Photosystem II
Photosystem II (or water-plastoquinone oxidoreductase) is the first
By replenishing lost electrons with electrons from the
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
The core of PSII consists of a pseudo-symmetric heterodimer of two homologous proteins D1 and D2.[2] Unlike the reaction centers of all other photosystems 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%).[3] Because of this, P680+ is highly oxidizing and can take part in the splitting of water.[2]
Photosystem II (of
4CaO
5 cluster (including two chloride ions), one non heme Fe2+
and two putative Ca2+
ions per monomer.[4] There are several crystal structures of photosystem II.[5] The PDB accession codes for this protein are 3WU2, 3BZ1, 3BZ2 (3BZ1 and 3BZ2 are monomeric structures of the Photosystem II dimer),[4] 2AXT, 1S5L, 1W5C, 1ILX, 1FE1, 1IZL
| Subunit | Family | Function |
|---|---|---|
| D1 (PsbA) | 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) | Photosystem II light-harvesting protein | Binds manganese center |
| CP47 (PsbB) | ||
| O | Manganese-stabilising protein (InterPro: IPR002628) | Manganese Stabilizing Protein |
| By convention, gene names are formed by Psb + subunit letter. For example, subunit O is PsbO. The exceptions are D1 (PsbA) and D2 (PsbD). | ||
| Cofactor | Function |
|---|---|
| Chlorophyll | Absorbs light energy and converts it to chemical energy |
Beta-carotene
|
Quench excess photoexcitation energy |
| Heme B559 | Bound to Cytochrome b559 (PsbE–PsbF) as a secondary/protective electron carrier |
| Pheophytin | Primary electron acceptor |
| Plastoquinone | Mobile intra-thylakoid membrane electron carrier |
| Manganese center | Also known as the oxygen evolving center, or OEC |
| Photosystem II | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Identifiers | |||||||||
ExPASy | NiceZyme view | ||||||||
| KEGG | KEGG entry | ||||||||
| MetaCyc | metabolic pathway | ||||||||
| PRIAM | profile | ||||||||
| PDB structures | RCSB PDB PDBe PDBsum | ||||||||
| |||||||||
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 +4)[6] 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
Water splitting
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.[15][16][17] 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.[18] 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[19]). Based on this experiment, Bessel Kok and co-workers [20] 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 S4-state), OEC returns to its basic S0-state. In the absence of light, the OEC will "relax" to the S1 state; the S1 state is often described as being "dark-stable". The S1 state is largely considered to consist of manganese ions with oxidation states of Mn3+, Mn3+, Mn4+, Mn4+.[21] Finally, the intermediate S-states[22] 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.[23]
Inhibitors
See also
- Oxygen evolution
- P680
- Photosynthesis
- Photosystem
- Photosystem I
- Photosystem II light-harvesting protein
- Reaction Centre
- Photoinhibition
References
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