Photosystem II - wikipedia Photosystem II This article is about the protein complex. For the Indonesian political party, see Indonesian Islamic Union Party. Cyanobacteria photosystem II, Dimer, PDB 2AXT Photosystem II ( or water - plastoquinone oxidoreductase ) is the first protein complex in the light - dependent reactions of oxygenic photosynthesis. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. Within the photosystem, enzymes capture photons of light to energize electrons that are then transferred through a variety of coenzymes and cofactors to reduce plastoquinone to plastoquinol. The energized electrons are replaced by oxidizing water to form hydrogen ions and molecular oxygen. By replenishing lost electrons with electrons from the splitting of water, photosystem II provides the electrons for all of photosynthesis to occur. The hydrogen ions ( protons ) generated by the oxidation of water help to create a proton gradient that is used by ATP synthase to generate ATP. The energized electrons transferred to plastoquinone are ultimately used to reduce NADP to NADPH or are used in cyclic photophosphorylation. Contents 1 Structure 2 Oxygen - evolving complex ( OEC ) 3 Water splitting 4 See also 5 References Structure Cyanobacterial photosystem II, Monomer, PDB 2AXT. The core of PSII consists of a pseudo-symmetric heterodimer of two homologous proteins D1 and D2. Unlike the reaction centers of all other photosystems which have a special pair of closely spaced chlorophyll molecules, the pigment that undergoes the initial photoinduced charge separation in PSII is a chlorophyll monomer. Because the positive charge is not shared across two molecules, the ionised pigment is highly oxidizing and can take part in the splitting of water. Photosystem II ( of cyanobacteria and green plants ) is composed of around 20 subunits ( depending on the organism ) as well as other accessory, light - harvesting proteins. Each photosystem II contains at least 99 cofactors : 35 chlorophyll a, 12 beta - carotene, two pheophytin, two plastoquinone, two heme, one bicarbonate, 20 lipid molecules, the Mn CaO cluster ( including two chloride ions ), one non heme Fe and two putative Ca ions per monomer. There are several crystal structures of photosystem II. The PDB accession codes for this protein are 3WU2, 3BZ1, 3BZ2 ( 3BZ1 and 3BZ2 are monomeric structures of the Photosystem II dimer ), 2AXT, 1S5L, 1W5C, 1ILX, 1FE1, 1IZL. Photosystem II Light Harvesting complex protein Identifiers Symbol PSII Pfam PF00421 InterPro IPR000932 TCDB 3. E. 2 OPM superfamily OPM protein 3arc Available protein structures : Pfam structures PDB RCSB PDB ; PDBe ; PDBj PDBsum structure summary Protein Subunits ( only with known function ) Subunit Function D1 Reaction center Protein, binds Chlorophyll P680, pheophytin, beta - carotene, quinone and manganese center D2 Reaction center Protein CP43 Binds manganese center CP47 PsbO Manganese Stabilizing Protein Coenzymes / Cofactors Molecule Function Chlorophyll Absorbs light energy and converts it to chemical energy Beta - carotene quench excess photoexcitation energy Heme b559 also Protoporphyrin IX containing iron Pheophytin Primary electron acceptor Plastoquinone Mobile intra-thylakoid membrane electron carrier Manganese center also known as the oxygen evolving center, or OEC Photosystem II Identifiers EC number 1.10. 3.9 Databases IntEnz IntEnz view BRENDA BRENDA entry ExPASy NiceZyme view KEGG KEGG entry MetaCyc metabolic pathway PRIAM profile PDB structures RCSB PDB PDBe PDBsum Search PMC articles PubMed articles NCBI proteins Oxygen - evolving complex ( OEC ) Proposed structure of Manganese Center Main article : oxygen - evolving complex The oxygen - evolving complex is the site of water oxidation. It is a metallo - oxo cluster comprising four manganese ions ( in oxidation states ranging from + 2 to + 4 ) and one divalent calcium ion. When it oxidizes water, producing oxygen gas and protons, it sequentially delivers the four electrons from water to a tyrosine ( D1 - Y161 ) sidechain and then to P680 itself. The first structural model of the oxygen - evolving complex are solved by X-ray crystallography from frozen protein crystals with a resolution of 3.8 Å in 2001. Over the next years the resolution of the model is gradually increased to 2.9 Å. While obtaining these structures, in itself was a great feat, they do not show the oxygen - evolving complex in full detail. In 2011 the OEC of PSII was resolved to a level of 1.9 Å revealing five oxygen atoms serving as oxo bridges linking the five metal atoms and four water molecules bound to the Mn4CaO5 cluster ; more than 1,300 water molecules were found in each photosystem II monomer, some forming extensive hydrogen - bonding networks that may serve as channels for protons, water or oxygen molecules. At this stage, it is suggested that the structures obtained by X-ray crystallography are biased, since there is evidence that the manganese atoms are reduced by the high - intensity X-rays used, altering the observed OEC structure. This incentivized researchers to take their crystals to a different X-ray facilities, called X-ray Free Electron Lasers, such as SLAC in the USA. In 2014 the structure observed in 2011 was confirmed. Knowing the structure of Photosystem II did not suffice to reveal how it works exactly. So now the race has started to solve the structure of Photosystem II at different stages in the mechanistic cycle ( discussed below ). Currently structures of the S1 state and the S3 state 's have been published almost simultaneously from two different groups, showing the addition of an oxygen molecule designated O6 between Mn1 and Mn4, suggesting that this may be the site on the oxygen evolving complex, where oxygen is produced. Water splitting Water - splitting process : Electron transport and regulation. The first level ( A ) shows the original Kok model of the S - states cycling, the second level ( B ) shows the link between the electron transport ( S - states advancement ) and the relaxation process of the intermediate S - states ( ( YzSn ), n = 0, 1, 2, 3 ) formation Photosynthetic water splitting ( or oxygen evolution ) is one of the most important reactions on the planet, since it is the source of nearly all the atmosphere 's oxygen. Moreover, artificial photosynthetic water - splitting may contribute to the effective use of sunlight as an alternative energy - source. The mechanism of water oxidation is still not fully elucidated, but we know many details about this process. The oxidation of water to molecular oxygen requires extraction of four electrons and four protons from two molecules of water. The experimental evidence that oxygen is released through cyclic reaction of oxygen evolving complex ( OEC ) within one PSII was provided by Pierre Joliot et al. They have shown that, if dark - adapted photosynthetic material ( higher plants, algae, and cyanobacteria ) is exposed to a series of single turnover flashes, oxygen evolution is detected with typical period - four damped oscillation with maxima on the third and the seventh flash and with minima on the first and the fifth flash ( for review, see ). Based on this experiment, Bessel Kok and co-workers introduced a cycle of five flash - induced transitions of the so - called S - states, describing the four redox states of OEC : When four oxidizing equivalents have been stored ( at the S - state ), OEC returns to its basic S - state. In the absence of light, the OEC will `` relax '' to the S state ; the S state is often described as being `` dark - stable ''. The S state is largely considered to consist of manganese ions with oxidation states of Mn, Mn, Mn, Mn. Finally, the intermediate S - states were proposed by Jablonsky and Lazar as a regulatory mechanism and link between S - states and tyrosine Z. In 2012, Renger expressed the idea of internal changes of water molecules into typical oxides in different S - states during water splitting. In 2017, Dolai showed almost complete concept of water splitting by the way of structural analysis of S - states and developed oxides from the water molecules. Dolai 's Diagram of S - states : Dolai 's diagrams are perfect pictures of different S - states in water - splitting during photosynthesis. These diagrams can express complete figures of intermediate S - states as well as the development of typical oxides ( monoxide ( 2H2O ), hydroxide ( OH. H O ), peroxide ( H O ), super oxide ( HO ) and di - oxygen ( O ) ) ; which are formed due to interchange of water molecules in S - states. The arrangement of hydrogen bonding is main criteria of the diagrams. The effect of photon ( hv ), emission of electron ( e ) and production of hydrogen ion ( H+ ) from water molecules can be shown by the diagrams in a more specific way of experimental detection. The diagrams almost completely analyze the pathway of water - splitting in photosynthesis. See also Biology portal Oxygen evolution P680 Photosynthesis Photosystem Photosystem I Photosystem II light - harvesting protein Reaction Centre Photoinhibition References Jump up ^ Loll B, Kern J, Saenger W, Zouni A, Biesiadka J ( December 2005 ). `` Towards complete cofactor arrangement in the 3.0 A resolution structure of photosystem II ''. Nature. 438 ( 7070 ) : 1040 -- 4. doi : 10.1038 / nature04224. PMID 16355230. ^ Jump up to : Rutherford AW, Faller P ( January 2003 ). `` Photosystem II : evolutionary perspectives ''. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. 358 ( 1429 ) : 245 -- 53. doi : 10.1098 / rstb. 2002.1186. PMC 1693113. PMID 12594932. ^ Jump up to : Guskov A, Kern J, Gabdulkhakov A, Broser M, Zouni A, Saenger W ( March 2009 ). `` Cyanobacterial photosystem II at 2.9 - A resolution and the role of quinones, lipids, channels and chloride ''. Nature Structural & Molecular Biology. 16 ( 3 ) : 334 -- 42. doi : 10.1038 / nsmb. 1559. PMID 19219048. Jump up ^ Yano, Junko ; Kern, Jan ; Yachandra, Vittal K. ; Nilsson, Håkan ; Koroidov, Sergey ; Messinger, Johannes ( 2015 ). `` Chapter 2, Section 3 X-Ray Diffraction and Spectroscopy of Photosystem II at Room Temperature Using Femtosecond X-Ray Pulses ''. In Peter M.H. Kroneck and Martha E. Sosa Torres. Sustaining Life on Planet Earth : Metalloenzymes Mastering Dioxygen and Other Chewy Gases. Metal Ions in Life Sciences. 15. Springer. p. 24. doi : 10.1007 / 978 - 3 - 319 - 12415 - 5_2. ^ Jump up to : Kuntzleman T, Yocum CF ( February 2005 ). `` Reduction - induced inhibition and Mn ( II ) release from the photosystem II oxygen - evolving complex by hydroquinone or NH2OH are consistent with a Mn ( III ) / Mn ( III ) / Mn ( IV ) / Mn ( IV ) oxidation state for the dark - adapted enzyme ''. Biochemistry. 44 ( 6 ) : 2129 -- 42. doi : 10.1021 / bi048460i. PMID 15697239. Jump up ^ Zouni A, Witt HT, Kern J, Fromme P, Krauss N, Saenger W, Orth P ( February 2001 ). `` Crystal structure of photosystem II from Synechococcus elongatus at 3.8 A resolution ''. Nature. 409 ( 6821 ) : 739 -- 43. doi : 10.1038 / 35055589. PMID 11217865. Jump up ^ Kamiya N, Shen JR ( January 2003 ). `` Crystal structure of oxygen - evolving photosystem II from Thermosynechococcus vulcanus at 3.7 - A resolution ''. Proceedings of the National Academy of Sciences of the United States of America. 100 ( 1 ) : 98 -- 103. doi : 10.1073 / pnas. 0135651100. PMC 140893. PMID 12518057. Jump up ^ Ferreira KN, Iverson TM, Maghlaoui K, Barber J, Iwata S ( March 2004 ). `` Architecture of the photosynthetic oxygen - evolving center ''. Science. 303 ( 5665 ) : 1831 -- 8. doi : 10.1126 / science. 1093087. PMID 14764885. Jump up ^ Guskov A, Kern J, Gabdulkhakov A, Broser M, Zouni A, Saenger W ( March 2009 ). `` Cyanobacterial photosystem II at 2.9 - A resolution and the role of quinones, lipids, channels and chloride ''. Nature Structural & Molecular Biology. 16 ( 3 ) : 334 -- 42. doi : 10.1038 / nsmb. 1559. PMID 19219048. Jump up ^ Umena Y, Kawakami K, Shen JR, Kamiya N ( May 2011 ). `` Crystal structure of oxygen - evolving photosystem II at a resolution of 1.9 Å ''. Nature. 473 ( 7345 ) : 55 -- 60. doi : 10.1038 / nature09913. PMID 21499260. Jump up ^ Suga M, Akita F, Hirata K, Ueno G, Murakami H, Nakajima Y, Shimizu T, Yamashita K, Yamamoto M, Ago H, Shen JR ( January 2015 ). `` Native structure of photosystem II at 1.95 Å resolution viewed by femtosecond X-ray pulses ''. Nature. 517 ( 7532 ) : 99 -- 103. doi : 10.1038 / nature13991. PMID 25470056. Jump up ^ Young ID, Ibrahim M, Chatterjee R, Gul S, Fuller F, Koroidov S, et al. ( December 2016 ). `` Structure of photosystem II and substrate binding at room temperature ''. Nature. 540 ( 7633 ) : 453 -- 457. doi : 10.1038 / nature20161. PMC 5201176. PMID 27871088. Jump up ^ Suga M, Akita F, Sugahara M, Kubo M, Nakajima Y, Nakane T, et al. ( March 2017 ). `` Light - induced structural changes and the site of O =O bond formation in PSII caught by XFEL ''. Nature. 543 ( 7643 ) : 131 -- 135. doi : 10.1038 / nature21400. PMID 28219079. Jump up ^ Joliot P. ; Barbieri G. ; Chabaud R. ( 1969 ). `` Un nouveau modele des centres photochimiques du systeme II ''. Photochemistry and Photobiology. 10 ( 5 ) : 309 -- 329. doi : 10.1111 / j. 1751 - 1097.1969. tb05696. x. Jump up ^ Joliot P ( 2003 ). `` Period - four oscillations of the flash - induced oxygen formation in photosynthesis ''. Photosynthesis Research. 76 ( 1 - 3 ) : 65 -- 72. doi : 10.1023 / A : 1024946610564. PMID 16228566. Jump up ^ Kok B, Forbush B, McGloin M ( June 1970 ). `` Cooperation of charges in photosynthetic O2 evolution - I. A linear four step mechanism ''. Photochemistry and Photobiology. 11 ( 6 ) : 457 -- 75. doi : 10.1111 / j. 1751 - 1097.1970. tb06017. x. PMID 5456273. Jump up ^ Jablonsky J, Lazar D ( April 2008 ). `` Evidence for intermediate S - states as initial phase in the process of oxygen - evolving complex oxidation ''. Biophysical Journal. 94 ( 7 ) : 2725 -- 36. Bibcode : 2008BpJ... 94.2725 J. doi : 10.1529 / biophysj. 107.122861. PMC 2267143. PMID 18178650. Jump up ^ Renger G ( August 2012 ). `` Mechanism of light induced water splitting in Photosystem II of oxygen evolving photosynthetic organisms ''. Biochimica et Biophysica Acta. 1817 ( 8 ) : 1164 -- 76. doi : 10.1016 / j. bbabio. 2012.02. 005. PMID 22353626. Jump up ^ Dolai, U ( 2017 ). `` Chemical Scheme of Water - Splitting Process during Photosynthesis by the way of Experimental Analysis ''. IOSR Journal of Pharmacy and Biological Sciences. 12 ( 6 ) : 65 -- 67. doi : 10.9790 / 3008 - 1206026567. ISSN 2319 - 7676. Oxidoreductases : diphenol family ( EC 1.10 ) 1.10. 1 Trans - acenaphthene - 1, 2 - diol dehydrogenase 1.10. 2 Coenzyme Q - cytochrome c reductase 1.10. 3 Catechol oxidase Laccase 1.10. 99 Cytochrome b6f complex Other Alternative oxidase Enzymes Activity Active site Binding site Catalytic triad Oxyanion hole Enzyme promiscuity Catalytically perfect enzyme Coenzyme Cofactor Enzyme catalysis Regulation Allosteric regulation Cooperativity Enzyme inhibitor Enzyme activator Classification EC number Enzyme superfamily Enzyme family List of enzymes Kinetics Enzyme kinetics Eadie -- Hofstee diagram Hanes -- Woolf plot Lineweaver -- Burk plot Michaelis -- Menten kinetics Types EC1 Oxidoreductases ( list ) EC2 Transferases ( list ) EC3 Hydrolases ( list ) EC4 Lyases ( list ) EC5 Isomerases ( list ) EC6 Ligases ( list ) Molecular and Cellular Biology portal Retrieved from `` https://en.wikipedia.org/w/index.php?title=Photosystem_II&oldid=843336082 '' Categories : Photosynthesis Light reactions Manganese enzymes EC 1.10. 3 Hidden categories : Protein pages needing a picture Talk Search Contents About Wikipedia Čeština Deutsch Eesti Français Italiano Polski Русский Српски / srpski Srpskohrvatski / српскохрватски Українська 3 more Edit links This page was last edited on 28 May 2018, at 13 : 28. 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Question: where do the electrons needed by photosystem two originate
Answer: from the splitting of water