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 Energy → Photosynthesis|
Expected Taxonomic Range:
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.
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), 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 188.8.131.52), 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.
Unification Links: AraCyc:PWY-101
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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
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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
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
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
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
Boudreaux01: Boudreaux B, MacMillan F, Teutloff C, Agalarov R, Gu F, Grimaldi S, Bittl R, Brettel K, Redding K (2001). "Mutations in both sides of the photosystem I reaction center identify the phylloquinone observed by electron paramagnetic resonance spectroscopy." J Biol Chem 276(40);37299-306. PMID: 11489879
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