1.97.1.12: photosystem I
This is an abbreviated version!
For detailed information about photosystem I, go to the full flat file.
Word Map on EC 1.97.1.12
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1.97.1.12
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photosystems
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chlorophyl
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thylakoids
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chloroplast
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light-harvesting
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cyanobacterium
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antenna
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spinach
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synechocystis
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synechococcus
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chlamydomonas
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photochemical
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photoinhibition
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plastoquinone
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light-induced
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ferredoxins
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reinhardtii
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photoautotrophic
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non-photochemical
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methylviologen
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pump-probe
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phycobilisomes
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thermoluminescence
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supercomplexes
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pigment-protein
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chlorophyll-proteins
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photooxidation
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oxygen-evolving
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lhcii
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photocurrent
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3-3,4-dichlorophenyl-1,1-dimethylurea
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grana
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water-splitting
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intersystem
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photoprotection
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phylloquinone
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excitonic
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light-driven
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photoreduced
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photoinhibitory
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photoreduction
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thermosynechococcus
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far-red
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picosecond
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flash-induced
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violaxanthin
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electrochromic
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photophosphorylation
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elongatus
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photochemistry
- 1.97.1.12
- photosystems
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chlorophyl
- thylakoids
- chloroplast
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light-harvesting
- cyanobacterium
- antenna
- spinach
- synechocystis
- synechococcus
- chlamydomonas
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photochemical
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photoinhibition
- plastoquinone
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light-induced
- ferredoxins
- reinhardtii
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photoautotrophic
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non-photochemical
- methylviologen
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pump-probe
- phycobilisomes
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thermoluminescence
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supercomplexes
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pigment-protein
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chlorophyll-proteins
- photooxidation
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oxygen-evolving
- lhcii
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photocurrent
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3-3,4-dichlorophenyl-1,1-dimethylurea
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grana
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water-splitting
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intersystem
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photoprotection
- phylloquinone
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excitonic
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light-driven
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photoreduced
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photoinhibitory
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photoreduction
- thermosynechococcus
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far-red
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picosecond
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flash-induced
- violaxanthin
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electrochromic
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photophosphorylation
- elongatus
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photochemistry
Reaction
Synonyms
OsCRR6, photosystem I P700 chlorophyll a apoprotein A2, PS I, PS-I, PS-I complex, PsaB, PsaF, PSI, PSI core complex
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General Information on EC 1.97.1.12 - photosystem I
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GENERAL INFORMATION 
ORGANISM
UNIPROT
COMMENTARY 
LITERATURE
evolution
it is proposed that PSI evolved stepwise from a trimeric form to tetrameric oligomer en route to becoming monomeric in plants/algae
malfunction
metabolism
physiological function
additional information
malfunction
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impairment of NDH-dependent cyclic electron flow in rice specifically causes a reduction in the electron transport rate through PS I (ETR I) at low light intensity with a concomitant reduction in CO2 assimilation rate, plant biomass and importantly, grain production, while there is no effect on PS II function at low or high light intensity. Oryza sativa crr6 mutant does not accumulate the NDH complex, CRR6 is specifically required for the assembly of NdhI in subcomplex A of chloroplast NDH, knockout of the crr6 gene in rice also leads to the lack of intact chloroplast NDH, no formation of a NDH-PS I supercomplex in the Crr6 mutant. Phenotype overview
malfunction
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photosynthetic performance is affected in WHY1 knockout mutant under different light conditions. Loss of WHIRLY1 decreases chloroplast NAD(P)H dehydrogenase-like complex (NDH) activity and the accumulation of NDH supercomplex. Loss of WHIRLY1 leads to a higher photochemical quantum yield of photosystem I Y(I) and electron transport rate and a lower non-photochemical quenching involved in the thermal dissipation of excitation energy of chlorophyll fluorescence than the wild-type. Several genes encoding the PSI-NDH complexes are also upregulated in kowhy1 and the whirly1whirly3 double mutant (ko1/3) but steady in oepWHY1 plants. Under high light conditions, both kowhy1 and ko1/3 plants show lower electron transport rate than wild-type which are contrary to that under normal light condition. Moreover, the expression of several PSI-NDH encoding genes and ERF109, which is related to jasmonate (JA) response, varies in kowhy1 under different light conditions. Loss of WHY1 seems to increase photosystem I quantum yield but not photosystem II quantum yield, phenotype, overview. Overexpression of WHY1 in both nucleus and plastid improves the transcription level of a number of genes encoding PSI core subunits which are essential to the formation of super complexes of PSI
malfunction
site-directed mutagenesis of residues involved in the respective phylloquinone-binding sites results in a specific alteration of the rates of semiquinone oxidation. Mutation in the PhQA binding pocket (PsaA-F689N) in PSI of Chlamydomonas reinhardtii reduces down PhQA- oxidation kinetics by almost two orders of magnitude. This creates an unprecedented situation in which the reduction of P700+ is faster than the oxidation of the semiquinone, thereby providing the opportunity to initiate a second photochemical event while PhQA- is still present in ETCA, kinetics, overview
malfunction
the PsaA-N604L mutation (near ec2B) results in a 50% reduction in the amount of electron transfer in the cofactor B-branch, while the PsaB-N591L mutation (near ec2A) results in a 70% reduction in the amount of electron transfer in the cofactor A-branch. The PsaB-N591L mutation had a significant effect upon trapping, while the PsaA-N604L mutation does not have a significant effect upon trapping
malfunction
the PsaA-N604L mutation (near ec2B) results in a 50% reduction in the amount of electron transfer in the cofactor B-branch, while the PsaB-N591L mutation (near ec2A) results in a 70% reduction in the amount of electron transfer in the cofactor A-branch. The PsaB-N591L mutation had a significant effect upon trapping, while the PsaA-N604L mutation does not have a significant effect upon trapping
malfunction
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photosynthetic performance is affected in WHY1 knockout mutant under different light conditions. Loss of WHIRLY1 decreases chloroplast NAD(P)H dehydrogenase-like complex (NDH) activity and the accumulation of NDH supercomplex. Loss of WHIRLY1 leads to a higher photochemical quantum yield of photosystem I Y(I) and electron transport rate and a lower non-photochemical quenching involved in the thermal dissipation of excitation energy of chlorophyll fluorescence than the wild-type. Several genes encoding the PSI-NDH complexes are also upregulated in kowhy1 and the whirly1whirly3 double mutant (ko1/3) but steady in oepWHY1 plants. Under high light conditions, both kowhy1 and ko1/3 plants show lower electron transport rate than wild-type which are contrary to that under normal light condition. Moreover, the expression of several PSI-NDH encoding genes and ERF109, which is related to jasmonate (JA) response, varies in kowhy1 under different light conditions. Loss of WHY1 seems to increase photosystem I quantum yield but not photosystem II quantum yield, phenotype, overview. Overexpression of WHY1 in both nucleus and plastid improves the transcription level of a number of genes encoding PSI core subunits which are essential to the formation of super complexes of PSI
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malfunction
Chlamydomonas reinhardtii KRC91-1A
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site-directed mutagenesis of residues involved in the respective phylloquinone-binding sites results in a specific alteration of the rates of semiquinone oxidation. Mutation in the PhQA binding pocket (PsaA-F689N) in PSI of Chlamydomonas reinhardtii reduces down PhQA- oxidation kinetics by almost two orders of magnitude. This creates an unprecedented situation in which the reduction of P700+ is faster than the oxidation of the semiquinone, thereby providing the opportunity to initiate a second photochemical event while PhQA- is still present in ETCA, kinetics, overview
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metabolism
cyclic electron flow (CEF) can be stimulated to provide significant additional protection in plants when they are confronted with acidic pH conditions. But at low pH, the photosynthetic efficiency greatly decreases resulting in excessive excitation pressure in PS II and consequently an increased risk of damage on the donor side of PS II
metabolism
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the physiological contribution of NDH-mediated CEF is greater in C4 photosynthesis than in C3 photosynthesis, suggesting that the mechanism of PET in C4 photosynthesis has changed from that in C3 photosynthesis accompanying the changes in the mechanism of CO2 assimilation
physiological function
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Cyclic electron flow (CEF) can produce ATP without producing NADPH by transporting electrons from the accepter side of PSI to plastoquinone in a cyclic manner and promoting proton translocation across the thylakoid membrane. CEF around PSI mediated by chloroplast NADH dehydrogenase-like complex (NDH), is an alternative pathway of photosynthetic electron transport (PET) and plays a crucial role in C4 photosynthesis, although the contribution of NDH-mediated CEF is small in C3 photosynthesis. NDH-suppressed plants grow poorly, especially under low light conditions. Electron donation to PSI from NDH occurs via the Cyt b6/f complex, the Cyt b6/f complex functions as a proton pump across the thylakoid membrane
physiological function
cyclic electron flow (CEF) PS I is a highly dynamic, regulated and large capacity pathway involving ferredoxin (Fd) and Cytb6f complex. In CEF, the plastoquinone (PQ) pool is reduced by Fd which is photoreduced by PS I. The reduced PQ pool is oxidized by the Cytb6f complex, where DpH is generated across thylakoid membranes through the Q cycle. This DpH drives ATP synthase and produces the ATP required for the net CO2 assimilation and also stimulates NPQ. It has been proposed that there are two physiological functions of CEF in thylakoid membrane. During photosynthesis, cyclic electron flow around PSI can generate a pH gradient across the thylakoid membrane that not only is used to synthesize ATP, but can also induce thermal dissipation and provide protection for the photosynthetic apparatus
physiological function
photosystem I (PS I) is a key pigment-protein complex of the electron transfer (ET) chain of oxygenic photosynthetic organisms. It includes both a large antenna system for harvesting solar energy and a photochemical reaction center catalyzing charge separation across the membrane. PS I is the key element of the energy-transducing pathways because the electron flow in thylakoids is controlled by the redox states of cofactors in the acceptor part of PS I
physiological function
photosystem I (PSI) is a large pigment-protein complex that functions as a light-driven oxidoreductase, catalyzing an otherwise uphill electron transfer from cytochrome c or plastocyanin to ferredoxin or flavodoxin
physiological function
photosystem I (PSI) is a large pigment-protein complex that functions as a light-driven oxidoreductase, catalyzing an otherwise uphill electron transfer from cytochrome c or plastocyanin to ferredoxin or flavodoxin
physiological function
photosystem I (PSI) is one of two large reaction centres responsible for converting light photons into the chemical energy needed to sustain life. In the thylakoid membranes of plants, PSI is found together with its integral light-harvesting antenna, light-harvesting complex I (LHCI), in a membrane supercomplex containing hundreds of light-harvesting pigments
physiological function
pigment-protein complex of photosystem I (PS I) is one of the crucial enzymes in the electron transfer (ET) chain in thylakoid membranes of oxygenic photosynthetic organisms. PS I complex catalyzes the light-driven ET from peripheral donor proteins plastocyanin or cytochrome c6 to ferredoxin or flavodoxin. Electron transport in PS I occurs through both branches of the redox cofactors A and B from P700 to FX
physiological function
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plastid-nucleus-located WHIRLY1 protein, which plays a role in regulating leaf senescence and is believed to associate with the increase of reactive oxygen species delivered from redox state of the photosynthetic electron transport chain, interacts with light-harvesting protein complex I (LHCA1) and affects the expression of genes encoding photosystem I (PSI) and light harvest complexes (LHCI). WHY1 might not regulate the NDH-PSI super complex genes directly in transcriptional level, but have a role in the formation of NDH-PSI super complex. WHY1 in chloroplast may have a protective effect on photosystem I, and WHIRLY1 may act as a communicator between the plastids and the nucleus
physiological function
the chloroplast must regulate supply of reducing equivalents and ATP to meet rapid changes in downstream metabolic demands. Cyclic electron flow around photosystem I (CEF) is proposed to balance the ATP/NADPH budget by using reducing equivalents to drive plastoquinone reduction, leading to the generation of proton motive force and subsequent ATP synthesis. Photosynthetic energy is stored by linear electron flow (LEF), which involves electron transfer through both photosystem II (PS II) and photosystem I (PS I). Multiple alternative electron pathways are identified or proposed including the water-water cycle, themalate shunt, the plastid terminal oxidase, and cyclic electron flow around photosystem I (CEF). Cyclic electron flow can alleviate an ATP deficit by passing electrons fromthe acceptor side of PS I back to PQ, driving the translocation of protons into the lumen without net reduction of NADP+. High rates of CEF are readily measured in vivo
physiological function
two functional electron transfer (ET) chains, related by a pseudo-C2 symmetry, are present in the reaction center of photosystem I (PSI). Due to slight differences in the environment around the cofactors of the two branches, there are differences in both the kinetics of ET and the proportion of ET that occurs on the two branches. The oxidation rates of the reduced phylloquinone (PhQ) cofactor differ by an order of magnitudes. The presence of PhQ-A does not impact the overall quantum yield and leads to an almost complete redistribution of the fractional utilization of the two functional ET chains, in favor of the one that does not bear the charged species, molecular mechanism that gives rise to the high quantum efficiency in PSI, overview
physiological function
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while linear electron transport generates both ATP and NADPH, cyclic electron transport around photosystem I (PS I) is exclusively involved in ATP synthesis without the accumulation of NADPH. The role of cyclic electron transport around PS I is proposed to be essential for balancing the ATP/NADPH production ratio and/or for protecting both photosystems from the damage via stromal overreduction. Two pathways of PS I cyclic electron transport have been proposed: the main pathway depends on PGR5 (proton gradient regulation 5) and PGRL1 (PGR5-like photosynthetic phenotype) proteins, whereas the minor pathway is mediated by a chloroplast NADH dehydrogenase-like (NDH) complex. The chloroplast NDH-dependent PS I cyclic electron transport plays a role in alleviation of oxidative damage in strong light. Significant physiological function for the chloroplast NDH at low light intensities commonly experienced during the reproductive and ripening stages of rice cultivation that have adverse effects crop yield. Regulation of photosynthetic electron transport in the thylakoid membrane of chloroplasts is fundamental for the maximum photosynthetic yield and plant growth. Formation of a NDH-PS I supercomplex
physiological function
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plastid-nucleus-located WHIRLY1 protein, which plays a role in regulating leaf senescence and is believed to associate with the increase of reactive oxygen species delivered from redox state of the photosynthetic electron transport chain, interacts with light-harvesting protein complex I (LHCA1) and affects the expression of genes encoding photosystem I (PSI) and light harvest complexes (LHCI). WHY1 might not regulate the NDH-PSI super complex genes directly in transcriptional level, but have a role in the formation of NDH-PSI super complex. WHY1 in chloroplast may have a protective effect on photosystem I, and WHIRLY1 may act as a communicator between the plastids and the nucleus
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physiological function
Chlamydomonas reinhardtii KRC91-1A
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two functional electron transfer (ET) chains, related by a pseudo-C2 symmetry, are present in the reaction center of photosystem I (PSI). Due to slight differences in the environment around the cofactors of the two branches, there are differences in both the kinetics of ET and the proportion of ET that occurs on the two branches. The oxidation rates of the reduced phylloquinone (PhQ) cofactor differ by an order of magnitudes. The presence of PhQ-A does not impact the overall quantum yield and leads to an almost complete redistribution of the fractional utilization of the two functional ET chains, in favor of the one that does not bear the charged species, molecular mechanism that gives rise to the high quantum efficiency in PSI, overview
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additional information
charge transfer and recombination reactions in intact and PsaC-depleted (FX-core) wild-type PS I complexes and in the menB PS I complexes containing plastoquinone (menB-PQ) and 2,3-dichloro-1,4-naphthoquinone (menB-Cl2NQ), overview. 2,3-Dichlorophenolindophenol reduced by ascorbate serves as an external electron donor for the photooxidized P700, while methylviologen plays a role of external acceptor capturing electrons from the photoreduced terminal FA/FB cluster
additional information
cyclic electron flow around photosystem I is enhanced at low pH of 5.5. The light response curve of PS I is significantly affected by acidification of the thylakoid membranes
additional information
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cyclic electron flow around photosystem I is enhanced at low pH of 5.5. The light response curve of PS I is significantly affected by acidification of the thylakoid membranes
additional information
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cytochrome c6 in the Phaeodactylum tricornutum alga, as inmost diatoms, is the only electron donor to photosystem I, and thus they lack plastocyanin as an alternative electron carrier
additional information
in photosystem I, light-induced electron transfer can occur in either of two symmetry-related branches of cofactors, each of which is composed of a pair of chlorophylls (ec2A/ec3A or ec2B/ec3B) and a phylloquinone (PhQA or PhQB). The axial ligand to the central Mg2+ of the ec2A and ec2B chlorophylls is a water molecule that is also H-bonded to a nearby Asn residue, an important interaction for charge separation by converting each of the Asn residues to a Leu in the cyanobacterium Synechocystis sp. PCC6803. Each branch of the reaction center appears to operate independently of the other in carrying out light-induced charge separation
additional information
in photosystem I, light-induced electron transfer can occur in either of two symmetry-related branches of cofactors, each of which is composed of a pair of chlorophylls (ec2A/ec3A or ec2B/ec3B) and a phylloquinone (PhQA or PhQB). The axial ligand to the central Mg2+ of the ec2A and ec2B chlorophylls is a water molecule that is also H-bonded to a nearby Asn residue, an important interaction for charge separation by converting each of the Asn residues to a Leu in the green alga, Chlamydomonas reinhardtii. Each branch of the reaction center appears to operate independently of the other in carrying out light-induced charge separation
additional information
lipid structure and enzyme complex structure analysis of photosystem I supercomplex including light-harvesting complex I, PSI-LHCI, overview. Foerster rate calculations reveal the importance of the luminal-side pigment junctions. Chlorophyll b molecules are coordinated by a mixture of polar and hydrophobic interactions
additional information
PS I binding analysis using redox-inactive gallium ferredoxin, overview. Gallium ferredoxin binds to PSI competitively with ferredoxin suggesting a single ferredoxin-binding site in PSI, kinetics
additional information
Thermosynechococcus vestitus
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PS I binding analysis using redox-inactive gallium ferredoxin, overview. Gallium ferredoxin binds to PSI competitively with ferredoxin suggesting a single ferredoxin?binding site in PSI, kinetics
additional information
the membrane-embedded core of each PS I monomer is formed by the two largest subunits, PsaA and PsaB, which bind electron transport cofactors arranged in two symmetrical branches, A and B, extending from P700, a pair of chlorophyll a molecules located on the lumenal side, to the [4Fe-4S] cluster FX, placed on the opposite stromal side of the complex. Each of the two branches, related by a pseudo-C2 rotation axis which passes through P700 and FX, carries two electronically coupled Chl a molecules (termed A0A or A0B) and one phylloquinone A1A or A1B
