Everything about Photosystem Ii totally explained
Photosystem II is the first protein complex in
Light Dependent Photosynthesis. It is located in the thylakoid membrane of plants, algea 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's 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
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
PDB accession codes for this protein are,,,, .
| Protein Subunits |
|
| Subunit |
Function |
| D1 |
Reaction center Protein, binds Chlorophyl P680, pheophytin,
beta-carotene,quinone and manganese center
|
| D2 |
Reaction center Protein |
| CP43 |
Binds manganese center |
| CP47 |
|
| cytochrome b559 alpha |
|
| cytochrome b559 beta |
|
| PsbH |
|
| PsbI |
|
| PsbJ |
|
| PsbK |
|
| PsbL |
|
| PsbM |
|
| PsbN |
|
| PsbO |
Manganese Stabilizing Protein |
| PsbT |
|
| PsbV |
|
| PsbX |
|
| PsbZ |
|
| Coenzymes/Cofactors |
|
| Molecule |
Function |
| Chlorophyl |
Absorbs light |
| Beta-Carotene |
|
| Heme b559 |
also Protopophyrin IX containing iron |
| Pheophytin |
|
| Plastoquinone |
|
| Manganese center |
also known as the oxygen evolving center, or OEC |
Electron transfer
Oxygen Evolving Complex (OEC)
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's 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.
Water Oxidation
Photosynthetic water oxidation (or
oxygen evolution) is arguably one of the most important reactions on the planet, since it's the source of practically all the atmosphere's oxygen. The mechanism of water oxidation is still not fully elucidated but there's significant evidence that it occurs by the S-state mechanism.
Further Information
Get more info on 'Photosystem Ii'.
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