Editing Photosystem II

{{short description|First protein complex in light-dependent reactions of oxygenic photosynthesis}}
{{Redirect|PSII|the video game console|PlayStation 2|the Indonesian political party|Indonesian Islamic Union Party|other uses|PS2 (disambiguation)}}
[[Image:PhotosystemII.PNG|right|thumb|400px|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, [[enzyme]]s capture [[photons]] of light to energize [[electrons]] that are then transferred through a variety of [[coenzymes]] and [[Cofactor (biochemistry)|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 [[Photodissociation|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 [[Adenosine triphosphate|ATP]].  The energized electrons transferred to plastoquinone are ultimately used to reduce {{chem|NADP|+}} to [[NADPH]] or are used in [[light-dependent reactions|non-cyclic electron flow]].<ref>{{cite journal | vauthors = Loll B, Kern J, Saenger W, Zouni A, Biesiadka J | title = Towards complete cofactor arrangement in the 3.0 A resolution structure of photosystem II | journal = Nature | volume = 438 | issue = 7070 | pages = 1040–4 | date = December 2005 | pmid = 16355230 | doi = 10.1038/nature04224 | bibcode = 2005Natur.438.1040L | s2cid = 4394735 }}</ref> [[DCMU]] is a chemical often used in laboratory settings to inhibit photosynthesis. When present, DCMU inhibits electron flow from photosystem II to plastoquinone.

== Structure of complex ==
[[Image:Photosystem-II 2AXT.PNG|right|thumb|400px|Cyanobacterial photosystem II, Monomer, PDB 2AXT.]]
[[File:Photosystem II - multilingual.svg|right|thumb|Schematic of PSII, highlighting electron transfer.]]
The core of PSII consists of a pseudo-symmetric heterodimer of two homologous proteins D1 and D2.<ref name=Rutherford2003>{{cite journal | vauthors = Rutherford AW, Faller P | title = Photosystem II: evolutionary perspectives | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 358 | issue = 1429 | pages = 245–53 | date = January 2003 | pmid = 12594932 | pmc = 1693113 | doi = 10.1098/rstb.2002.1186 | authorlink1 = Bill Rutherford }}</ref> Unlike the reaction centers of all other [[photosystem]]s in which the positive charge sitting on the chlorophyll dimer that undergoes the initial photoinduced charge separation is equally shared by the two monomers, in intact PSII the charge  is mostly localized on one chlorophyll center (70−80%).<ref>{{cite journal | vauthors = Okubo T, Tomo T, Sugiura M, Noguchi T | title = Perturbation of the structure of P680 and the charge distribution on its radical cation in isolated reaction center complexes of photosystem II as revealed by fourier transform infrared spectroscopy | journal = Biochemistry | volume = 46 | issue = 14 | pages = 4390–7 | date = April 2007 | pmid = 17371054 | doi = 10.1021/bi700157n }}</ref> Because of this, P680<sup>+</sup> is highly oxidizing and can take part in the splitting of water.<ref name=Rutherford2003/>

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 lipids, the {{chem|Mn|4|Ca|O|5}} cluster (including two chloride ions), one non heme {{chem|Fe|2+}} and two putative {{chem|Ca|2+}} ions per monomer.<ref name=Guskov09/>  There are several crystal structures of photosystem II.<ref>
{{cite book | vauthors = Junko Y, Kern J, Yachandra VK, Nilsson H, Koroidov S, Messinger J | veditors = Kroneck PM, Sosa Torres ME| title = Sustaining Life on Planet Earth: Metalloenzymes Mastering Dioxygen and Other Chewy Gases | series = Metal Ions in Life Sciences | volume = 15 | pages = 13–43 | year = 2015 | publisher = Springer | chapter = Chapter 2, Section 3 ''X-Ray Diffraction and Spectroscopy of Photosystem II at Room Temperature Using Femtosecond X-Ray Pulses'' | doi = 10.1007/978-3-319-12415-5_2 | pmid = 25707465 | pmc = 4688042 | isbn = 978-3-319-12414-8 }}
</ref> The [[Protein Data Bank|PDB]] accession codes for this protein are {{PDB link|3WU2}}, {{PDB link|3BZ1}}, {{PDB link|3BZ2}} (3BZ1 and 3BZ2 are monomeric structures of the Photosystem II dimer),<ref name=Guskov09>{{cite journal | vauthors = Guskov A, Kern J, Gabdulkhakov A, Broser M, Zouni A, Saenger W | title = Cyanobacterial photosystem II at 2.9-A resolution and the role of quinones, lipids, channels and chloride | journal = Nature Structural & Molecular Biology | volume = 16 | issue = 3 | pages = 334–42 | date = March 2009 | pmid = 19219048 | doi = 10.1038/nsmb.1559 | s2cid = 23034289 }}</ref> {{PDB link|2AXT}}, {{PDB link|1S5L}}, {{PDB link|1W5C}}, {{PDB link|1ILX}}, {{PDB link|1FE1}}, {{PDB link|1IZL}}. 
{| class="wikitable"
|+Protein Subunits (only with known function)
|-
!'''Subunit'''
!'''Family'''
!'''Function'''
|-
|D1 (PsbA)
|rowspan=2|[[Photosynthetic reaction centre protein family]]
|Reaction center protein, binds Chlorophyll P680, pheophytin, beta-carotene, quinone and manganese center
|-
|D2 (PsbD)
|Reaction center protein
|-
|CP43 (PsbC)
|rowspan=2|[[Photosystem II light-harvesting protein]]
|Binds manganese center
|-
|CP47 (PsbB)
|
|-
|O
| [[Manganese-stabilising protein]] ({{InterPro|IPR002628}})
|Manganese Stabilizing Protein
|-
|style="text-align:center" colspan=3|By convention, gene names are formed by Psb + subunit letter. For example, subunit O is ''PsbO''. The exceptions are D1 (''PsbA'') and D2 (''PsbD'').
|}

{|class=wikitable
|+Coenzymes/Cofactors
|-
!'''Cofactor'''
!'''Function'''
|-
|[[Chlorophyll]]
|Absorbs light energy and converts it to chemical energy
|-
|[[Beta-carotene]]
|Quench excess photoexcitation energy
|-
|[[Heme B]]559
|Bound to [[Cytochrome b559]] (PsbE&ndash;PsbF) as a secondary/protective electron carrier
|-
|[[Pheophytin]]
|Primary electron acceptor
|-
|[[Plastoquinone]]
|Mobile intra-thylakoid membrane electron carrier
|-
|[[Oxygen-evolving complex|Manganese center]]
|Also known as the oxygen evolving center, or OEC
|}

{{Infobox enzyme
| Name       = Photosystem II
| EC_number  = 1.10.3.9
| CAS_number =
| GO_code    = 
| image      = 
| width      = 
| caption    =  
}}

==Oxygen-evolving complex (OEC)==
[[File:Manganese cluster in the oxygen-evolving complex.svg|thumb|200px|left|Proposed structure of Manganese Center]]
{{main|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 +3 to +4)<ref>{{cite journal |doi=10.1146/annurev-biochem-011520-104801 |title=Current Understanding of the Mechanism of Water Oxidation in Photosystem II and Its Relation to XFEL Data |year=2020 |last1=Cox |first1=Nicholas |last2=Pantazis |first2=Dimitrios A. |last3=Lubitz |first3=Wolfgang |journal=Annual Review of Biochemistry |volume=89 |pages=795–820 |pmid=32208765 |s2cid=214645936 |doi-access=free }}</ref> 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. It is composed of three protein subunits, OEE1 (PsbO), OEE2 (PsbP) and OEE3 (PsbQ); a fourth PsbR peptide is associated nearby.

The first structural model of the oxygen-evolving complex was solved using [[X-ray crystallography]] from frozen protein crystals with a resolution of 3.8[[Ångström|Å]] in 2001.<ref>{{cite journal | vauthors = Zouni A, Witt HT, Kern J, Fromme P, Krauss N, Saenger W, Orth P | title = Crystal structure of photosystem II from Synechococcus elongatus at 3.8 A resolution | language = En | journal = Nature | volume = 409 | issue = 6821 | pages = 739–43 | date = February 2001 | pmid = 11217865 | doi = 10.1038/35055589 | bibcode = 2001Natur.409..739Z | s2cid = 4344756 }}</ref> Over the next years the resolution of the model was gradually increased to 2.9[[Ångström|Å]].<ref>{{cite journal | vauthors = Kamiya N, Shen JR | title = Crystal structure of oxygen-evolving photosystem II from Thermosynechococcus vulcanus at 3.7-A resolution | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 100 | issue = 1 | pages = 98–103 | date = January 2003 | pmid = 12518057 | pmc = 140893 | doi = 10.1073/pnas.0135651100 | bibcode = 2003PNAS..100...98K | doi-access = free }}</ref><ref>{{cite journal | vauthors = Ferreira KN, Iverson TM, Maghlaoui K, Barber J, Iwata S | title = Architecture of the photosynthetic oxygen-evolving center | journal = Science | volume = 303 | issue = 5665 | pages = 1831–8 | date = March 2004 | pmid = 14764885 | doi = 10.1126/science.1093087 | bibcode = 2004Sci...303.1831F | s2cid = 31521054 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Guskov A, Kern J, Gabdulkhakov A, Broser M, Zouni A, Saenger W | title = Cyanobacterial photosystem II at 2.9-A resolution and the role of quinones, lipids, channels and chloride | language = En | journal = Nature Structural & Molecular Biology | volume = 16 | issue = 3 | pages = 334–42 | date = March 2009 | pmid = 19219048 | doi = 10.1038/nsmb.1559 | s2cid = 23034289 }}</ref> While obtaining these structures was in itself a great feat, they did not show the oxygen-evolving complex in full detail. In 2011 the OEC of PSII was resolved to a level of 1.9Å revealing five oxygen atoms serving as oxo bridges linking the five metal atoms and four water molecules bound to the {{chem|Mn|4|Ca|O|5}} 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.<ref name="pmid21499260">{{cite journal | vauthors = Umena Y, Kawakami K, Shen JR, Kamiya N | title = Crystal structure of oxygen-evolving photosystem II at a resolution of 1.9 Å | journal = Nature | volume = 473 | issue = 7345 | pages = 55–60 | date = May 2011 | pmid = 21499260 | doi = 10.1038/nature09913 | bibcode = 2011Natur.473...55U | s2cid = 205224374 | url = http://ousar.lib.okayama-u.ac.jp/files/public/4/47455/20160528084139320094/Nature_473_55–60.pdf }}</ref> 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 laser|X-ray Free Electron Lasers]], such as [[SLAC National Accelerator Laboratory|SLAC]] in the USA. In 2014 the structure observed in 2011 was confirmed.<ref>{{cite journal | vauthors = Suga M, Akita F, Hirata K, Ueno G, Murakami H, Nakajima Y, Shimizu T, Yamashita K, Yamamoto M, Ago H, Shen JR | title = Native structure of photosystem II at 1.95 Å resolution viewed by femtosecond X-ray pulses | language = En | journal = Nature | volume = 517 | issue = 7532 | pages = 99–103 | date = January 2015 | pmid = 25470056 | doi = 10.1038/nature13991 | bibcode = 2015Natur.517...99S | s2cid = 205241611 | url = http://ousar.lib.okayama-u.ac.jp/53637 | url-access = subscription }}</ref> 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,<ref>{{cite journal | vauthors = Young ID, Ibrahim M, Chatterjee R, Gul S, Fuller F, Koroidov S, Brewster AS, Tran R, Alonso-Mori R, Kroll T, Michels-Clark T, Laksmono H, Sierra RG, Stan CA, Hussein R, Zhang M, Douthit L, Kubin M, de Lichtenberg C, Long Vo P, Nilsson H, Cheah MH, Shevela D, Saracini C, Bean MA, Seuffert I, Sokaras D, Weng TC, Pastor E, Weninger C, Fransson T, Lassalle L, Bräuer P, Aller P, Docker PT, Andi B, Orville AM, Glownia JM, Nelson S, Sikorski M, Zhu D, Hunter MS, Lane TJ, Aquila A, Koglin JE, Robinson J, Liang M, Boutet S, Lyubimov AY, Uervirojnangkoorn M, Moriarty NW, Liebschner D, Afonine PV, Waterman DG, Evans G, Wernet P, Dobbek H, Weis WI, Brunger AT, Zwart PH, Adams PD, Zouni A, Messinger J, Bergmann U, Sauter NK, Kern J, Yachandra VK, Yano J | display-authors = 6 | title = Structure of photosystem II and substrate binding at room temperature | language = En | journal = Nature | volume = 540 | issue = 7633 | pages = 453–457 | date = December 2016 | pmid = 27871088 | pmc = 5201176 | doi = 10.1038/nature20161 | bibcode = 2016Natur.540..453Y }}</ref><ref>{{cite journal | vauthors = Suga M, Akita F, Sugahara M, Kubo M, Nakajima Y, Nakane T, Yamashita K, Umena Y, Nakabayashi M, Yamane T, Nakano T, Suzuki M, Masuda T, Inoue S, Kimura T, Nomura T, Yonekura S, Yu LJ, Sakamoto T, Motomura T, Chen JH, Kato Y, Noguchi T, Tono K, Joti Y, Kameshima T, Hatsui T, Nango E, Tanaka R, Naitow H, Matsuura Y, Yamashita A, Yamamoto M, Nureki O, Yabashi M, Ishikawa T, Iwata S, Shen JR | display-authors = 6 | title = Light-induced structural changes and the site of O=O bond formation in PSII caught by XFEL | language = En | journal = Nature | volume = 543 | issue = 7643 | pages = 131–135 | date = March 2017 | pmid = 28219079 | doi = 10.1038/nature21400 | bibcode = 2017Natur.543..131S | s2cid = 205254025 | url = http://ousar.lib.okayama-u.ac.jp/55218 | url-access = subscription }}</ref> suggesting that this may be the site on the oxygen evolving complex, where oxygen is produced.

==Water splitting==
[[Image:Water splitting process.jpg|thumb|200px|left|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 understood in substantial detail.<ref>{{cite journal | vauthors = Vinyard DJ, Brudvig GW | title = Progress Toward a Molecular Mechanism of Water Oxidation in Photosystem II | journal = Annual Review of Physical Chemistry | volume = 68 | issue = 1 | pages = 101–116 | date = May 2017 | pmid = 28226223 | doi = 10.1146/annurev-physchem-052516-044820 | bibcode = 2017ARPC...68..101V }}</ref><ref>{{cite journal | vauthors = Cox N, Pantazis DA, Lubitz W | title = Current Understanding of the Mechanism of Water Oxidation in Photosystem II and Its Relation to XFEL Data | journal = Annual Review of Biochemistry | volume = 89 | issue = 1 | pages = 795–820 | date = June 2020 | pmid = 32208765 | doi = 10.1146/annurev-biochem-011520-104801 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Ibrahim M, Fransson T, Chatterjee R, Cheah MH, Hussein R, Lassalle L, Sutherlin KD, Young ID, Fuller FD, Gul S, Kim IS, Simon PS, de Lichtenberg C, Chernev P, Bogacz I, Pham CC, Orville AM, Saichek N, Northen T, Batyuk A, Carbajo S, Alonso-Mori R, Tono K, Owada S, Bhowmick A, Bolotovsky R, Mendez D, Moriarty NW, Holton JM, Dobbek H, Brewster AS, Adams PD, Sauter NK, Bergmann U, Zouni A, Messinger J, Kern J, Yachandra VK, Yano J | display-authors = 6 | title = Untangling the sequence of events during the S<sub>2</sub> → S<sub>3</sub> transition in photosystem II and implications for the water oxidation mechanism | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 117 | issue = 23 | pages = 12624–12635 | date = June 2020 | pmid = 32434915 | pmc = 7293653 | doi = 10.1073/pnas.2000529117 | doi-access = free | bibcode = 2020PNAS..11712624I }}</ref> 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.<ref>{{cite journal |doi=10.1111/j.1751-1097.1969.tb05696.x |author1 =Joliot P. |author2 =Barbieri G. |author3 =Chabaud R. |title=Un nouveau modele des centres photochimiques du systeme II |journal=Photochemistry and Photobiology |volume=10 |issue=5 |pages=309–329 |year=1969 |s2cid =96744015 }}</ref> 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<ref>{{cite journal | vauthors = Joliot P | title = Period-four oscillations of the flash-induced oxygen formation in photosynthesis | journal = Photosynthesis Research | volume = 76 | issue = 1–3 | pages = 65–72 | year = 2003 | pmid = 16228566 | doi = 10.1023/A:1024946610564 | s2cid = 8742213 }}</ref>). Based on this experiment, Bessel Kok and co-workers <ref>{{cite journal | vauthors = Kok B, Forbush B, McGloin M | title = Cooperation of charges in photosynthetic O2 evolution-I. A linear four step mechanism | journal = Photochemistry and Photobiology | volume = 11 | issue = 6 | pages = 457–75 | date = June 1970 | pmid = 5456273 | doi = 10.1111/j.1751-1097.1970.tb06017.x | s2cid = 31914925 }}</ref> 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<sub>4</sub>-state), OEC returns to its basic S<sub>0</sub>-state. In the absence of light, the OEC will "relax" to the S<sub>1</sub> state; the S<sub>1</sub> state is often described as being "dark-stable". The S<sub>1</sub> state is largely considered to consist of manganese ions with oxidation states of Mn<sup>3+</sup>, Mn<sup>3+</sup>, Mn<sup>4+</sup>, Mn<sup>4+</sup>.<ref name=":0">{{cite journal|vauthors=Kuntzleman T, Yocum CF|date=February 2005|title=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|journal=Biochemistry|volume=44|issue=6|pages=2129–42|doi=10.1021/bi048460i|pmid=15697239}}</ref> Finally, the '''intermediate S-states'''<ref>{{cite journal | vauthors = Jablonsky J, Lazar D | title = Evidence for intermediate S-states as initial phase in the process of oxygen-evolving complex oxidation | journal = Biophysical Journal | volume = 94 | issue = 7 | pages = 2725–36 | date = April 2008 | pmid = 18178650 | pmc = 2267143 | doi = 10.1529/biophysj.107.122861 | bibcode = 2008BpJ....94.2725J }}</ref> 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.<ref>{{cite journal | vauthors = Renger G | title = Mechanism of light induced water splitting in Photosystem II of oxygen evolving photosynthetic organisms | journal = Biochimica et Biophysica Acta (BBA) - Bioenergetics | volume = 1817 | issue = 8 | pages = 1164–76 | date = August 2012 | pmid = 22353626 | doi = 10.1016/j.bbabio.2012.02.005 | doi-access = free }}</ref>

==Inhibitors==
[[Enzyme inhibitor|Inhibitors]] of PSII are used as herbicides. There are two main chemical families, the [[1,3,5-Triazine|triazines]] derived from [[cyanuric chloride]]<ref>{{cite web |url=http://www.alanwood.net/pesticides/class_herbicides.html#chlorotriazine_herbicides |title=Chlorotraizine herbicides |website=alanwood.net|access-date=2021-03-26}}</ref> of which [[atrazine]] and [[simazine]] are the most commonly used and the [[aryl]] [[ureas]] which include [[chlortoluron]] and [[diuron]] (DCMU).<ref>{{cite web |url=http://www.alanwood.net/pesticides/class_herbicides.html#urea_herbicides |title=Urea herbicides |website=alanwood.net|access-date=2021-03-26}}</ref><ref>{{cite book |doi=10.1016/B978-0-444-89440-3.50018-7 |chapter=Herbicides of photosystem II |title=The Photosystems |year=1992 | vauthors = Oettmeier W |pages=349–408 |isbn=9780444894403 }}</ref>

== See also ==
*[[Oxygen evolution]]
*[[P680]]
*[[Photosynthesis]]
*[[Photosystem]]
*[[Photosystem I]]
*[[Photosystem II light-harvesting protein]]
*[[Reaction Centre]]
*[[Photoinhibition]]

== References ==
{{Reflist|35em}}

{{Diphenol family oxidoreductases}}
{{Enzymes}}
{{Multienzyme complexes}}

[[Category:Photosynthesis]]
[[Category:Light reactions]]
[[Category:Manganese enzymes]]
[[Category:EC 1.10.3]]