Photosystem I
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Photosystem I (PSI, or plastocyanin–ferredoxin oxidoreductase) is one of two photosystems in the photosynthetic light reactions of algae, plants, and cyanobacteria. Photosystem I <ref name="pmid3333014">Template:Cite journal</ref> is an integral membrane protein complex that uses light energy to catalyze the transfer of electrons across the thylakoid membrane from plastocyanin to ferredoxin. Ultimately, the electrons that are transferred by Photosystem I are used to produce the moderate-energy hydrogen carrier NADPH.<ref>Template:Cite journal</ref> The photon energy absorbed by Photosystem I also produces a proton-motive force that is used to generate ATP. PSI is composed of more than 110 cofactors, significantly more than Photosystem II.<ref name="Nelson_2006">Template:Cite journal</ref>
HistoryEdit
This photosystem is known as PSI because it was discovered before Photosystem II, although future experiments showed that Photosystem II is actually the first enzyme of the photosynthetic electron transport chain. Aspects of PSI were discovered in the 1950s, but the significance of these discoveries was not yet recognized at the time.<ref name="Fromme">Template:Cite journal</ref> Louis Duysens first proposed the concepts of Photosystems I and II in 1960, and, in the same year, a proposal by Fay Bendall and Robert Hill assembled earlier discoveries into a coherent theory of serial photosynthetic reactions.<ref name="Fromme"/> Hill and Bendall's hypothesis was later confirmed in experiments conducted in 1961 by the Duysens and Witt groups.<ref name="Fromme"/>
Components and actionEdit
Two main subunits of PSI, PsaA and PsaB, are closely related proteins involved in the binding of the vital electron transfer cofactors P700, Acc, A0, A1, and Fx. PsaA and PsaB are both integral membrane proteins of 730 to 750 amino acids that contain 11 transmembrane segments. A [4Fe-4S] iron-sulfur cluster called Fx is coordinated by four cysteines; two cysteines are provided each by PsaA and PsaB. The two cysteines in each are proximal and located in a loop between the ninth and tenth transmembrane segments. A leucine zipper motif seems to be present <ref name="pmid2186925">Template:Cite journal</ref> downstream of the cysteines and could contribute to dimerisation of PsaA/PsaB. The terminal electron acceptors FA and FB, also [4Fe-4S] iron-sulfur clusters, are located in a 9-kDa protein called PsaC that binds to the PsaA/PsaB core near FX.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
| Protein subunits | Description |
|---|---|
| PsaA | Related large transmembrane proteins involved in the binding of P700, A0, A1, and Fx. Part of the photosynthetic reaction centre protein family. |
| PsaB | |
| PsaC | Iron-sulfur center; apoprotein for Fa and Fb |
| PsaD | Required for assembly, helps bind ferredoxin. Template:InterPro |
| PsaE | Template:InterPro |
| PsaI | May stabilize PsaL. Stabilizes light-harvesting complex II binding.<ref>Template:Cite journal</ref> Template:InterPro |
| PsaJ | Template:InterPro |
| PsaK | Template:InterPro |
| PsaL | Template:InterPro |
| PsaM | Template:InterPro |
| PsaX | Template:InterPro |
| cytochrome b6f complex | Soluble protein |
| Fa | From PsaC; In electron transport chain (ETC) |
| Fb | From PsaC; In ETC |
| Fx | From PsaAB; In ETC |
| Ferredoxin | Electron carrier in ETC |
| Plastocyanin | Soluble protein |
| Lipids | Description |
| MGDG II | Monogalactosyldiglyceride lipid |
| PG I | Phosphatidylglycerol phospholipid |
| PG III | Phosphatidylglycerol phospholipid |
| PG IV | Phosphatidylglycerol phospholipid |
| Pigments | Description |
| Chlorophyll a | 90 pigment molecules in antenna system |
| Chlorophyll a | 5 pigment molecules in ETC |
| Chlorophyll a0 | Early electron acceptor of modified chlorophyll in ETC |
| Chlorophyll a′ | 1 pigment molecule in ETC |
| β-Carotene | 22 carotenoid pigment molecules |
| Coenzymes and cofactors | Description |
| QK-A | Early electron acceptor vitamin K1 phylloquinone in ETC |
| QK-B | Early electron acceptor vitamin K1 phylloquinone in ETC |
| FNR | [[Ferredoxin-NADP+ reductase|Ferredoxin-Template:Chem oxidoreductase enzyme]] |
| Template:Chem | Calcium ion |
| Template:Chem | Magnesium ion |
PhotonEdit
Photoexcitation of the pigment molecules in the antenna complex induces electron and energy transfer.<ref name = "Raven"/>
Antenna complexEdit
The antenna complex is composed of molecules of chlorophyll and carotenoids mounted on two proteins.<ref name="Taiz">Template:Cite bookTemplate:Dead link</ref> These pigment molecules transmit the resonance energy from photons when they become photoexcited. Antenna molecules can absorb all wavelengths of light within the visible spectrum.<ref name="kent">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The number of these pigment molecules varies from organism to organism. For instance, the cyanobacterium Synechococcus elongatus (Thermosynechococcus elongatus) has about 100 chlorophylls and 20 carotenoids, whereas spinach chloroplasts have around 200 chlorophylls and 50 carotenoids.<ref name="kent"/><ref name="Nelson_2006"/> Located within the antenna complex of PSI are molecules of chlorophyll called P700 reaction centers. The energy passed around by antenna molecules is directed to the reaction center. There may be as many as 120 or as few as 25 chlorophyll molecules per P700.<ref name="Shubin">Template:Cite journal</ref>
P700 reaction centerEdit
Template:Main article The P700 reaction center is composed of modified chlorophyll a that best absorbs light at a wavelength of 700 nm.<ref name="rutherford">Template:Cite journal</ref> P700 receives energy from antenna molecules and uses the energy from each photon to raise an electron to a higher energy level (P700*). These electrons are moved in pairs in an oxidation/reduction process from P700* to electron acceptors, leaving behind P700+. The pair of P700* - P700+ has an electric potential of about −1.2 volts. The reaction center is made of two chlorophyll molecules and is therefore referred to as a dimer.<ref name="Taiz"/> The dimer is thought to be composed of one chlorophyll a molecule and one chlorophyll a′ molecule. However, if P700 forms a complex with other antenna molecules, it can no longer be a dimer.<ref name="Shubin"/>
Modified chlorophyll A0 and A1Edit
The two modified chlorophyll molecules are early electron acceptors in PSI. They are present one per PsaA/PsaB side, forming two branches electrons can take to reach Fx. A0 accepts electrons from P700*, passes it to A1 of the same side, which then passes the electron to the quinone on the same side. Different species seems to have different preferences for either A/B branch.<ref name=Grotjohann>Template:Cite book</ref>
PhylloquinoneEdit
A phylloquinone, sometimes called vitamin K1,<ref name="itoh">Template:Cite journal</ref> is the next early electron acceptor in PSI. It oxidizes A1 in order to receive the electron and in turn is re-oxidized by Fx, from which the electron is passed to Fb and Fa.<ref name="itoh"/><ref>Template:Cite journal</ref> The reduction of Fx appears to be the rate-limiting step.<ref name=Grotjohann/>
Iron–sulfur complexEdit
Three proteinaceous iron–sulfur reaction centers are found in PSI. Labeled Fx, Fa, and Fb, they serve as electron relays.<ref name="Vassiliev"/> Fa and Fb are bound to protein subunits of the PSI complex and Fx is tied to the PSI complex.<ref name="Vassiliev"/> Various experiments have shown some disparity between theories of iron–sulfur cofactor orientation and operation order.<ref name="Vassiliev">Template:Cite journal</ref> In one model, Fx passes an electron to Fa, which passes it on to Fb to reach the ferredoxin.<ref name=Grotjohann/>
FerredoxinEdit
Ferredoxin (Fd) is a soluble protein that facilitates reduction of Template:Chem to NADPH.<ref name="Forti">Template:Cite journal</ref> Fd moves to carry an electron either to a lone thylakoid or to an enzyme that reduces Template:Chem.<ref name="Forti"/> Thylakoid membranes have one binding site for each function of Fd.<ref name="Forti"/> The main function of Fd is to carry an electron from the iron-sulfur complex to the enzyme [[ferredoxin-NADP+ reductase|ferredoxin–Template:Chem reductase]].<ref name="Forti"/>
Ferredoxin–Template:Chem reductase (FNR)Edit
This enzyme transfers the electron from reduced ferredoxin to Template:Chem to complete the reduction to NADPH.<ref name="Madoz">Template:Cite journal</ref> FNR may also accept an electron from NADPH by binding to it.<ref name="Madoz"/>
PlastocyaninEdit
Plastocyanin is an electron carrier that transfers the electron from cytochrome b6f to the P700 cofactor of PSI in its ionized state P700+.<ref name = "Raven">Template:Cite book</ref><ref>Template:Cite journal</ref>
Ycf4 protein domainEdit
The Ycf4 protein domain found on the thylakoid membrane is vital to photosystem I. This thylakoid transmembrane protein helps assemble the components of photosystem I. Without it, photosynthesis would be inefficient.<ref name="pmid9321389">Template:Cite journal</ref>
EvolutionEdit
Molecular data show that PSI likely evolved from the photosystems of green sulfur bacteria. The photosystems of green sulfur bacteria and those of cyanobacteria, algae, and higher plants are not the same, but there are many analogous functions and similar structures. Three main features are similar between the different photosystems.<ref name="Lockau">Template:Cite journal</ref> First, redox potential is negative enough to reduce ferredoxin.<ref name="Lockau"/> Next, the electron-accepting reaction centers include iron–sulfur proteins.<ref name="Lockau"/> Last, redox centres in complexes of both photosystems are constructed upon a protein subunit dimer.<ref name="Lockau"/> The photosystem of green sulfur bacteria even contains all of the same cofactors of the electron transport chain in PSI.<ref name="Lockau"/> The number and degree of similarities between the two photosystems strongly indicates that PSI and the analogous photosystem of green sulfur bacteria evolved from a common ancestral photosystem.
See alsoEdit
ReferencesEdit
External linksEdit
- Photosystem I: Molecule of the Month in the Protein Data Bank Template:Webarchive
- Photosystem I in A Companion to Plant Physiology
- James Barber FRS Photosystems I & II
Template:Other oxidoreductases Template:Enzymes Template:Multienzyme complexes