Photosystem I
The photosystem I is an integral part of photosynthesis, which is the formation of organic substances with the aid of light as the power source , which in plants , algae , photosynthetic protists and photosynthetic bacteria occurs. It is a particularly structured material system , consisting of a light collecting complex and a reaction complex. In photosystem I, a strong reducing agent ( NADPH ) is formed for the synthesis of organic substances from carbon dioxide and water and light energy is converted into an energy source suitable for this biosynthesis, adenosine triphosphate (ATP).
properties
Photosystem I (PS I) is a protein complex of several membrane proteins , in the course of photosynthesis an oxidation of plastocyanin and the reduction of ferredoxin by absorbed photons catalyzed . It is located in the thylakoid membrane of the chloroplasts in plant cells and consists of 15 proteins. It is used to absorb photons and convey them to the reaction center P 700, a complex of molecules containing chlorophyll-a , which is characterized by absorption at a wavelength of around 700 nm. It acts as a photosensitizer . After excitation by photons, a secondary radical pair is formed within 100 picoseconds , which decays in 300 nanoseconds at room temperature or in 300 μs at 77 Kelvin.
The other chlorophyll-a molecules, like the other chlorophylls , biliproteins and carotenoids, have an antenna function, i. that is, they transfer the absorbed radiant energy to the reaction center. They are arranged in two parallel layers ( luminal and stromal ) for light collection in the thylakoid membrane . In the area of electron transport, on the other hand, they are arranged in a pseudo C2 symmetry orthogonally to the membrane. The most important thing for photosynthesis is the transfer of the photochemical excitation energy to the neighboring molecules. The fluorescence spectrum of the donor molecule must overlap with the absorption spectrum of the recipient molecule and the chlorophyll molecules must be tightly packed in the membrane. Both energy transfer and electron transfer processes take place.
At room temperature the chlorophyll-a fluorescence spectrum shows a strong absorption band at 685 nm and a weaker one at 740 nm. If the spectrum is determined at low temperatures, bands at 685, 695 and 720 nm result, the first two being pigment system II, which are assigned to pigment system I at 720 nm.
The pigment composition of the photosystems in higher plants is as follows:
- Photosystem I: carotenes , chlorophyll a, chlorophyll b ;
- Photosystem II : xanthophyll , chlorophyll a, chlorophyll b
The photosystems differ not only in the absorption spectrum of the pigment in the reaction center, but also in the composition of the antenna pigments. Photosystem II , for example, is richer in chlorophyll b. Photosystem I is less sensitive to temperature increases compared to photosystem II.
composition
Two proteins of photosystem I, PsaA and PsaB, each have eleven transmembrane helices and a mass of about 80 kDa and form a heterodimer and bind about 80 chlorophyll-a molecules as a cofactor , about 20 β-carotenes for light absorption and about six chlorophyll-a Molecules, two phylloquinones, and a 4Fe-4S cluster for electron transfer. The other thirteen proteins are relatively small, with masses between four and eighteen kilodaltons.
| Subunit | description |
|---|---|
| A. | 83 kDa , 751 amino acids |
| B. | 82.5 kDa, 735 amino acids |
| C. | 8.9 kDa, electron transfer from P 700 to ferredoxin |
| D. | 19 kDa |
| E. | 7.5 kDa |
| F. | 19 kDa |
| G | 8 kDa, only in plants |
| H | 10 kDa, in plants |
| I. | 5 kDa |
| J | 5 kDa, binds three chlorophyll molecules |
| K | 8.5 kDa, binds two chlorophyll molecules |
| L. | 16 kDa |
| M. | 3.5 kDa, only in cyanobacteria |
| N | 9 kDa, in plants and algae |
| O | |
| X | 4 kDa, only in cyanobacteria |
| Pigments | |
| Chlorophyll a | 95 molecules in the antenna system |
| Chlorophyll a | 2 molecules |
| Chlorophyll a 0 | Chlorophyll а 695 - primary electron acceptor |
| Chlorophyll a and a ' | P 700 |
| β-carotene | 22 molecules |
| Coenzymes / Cofactors | |
| F a | Fe 4 S 4 iron-sulfur clusters |
| F b | Fe 4 S 4 iron-sulfur clusters |
| F x | Fe 4 S 4 iron-sulfur clusters |
| Ferredoxin | Electron carrier |
| Plastocyanin | Soluble protein, contains a copper ion |
| Q K -A | Phylloquinone - an electron acceptor in electron transport (subunit A) |
| Q K -B | Phylloquinone - an electron acceptor in electron transport (subunit B) |
| Ca 2+ | Calcium ion |
| Mg 2+ | Magnesium ion |
Photon and electron transport
The substances of the photosystems that act as antennas are grouped together as a light-harvesting complex (LHC). This enlarges the light-absorbing area. The reaction centers are the actual redox systems of photochemical electron transport. The reaction center P 700 of photosystem I is a strong reducing agent (standard reduction potential in the ground state is approx. +0.45 V, in the photochemically excited state the value is −0.6 V). It absorbs two photons and then acts as a primary electron donor and, when excited twice with photons, transfers a total of two electrons to primary acceptor A, an iron-sulfur protein. The primary acceptor is able to reduce the ferredoxin lying on the membrane . This means that the ferredoxin is the electron acceptor here . From here the electrons are transferred to an FAD -containing enzyme.
This catalyzes the following reaction:
- NADP + changes into NADPH + H + by taking up two protons and two electrons .
The reaction product of photosystem I is the energy store ATP:
- In a phosphorylation reaction, ATP is formed from ADP and phosphate.
ATP is the energy supplier for the processes occurring in the dark reaction ( breathing ). The electron balance takes place in the further course in cooperation with the photosystem II by the plastocyanin acting as a donor. It is a protein with redox properties that contains two copper ions bound in its active center . This receives its electrons from the cytochrome b complex, which in turn is reduced by the plastoquinone . The plastoquinone itself changes into plastoquinol when the electrons are accepted. The chain is closed and the electron balance is balanced. During the overall reaction of photosynthesis, oxygen is also produced, namely during the light reaction in photosystem II. It is produced from the water. Carotene, which is present in photosystem I at the same time as the chlorophyll, has an absorbing effect and removes the singlet oxygen formed , which is toxic for the leaves of the plant.
Different dark reactions can follow: for example the enzymatic utilization of the bound carbon, which turns photosynthesis into a cycle that is called the Calvin cycle after its main discoverer . Overall, the process taking place in photosystem I can be classified as an electron transport chain . It can be noted that photosynthesis is a photoreaction with a quantum yield of over 90%.
The proton gradient required for ATP production can be generated not only via the linear electron transport chain, but also via cyclic electron transport in photosystem I.
Development history
Molecular biological data support the statement that the photosystem I probably developed from the photosystem of the green sulfur bacteria and Heliobacteria . It is similar in that the redox potential is negative enough to reduce ferredoxin. In addition, all three electron transport chains contain iron-sulfur proteins.
literature
- X. Qin, M. Suga, T. Kuang, J.-R. Shen: Structural basis for energy transfer pathways in the plant PSI-LHCI supercomplex. In: Science. 348, 2015, p. 989, doi: 10.1126 / science.aab0214 .
- Dieter Wöhrle, Michael W. Tausch, Wolf-Dieter Stohrer: Photochemistry: Concepts, Methods, Experiments . J. Wiley & Sons, Weinheim et al. O. 1998, ISBN 3-527-29545-3 .
- Peter Karlson: Short textbook of biochemistry for physicians and natural scientists . Thieme Verlag, Stuttgart 1977, ISBN 3-13-357810-3 .
- W. Baumeister: rororo-Pflanzenlexikon , Vol. 1: General botany . 59–62. Thousand. rororo 1979, ISBN 3-499-16100-1 .
- Wilhelm Nultsch: General botany: short textbook for physicians and natural scientists . 8th edition. Thieme Verlag, Stuttgart, New York 1986, ISBN 3-13-383308-1 , pp. 257,261.
- A. Gilbert, J. Baggott: Essentials of Molecular Photochemistry . Oxford Blackwill Scientific Publications, 1991, ISBN 0-632-02429-1 .
- R. Croce, H. van Amerongen: Light-harvesting in photosystem I. In: Photosynthesis research. Volume 116, number 2–3, October 2013, pp. 153–166, doi: 10.1007 / s11120-013-9838-x , PMID 23645376 , PMC 3825136 (free full text).
- T. Roach, A. Krieger-Liszkay: Regulation of photosynthetic electron transport and photoinhibition. In: Current protein & peptide science. Volume 15, number 4, 2014, pp. 351-362, PMID 24678670 , PMC 4030316 (free full text).
Individual evidence
- ^ A b H. Yang, J. Liu, X. Wen, C. Lu: Molecular mechanism of photosystem I assembly in oxygenic organisms. In: Biochimica et Biophysica Acta . [Electronic publication before printing] January 2015, doi: 10.1016 / j.bbabio.2014.12.011 , PMID 25582571 .
- ^ A b c d e S. Caffarri, T. Tibiletti, RC Jennings, S. Santabarbara: A comparison between plant photosystem I and photosystem II architecture and functioning. In: Current protein & peptide science. Volume 15, number 4, 2014, pp. 296–331, PMID 24678674 , PMC 4030627 (free full text).
- ^ G. Hastings: Vibrational spectroscopy of photosystem I. In: Biochimica et Biophysica Acta . Volume 1847, number 1, January 2015, pp. 55-68, doi: 10.1016 / j.bbabio.2014.07.014 , PMID 25086273 .
- ^ Eduard Strasburger, Dietrich von Denffer: Textbook of botany for universities . 32nd edition. G. Fischer Verlag, Stuttgart, New York 1983, ISBN 3-437-20295-2 .
- ^ S. Mathur, D. Agrawal, A. Jajoo: Photosynthesis: response to high temperature stress. In: Journal of Photochemistry and Photobiology . B, Biology. Volume 137, August 2014, pp. 116-126, doi: 10.1016 / j.jphotobiol.2014.01.010 , PMID 24796250 .
- ↑ Yuri Munekage, Mihoko Hashimoto, Chikahiro Miyake, Ken-Ichi Tomizawa, Tsuyoshi Endo, Masao Tasaka, Toshiharu Shikanai: Cyclic electron flow around photosystem I is essential for photosynthesis . In: Nature . tape 429 , no. 6991 , June 3, 2004, p. 579 , doi : 10.1038 / nature02598 (English, PDF ).
- ↑ Wolfgang Lockau, Wolfgang Nitschke: photosystem I and its bacterial counterparts . In: Physiologia Plantarum . tape 88 , no. 2 , June 1993, p. 372 , doi : 10.1111 / j.1399-3054.1993.tb05512.x ( PDF ).