| Photosystem I | |
| Ec Number: | 1.97.1.12 |
Photosystem I (PSI, or plastocyanin–ferredoxin oxidoreductase) is one of two photosystems in the photosynthetic light reactions of algae, plants, and cyanobacteria. Photosystem I [1] 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.[2] 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.[3]
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.[4] 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.[4] Hill and Bendall's hypothesis was later confirmed in experiments conducted in 1961 by the Duysens and Witt groups.[4]
Two main subunits of PSI, PsaA and PsaB, are closely related proteins involved in the binding of the vital electron transfer cofactors P, Acc, A, A, and F. 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 F 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 [5] downstream of the cysteines and could contribute to dimerisation of PsaA/PsaB. The terminal electron acceptors F and F, also [4Fe-4S] iron-sulfur clusters, are located in a 9-kDa protein called PsaC that binds to the PsaA/PsaB core near F.[6] [7]
| Protein subunits | Description | |||
|---|---|---|---|---|
| row 2, cell 1 | PsaA | Related large transmembrane proteins involved in the binding of P700, A0, A1, and Fx. Part of the photosynthetic reaction centre protein family. | ||
| row 3, cell 1 | PsaB | |||
| row 4, cell 1 | PsaC | row 4, cell 2 | Iron-sulfur center; apoprotein for Fa and Fb | |
| row 5, cell 1 | PsaD | row 5, cell 2 | Required for assembly, helps bind ferredoxin. | |
| row 6, cell 1 | PsaE | row 6, cell 2 | ||
| row 7, cell 1 | PsaI | row 7, cell 2 | May stabilize PsaL. Stabilizes light-harvesting complex II binding.[9] | |
| row 8, cell 1 | PsaJ | row 8, cell 2 | ||
| row 9, cell 1 | PsaK | row 9, cell 2 | ||
| row 10, cell 1 | PsaL | row 10, cell 2 | ||
| row 11, cell 1 | PsaM | row 11, cell 2 | ||
| row 12, cell 1 | PsaX | row 12, cell 2 | ||
| row 13, cell 1 | cytochrome b6f complex | row 13, cell 2 | Soluble protein | |
| row 14, cell 1 | Fa | row 14, cell 2 | From PsaC; In electron transport chain (ETC) | |
| row 15, cell 1 | Fb | row 15, cell 2 | From PsaC; In ETC | |
| row 16, cell 1 | Fx | row 16, cell 2 | From PsaAB; In ETC | |
| row 17, cell 1 | Ferredoxin | row 17, cell 2 | Electron carrier in ETC | |
| row 18, cell 1 | Plastocyanin | row 18, cell 2 | Soluble protein | |
| Lipids | Description | |||
| row 19, cell 1 | MGDG II | row 19, cell 2 | Monogalactosyldiglyceride lipid | |
| row 20, cell 1 | PG I | row 20, cell 2 | Phosphatidylglycerol phospholipid | |
| row 21, cell 1 | PG III | row 21, cell 2 | Phosphatidylglycerol phospholipid | |
| row 22, cell 1 | PG IV | row 22, cell 2 | Phosphatidylglycerol phospholipid | |
| Pigments | Description | |||
| row 23, cell 1 | Chlorophyll a | row 23, cell 2 | 90 pigment molecules in antenna system | |
| row 24, cell 1 | Chlorophyll a | row 24, cell 2 | 5 pigment molecules in ETC | |
| row 25, cell 1 | Chlorophyll a0 | row 25, cell 2 | Early electron acceptor of modified chlorophyll in ETC | |
| row 26, cell 1 | Chlorophyll a′ | row 26, cell 2 | 1 pigment molecule in ETC | |
| row 27, cell 1 | β-Carotene | row 27, cell 2 | 22 carotenoid pigment molecules | |
| Coenzymes and cofactors | Description | |||
| row 28, cell 1 | QK-A | row 28, cell 2 | Early electron acceptor vitamin K1 phylloquinone in ETC | |
| row 29, cell 1 | QK-B | row 29, cell 2 | Early electron acceptor vitamin K1 phylloquinone in ETC | |
| row 30, cell 1 | FNR | row 30, cell 2 | Ferredoxin- oxidoreductase enzyme | |
| row 31, cell 1 | row 31, cell 2 | Calcium ion | ||
| row 32, cell 1 | row 32, cell 2 | Magnesium ion |
Photoexcitation of the pigment molecules in the antenna complex induces electron and energy transfer.
The antenna complex is composed of molecules of chlorophyll and carotenoids mounted on two proteins.[10] 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.[11] 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.[11] [3] 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.[12]
See main article: article and P700. The P700 reaction center is composed of modified chlorophyll a that best absorbs light at a wavelength of 700 nm.[13] 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.[10] 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.[12]
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 F. A accepts electrons from P700*, passes it to A 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.[14]
A phylloquinone, sometimes called vitamin K,[15] is the next early electron acceptor in PSI. It oxidizes A in order to receive the electron and in turn is re-oxidized by F, from which the electron is passed to F and F.[15] [16] The reduction of Fx appears to be the rate-limiting step.[14]
Three proteinaceous iron–sulfur reaction centers are found in PSI. Labeled F, F, and F, they serve as electron relays. F and F are bound to protein subunits of the PSI complex and F is tied to the PSI complex. Various experiments have shown some disparity between theories of iron–sulfur cofactor orientation and operation order.[17] In one model, F passes an electron to F, which passes it on to F to reach the ferredoxin.[14]
Ferredoxin (Fd) is a soluble protein that facilitates reduction of to NADPH.[18] Fd moves to carry an electron either to a lone thylakoid or to an enzyme that reduces .[18] Thylakoid membranes have one binding site for each function of Fd.[18] The main function of Fd is to carry an electron from the iron-sulfur complex to the enzyme ferredoxin– reductase.[18]
This enzyme transfers the electron from reduced ferredoxin to to complete the reduction to NADPH.[19] FNR may also accept an electron from NADPH by binding to it.[19]
Plastocyanin is an electron carrier that transfers the electron from cytochrome b6f to the P700 cofactor of PSI in its ionized state P700.[20] [21]
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.[22]
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.[23] First, redox potential is negative enough to reduce ferredoxin.[23] Next, the electron-accepting reaction centers include iron–sulfur proteins.[23] Last, redox centres in complexes of both photosystems are constructed upon a protein subunit dimer.[23] The photosystem of green sulfur bacteria even contains all of the same cofactors of the electron transport chain in PSI.[23] 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.