Photosystem II
1126110
217570078
2008-06-06T16:59:06Z
137.224.252.10
[[Image:Thylakoid membrane.png|thumb|400px|right|Light-dependent reactions of photosynthesis at the thylakoid membrane]]
[[Image:PhotosystemII.PNG|right|thumb|400px|Cyanobacteria photosystem II, Dimer, PDB 2AXT.]]
'''Photosystem II''' is the first protein complex in [[Light Dependent Photosynthesis]]. It is located in the thylakoid membrane of plants, algae and cyanobacteria. The enzyme uses [[photons]] of light to energize electrons which 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 obtaining these electrons from 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 NADP<sup>+</sup> to [[NADPH]] or are used in [[cyclic photophosphorylation]].
==Structure==
[[Image:PSIIpic.PNG|right|thumb|400px|Cyanobacteria photosystem II, Monomer, PDB 2AXT.]]
Photosystem II is composed of 16-18 subunits as well as other accessory, light harvesting proteins. There are several crystal structures of photosystem II. The [[Protein Data Bank|PDB]] accession codes for this protein are {{PDB link|2AXT}}, {{PDB link|1W5C}}, {{PDB link|1ILX}}, {{PDB link|1FE1}}, {{PDB link|1IZL}}.
{| class="wikitable"
|-
! Protein Subunits
!
|-
| row 2,cell 1|<b>Subunit</b>
| row 2,cell 2|<b>Function</b>
|-
| row 3, cell 1| D1
| row 3, cell 2|Reaction center Protein, binds Chlorophyl P680, pheophytin,<br>
beta-carotene,quinone and manganese center
|-
| row 4, cell 1|D2
| row 4, cell 2|Reaction center Protein
|-
| row 5, cell 1|CP43
| row 5, cell 2|Binds manganese center
|-
| row 6, cell 1|CP47
| row 6, cell 2|
|-
| row 7, cell 1|[[cytochrome b559]] alpha
| row 7, cell 2|
|-
| row 8, cell 1|cytochrome b559 beta
| row 8, cell 2|
|-
| row 9, cell 1|PsbH
| row 9, cell 2|
|-
| row 10, cell 1|PsbI
| row 10, cell 2|
|-
| row 11, cell 1|PsbJ
| row 11, cell 2|
|-
| row 12, cell 1|PsbK
| row 12, cell 2|
|-
| row 13, cell 1|PsbL
| row 13, cell 2|
|-
| row 14, cell 1|PsbM
| row 14, cell 2|
|-
| row 15, cell 1|PsbN
| row 15, cell 2|
|-
| row 16, cell 1|PsbO
| row 16, cell 2|Manganese Stabilizing Protein
|-
| row 17, cell 1|PsbT
| row 17, cell 2|
|-
| row 18, cell 1|PsbV
| row 18, cell 2|
|-
| row 19, cell 1|PsbX
| row 19, cell 2|
|-
| row 20, cell 1|PsbZ
| row 20, cell 2|
|-
! Coenzymes/Cofactors
!
|-
| row 22, cell 1|<b>Molecule</b>
| row 22, cell 2|<b>Function</b>
|-
| row 23, cell 1|[[Chlorophyl]]
| row 23, cell 2|Absorbs light
|-
| row 24, cell 1|[[Beta-Carotene]]
| row 24, cell 2|
|-
| row 25, cell 1|[[Heme]] b559
| row 25, cell 2|also [[Protopophyrin IX]] containing iron
|-
| row 26, cell 1|[[Pheophytin]]
| row 26, cell 2|
|-
| row 27, cell 1|[[Plastoquinone]]
| row 27, cell 2|
|-
| row 28, cell 1|[[Manganese center]]
| row 28, cell 2|also known as the oxygen evolving center, or OEC
|}
== Electron transfer ==
==Oxygen Evolving Complex (OEC)==
[[Image:Mncenter.PNG|right|thumb|200px|Proposed structure of Manganese Center]]
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 +5) and one divalent calcium ion. When it oxidizes water, producing dioxygen gas and protons, it sequentially delivers the four electrons from water to a tyrosine (D1-T161) sidechain and thence to P680 itself. The structure of the oxygen evolving complex is still contentious. The structures obtained by [[X-ray crystallography]] are particularly controversial, since there is evidence that the manganese atoms are reduced by the high-intensity [[X-rays]] used, altering the observed OEC structure. However, crystallography in combination with a variety of other (less damaging) spectroscopic methods such as [[EXAFS]] and [[electron paramagnetic resonance]] have given a fairly clear idea of the structure of the cluster. One possibility is the cubane-like structure shown on the right.<ref>K. N. Ferreira, T. M. Iverson, K. Maghlaoui, J. Barber, S. Iwata; Science (2004) 303; pp. 1831-1838</ref>
==Water Oxidation==
[[image:Oxygen evolving complex.png|thumb|300px|Oxygen evolution by water oxidation during [[photosynthesis]]. The jagged lines represent four [[photon]]s oxidizing the central cluster of the [[oxygen evolving complex]] by exciting and removing four electrons through a cycle of ''S-states''.]]
Photosynthetic water oxidation (or [[oxygen evolution]]) is arguably one of the most important reactions on the planet, since it is the source of practically all the atmosphere's oxygen. The mechanism of water oxidation is still not fully elucidated but there is significant evidence that it occurs by the S-state mechanism.
==See Also==
*[[Photosynthesis]]
*[[Oxygen Evolution]]
*[[Photosystem]]
*[[Reaction Centre]]
*[[P680]]
==References==
{{Reflist}}
==External Links==
http://www.bio.ic.ac.uk/research/barber/psIIimages/PSII.html
[[Category:Photosynthesis]]