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MetaCyc Pathway: photosynthesis light reactions
Inferred from experiment

Enzyme View:

Pathway diagram: photosynthesis light reactions

This view shows enzymes only for those organisms listed below, in the list of taxa known to possess the pathway. If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.

Superclasses: Generation of Precursor Metabolites and EnergyElectron Transfer
Generation of Precursor Metabolites and EnergyPhotosynthesis

Some taxa known to possess this pathway include : Arabidopsis thaliana col, Chlamydomonas reinhardtii, Synechococcus elongatus PCC 7942, Synechocystis sp. PCC 6803

Expected Taxonomic Range: Bacillariophyta, Bacteria , Chlorarachniophyceae, Chlorophyta, Chromerida, Chrysophyceae, Cryptophyta, Cyanobacteria, Dictyochophyceae, Dinophyceae, Euglenozoa, Eustigmatophyceae, Glaucocystophyceae, Haptophyceae, Phaeophyceae, Raphidophyceae, Rhodophyta, Viridiplantae, Xanthophyceae

Photosynthesis is composed of two processes, the light reactions and the dark reactions. The light reactons take place in the two photosystems - photosystem I and photosystem II, where light energy is harvested and is used to power the transfer of electrons from water, via a series of electron donors and acceptors, to the final acceptor NADP+, which is reduced to NADPH. The NADPH generated by the light reactions is used for sugar synthesis in the dark reactions.

The light reactions also generate a proton motive force across the thylakoid membrane, and the proton gradient is used to synthesize ATP.

There are two general chemical reactions involved in the light reactions: water oxidation in photosystem II, and NADP reduction in photosystem I. Both of the photosystems are large multiprotein complexes contained within the thylakoid membranes of all types of plants, algae and cyanobacteria, and both photosystems capture the light by means of large antennae systems, consisting of chlorophylls and carotenoids.

Photosystem II (PSII) utilizes solar energy for splitting water molecules, and transfers the electrons obtained from the water molecule via an electron transfer chain that ends with a plastoquinone.

The excitation of the photosystem II reaction center (RC) via light absorption by chlorophylls in the antenna drives the transfer of a very high potential electron from the cluster of four chlorophylls bound to the D1- and D2-proteins (known as P680) to a pheophytin acceptor, resulting in a P680* radical. The radical transfers the electron to a firmly bound a plastoquinone (PQ) called QA, which transfers it to a second, not firmly bound PQ, called QB. When QB is fully reduced and protonated to a plastoquinol (PQH2) form, it diffuses from the QB-binding site into the lipid matrix of the membrane (for more information see photosystem II).

The path between the two photosystems is not direct. The QB plastoquinone first transfers the electrons to a third complex, the plastoquinol--plastocyanin reductase (better known as the cytochrome b6f complex monomer), where the electrons are transferred to the protein plastocyanin. Finally plastocyanin travels to photosystem I, and delivers the electrons to one of its components, P700, which is located on the inside of the thylakoids (the lumen).

photosystem I (PS I) catalyzes the light driven electron transfer from plastocyanin to a ferredoxin, which is on the stromal side of the membrane. As in photosystem II, photosystem I performs both light capturing and electron transfer. The light capturing is performed by the large antenna system that consists of 90 antenna chlorophylls and 22 carotenoids. The electron transport chain consists of six chlorophylls, two phylloquinones and three [4Fe-4S] iron-sulfur clusters.

The electron transfer chain of PS I starts with P700, a heterodimer of a chlorophyll a and a chlorophyll a' (the C13 epimer of chlorophyll a). As in photosystem II, a high energy electron is removed from P700 after excitation by light, and that electron is transferred through the system. The role of the electron delivered from photosystem II by plastocyanin is to re-reduce P700.

From P700 the electron is transferred stepwise to A (a chlorophyll a molecule), A0 (another chlorophyll a molecule), A1 (a phylloquinone molecule) and then through three 4Fe4S clusters, named FX, FA and FB. From the terminal 4Fe4S cluster, FB, the electron is transferred to the 2Fe2S cluster of a ferredoxin, which leaves photosystem I and transfers the electron to the ferredoxin-NADP oxidoreductase (EC, where NADP+ is finally reduced to NADPH. Under conditions of iron limitation, a flavodoxin may replace the ferredoxin [Grotjohann05].

Cyanobacterial PS I can exist in both trimeric and monomeric forms. The trimeric form has been shown to be the prominent oligomeric state at low light intensity. The monomeric unit of PSI from Synechococcus elongatus consists of 12 protein subunits, to which 127 cofactors are non-covalently bound.

Some components of photosystem I and photosystem II in plants are different from those in photosynthetic microbes.

Citations: [Dey97]

Superpathways: superpathway of photosynthetic hydrogen production, photosynthetic 3-hydroxybutanoate biosynthesis (engineered), 1-butanol autotrophic biosynthesis, oxygenic photosynthesis, ethylene biosynthesis V (engineered)

Unification Links: AraCyc:PWY-101

Created 26-Apr-2002 by Zhang P, TAIR
Revised 20-May-2008 by Caspi R, SRI International
Revised 13-Jan-2011 by Caspi R, SRI International


Collins81: Collins MD, Jones D (1981). "Distribution of isoprenoid quinone structural types in bacteria and their taxonomic implication." Microbiol Rev 45(2);316-54. PMID: 7022156

Dey97: Dey, PM, Harborne, JB "Plant Biochemistry." Academic Press 1997.

Grotjohann05: Grotjohann I, Fromme P (2005). "Structure of cyanobacterial photosystem I." Photosynth Res 85(1);51-72. PMID: 15977059

Park06a: Park YJ, Yoo CB, Choi SY, Lee HB (2006). "Purifications and characterizations of a ferredoxin and its related 2-oxoacid:ferredoxin oxidoreductase from the hyperthermophilic archaeon, Sulfolobus solfataricus P1." J Biochem Mol Biol 39(1);46-54. PMID: 16466637

Vermaas01: Vermaas WFJ (2001). "Photosynthesis and Respiration in Cyanobacteria." Encyclopedia of Life Sciences, Macmillan Publishers Ltd, Nature Publishing Group.

Other References Related to Enzymes, Genes, Subpathways, and Substrates of this Pathway

Albus10: Albus CA, Ruf S, Schottler MA, Lein W, Kehr J, Bock R (2010). "Y3IP1, a nucleus-encoded thylakoid protein, cooperates with the plastid-encoded Ycf3 protein in photosystem I assembly of tobacco and Arabidopsis." Plant Cell 22(8);2838-55. PMID: 20807881

Alizadeh94: Alizadeh S, Nechushtai R, Barber J, Nixon P (1994). "Nucleotide sequence of the psbE, psbF and trnM genes from the chloroplast genome of Chlamydomonas reinhardtii." Biochim Biophys Acta 1188(3);439-42. PMID: 7803458

Allahverdiyeva07: Allahverdiyeva Y, Mamedov F, Suorsa M, Styring S, Vass I, Aro EM (2007). "Insights into the function of PsbR protein in Arabidopsis thaliana." Biochim Biophys Acta 1767(6);677-85. PMID: 17320041

Allakhverdiev11: Allakhverdiev SI, Tsuchiya T, Watabe K, Kojima A, Los DA, Tomo T, Klimov VV, Mimuro M (2011). "Redox potentials of primary electron acceptor quinone molecule (QA)- and conserved energetics of photosystem II in cyanobacteria with chlorophyll a and chlorophyll d." Proc Natl Acad Sci U S A 108(19);8054-8. PMID: 21521792

Allen04: Allen JF (2004). "Cytochrome b6f: structure for signalling and vectorial metabolism." Trends Plant Sci 9(3);130-7. PMID: 15003236

Armbruster10: Armbruster U, Zuhlke J, Rengstl B, Kreller R, Makarenko E, Ruhle T, Schunemann D, Jahns P, Weisshaar B, Nickelsen J, Leister D (2010). "The Arabidopsis thylakoid protein PAM68 is required for efficient D1 biogenesis and photosystem II assembly." Plant Cell 22(10);3439-60. PMID: 20923938

Bairoch93a: Bairoch A, Boeckmann B (1993). "The SWISS-PROT protein sequence data bank, recent developments." Nucleic Acids Res. 21:3093-3096. PMID: 8332529

Bals10: Bals T, Dunschede B, Funke S, Schunemann D (2010). "Interplay between the cpSRP pathway components, the substrate LHCP and the translocase Alb3: an in vivo and in vitro study." FEBS Lett 584(19);4138-44. PMID: 20828566

Baniulis: Baniulis D, Yamashita E, Zhang H, Hasan SS, Cramer WA "Structure-function of the cytochrome b6f complex." Photochem Photobiol 84(6);1349-58. PMID: 19067956

Baniulis11: Baniulis D, Zhang H, Zakharova T, Hasan SS, Cramer WA (2011). "Purification and crystallization of the cyanobacterial cytochrome b6f complex." Methods Mol Biol 684;65-77. PMID: 20960122

Barber02: Barber J (2002). "Photosystem II: a multisubunit membrane protein that oxidises water." Curr Opin Struct Biol 12(4);523-30. PMID: 12163077

Barber06a: Barber J (2006). "Photosystem II: an enzyme of global significance." Biochem Soc Trans 34(Pt 5);619-31. PMID: 17052167

Barber99: Barber J, Kuhlbrandt W (1999). "Photosystem II." Curr Opin Struct Biol 9(4);469-75. PMID: 10449373

Barros99: Barros MH, Nobrega FG (1999). "YAH1 of Saccharomyces cerevisiae: a new essential gene that codes for a protein homologous to human adrenodoxin." Gene 233(1-2);197-203. PMID: 10375636

Bellafiore02: Bellafiore S, Ferris P, Naver H, Gohre V, Rochaix JD (2002). "Loss of Albino3 leads to the specific depletion of the light-harvesting system." Plant Cell 14(9);2303-14. PMID: 12215522

BerryLowe92: Berry-Lowe SL, Johnson CH, Schmidt GW (1992). "Nucleotide Sequence of the psbB Gene of Chlamydomonas reinhardtii Chloroplasts." Plant Physiol 98(4);1541-3. PMID: 16668833

Berthold95: Berthold DA, Schmidt CL, Malkin R (1995). "The deletion of petG in Chlamydomonas reinhardtii disrupts the cytochrome bf complex." J Biol Chem 270(49);29293-8. PMID: 7493961

Bertsch91: Bertsch J, Malkin R (1991). "Nucleotide sequence of the petA (cytochrome f) gene from the green alga, Chlamydomonas reinhardtii." Plant Mol Biol 17(1);131-3. PMID: 1868213

Bingham91: Bingham SE, Xu RH, Webber AN (1991). "Transformation of chloroplasts with the psaB gene encoding a polypeptide of the photosystem I reaction center." FEBS Lett 292(1-2);137-40. PMID: 1959594

Boekema95: Boekema EJ, Hankamer B, Bald D, Kruip J, Nield J, Boonstra AF, Barber J, Rogner M (1995). "Supramolecular structure of the photosystem II complex from green plants and cyanobacteria." Proc Natl Acad Sci U S A 92(1);175-9. PMID: 7816811

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Please cite the following article in publications resulting from the use of MetaCyc: Caspi et al, Nucleic Acids Research 42:D459-D471 2014
Page generated by Pathway Tools version 19.5 (software by SRI International) on Tue May 3, 2016, biocyc14.