Photosystem II

Light-dependent reactions of photosynthesis at the thylakoid membrane

Cyanobacteria photosystem II, Dimer, PDB 2AXT.

Photosystem II is the first protein complex in the Light-dependent reactions. 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 ATP. The energized electrons transferred to plastoquinone are ultimately used to reduce NADP⁺ to NADPH or are used in Cyclic Photophosphorylation.

Structure

Cyanobacteria photosystem II, Monomer, PDB 2AXT.

Photosystem II (of cyanobacteria and green plants) is composed of 20 subunits as well as other accessory, light harvesting proteins. Each photosystem II contains at least 99 cofactors--35 chlorophyll a, 12 beta-carotene, two pheophytin, three plastoquinone, two heme, bicarbonate, 25 lipid and seven n-dodecyl-beta-D-maltoside detergent molecules, the six components of the Mn₄Ca cluster (including chloride ion), and one Fe²⁺ and two putative Ca²⁺ ion per monomer (Guskov et al., 2009). There are several crystal structures of photosystem II. The PDB accession codes for this protein are 3BZ1, 3BZ2(3BZ1 and 3BZ2 are monomeric structures of the Photosystem II dimer) ^[1] 2AXT, 1S5L, 1W5C, 1ILX, 1FE1, 1IZL.

Oxygen Evolving Complex (OEC)

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-Y161) 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.^[2]

Water Oxidation

Photosynthetic water oxidation (or oxygen evolution) is arguably one of the most important reactions on the planet, since it is the source of nearly 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.^{[citation needed]}

See also

• Photosynthesis
• Oxygen evolution
• Photosystem
• Reaction Centre
• P680

References

Loll, B., Kern, J., Saenger, W., Zouni, A., and J. Biesiadka. 2005. Towards complete cofactor arrangement in the 3.0 A resolution structure of photosystem II. Nature 438: 1040-1044.

External links

• http://www.bio.ic.ac.uk/research/barber/psIIimages/PSII.html