Photosystem II: Difference between revisions

<Structure load='1s5l' size='400' frame='true' align='right' caption='Photosystem II, [[1s5l]]' scene='' />

-->

This structure of Photosystem II was crystallized from the cyanobacteria, ''Thermosynechococcus elongatus'', at 3.0Å <ref>PMID: 16355230</ref> and at 3.50 Å <ref>PMID: 14764885</ref>. PDB codes are [[2axt]] and [[1s5l]], respectively.  Cyanobacteria and plants both contain Photosystem II while photosynthetic bacteria contain the bacterial reaction center.  This photosynthetic protein complex is associated with a variety of functional ligands. It is a <scene name='Photosystem_II/Psii_dimer/1'>dimer</scene> composed mainly of alpha-helices.  Nineteen <scene name='Photosystem_II/Protein_only/1'>subunits</scene> are in each monomer, with multiple extrinsic subunits associated with the oxygen evolving complex missing from this crystallization.  Photosystem II is a membrane bound protein complex that in plants is associated with the thylakoid membrane of chloroplasts.  <scene name='Photosystem_II/Hydrophobic_polar/1'>Polar and hydrophobic</scene> regions correlate with membrane associated nature of the protein.  '''<FONT COLOR="#616D7E">Hydrophobic</FONT>''' helices make up the transmembranal portion, while '''<FONT COLOR="#C031C7">polar</FONT>''' residues are concentrated externally on either side of the membrane.

Photosystem II is an integral part of photosynthesis, the conversion of light energy into chemical energy by living organisms.  Photosystem II is linked to a variety of other proteins, including Photosytem I.  These proteins ultimately produce NADPH and ATP that power the Calvin cycle.  Using this energy, glucose is synthesized from carbon dioxide and water.

<scene name='Photosystem_II/Chlorophyll_green/4'>Chlorophyll</scene> surround Photosystem II and capture energy from sunlight, exciting electrons.  Chlorophyll are highly conjugated and absorb visible light, along with accessory light harvesting pigments such as <scene name='Photosystem_II/Betacarotene/3'>beta carotene</scene>. Beta carotene absorbs visible light of other wavelengths and also protects Photosystem II by destroying reactive oxygen species that result from this photoexcitation. [[Image:b-car.svg.png|b-car.svg.png|thumb|right|400px|structure of beta carotene]]  Electrons are passed from chlorophyll to <scene name='Photosystem_II/Pheophytin_purple/5'>pheophytin</scene>.  Pheophytin are very similar to chlorophyll except they  contain 2 H⁺ instead of a Mg²⁺ ion.  From the pheophytin, electrons transferred to <scene name='Photosystem_II/Quinone_pink/5'>plastoquinones</scene>, which are reduced.  Located between each pair of quinones, an iron helps to transfer the electron. These plastoquinones eventually move to a plastoquinone pool which travels to another large protein subunit, cytochrome b ₆/ f.  Eventually these electrons reduce NADP⁺ to NADPH.  The <scene name='Photosystem_II/Electron_pathway/3'>electron pathway</scene> through Photosystem II is shown, with '''<FONT COLOR="#F87217">beta-carotenes</FONT>''', '''<FONT COLOR="#571B7e">pheophytins</FONT>''', '''<FONT COLOR="#E42217">iron</FONT>''' and '''<FONT COLOR="#F535AA">plasotoquinones</FONT>'''.

Another important facet of Photosystem II is its ability to oxidize water to oxygen with its <scene name='Photosystem_II/Oxygen_evolving_centers/11'>oxygen evolving centers</scene>.  These centers are <scene name='Photosystem_II/Single_oxygen_evolving/1'>cubane-like</scene> structures with 3 '''<FONT COLOR="#8D38C9">manganese</FONT>''', 4 '''<FONT COLOR="#C11B17">oxygen</FONT>''' and a '''<FONT COLOR="#59E817">calcium</FONT>''' linked to a fourth manganese<ref>PMID: 14764885</ref>.  Oxidation of water leaves 2 H ⁺ on the lumenal side of the membrane, helping to establish the proton gradient essential for ATP synthesis in the CF₁CF₀-ATP sythase protein.

[[3arc]], [[3a0b]], [[3a0h]] – PSII – ''Thermosynechococcus vulcanos''<br />