef) involving the NADPH dehydrogenase complex. 2008 Nov 15