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| Photosystem I (PS I) is the second ] in the photosynthetic light reactions of ], ]s, and some ]. Photosystem I is so named because it was discovered before ]. Aspects of PS I were discovered in the 1950s but the significances of these discoveries was not yet known<ref>Fromme, Petra, Paul Mathis. “Unraveling the Photosystem I Reaction Center: a History, or the Sum of Many Efforts.” Photosynthesis Research Vol. 80 issues 1-3 (2004): 109-124</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 cohesive theory of serial ] reactions<ref>Fromme, Petra, Paul Mathis. “Unraveling the Photosystem I Reaction Center: a History, or the Sum of Many Efforts.” Photosynthesis Research Vol. 80 issues 1-3 (2004): 109-124</ref>. Hill and Bendall’s hypothesis was later justified in experiments conducted in 1961 by Duysens and Witt groups<ref>Fromme, Petra, Paul Mathis. “Unraveling the Photosystem I Reaction Center: a History, or the Sum of Many Efforts.” Photosynthesis Research Vol. 80 issues 1-3 (2004): 109-124</ref>. | |||
| ===General overview=== | |||
| ====What is Photosystem I==== | |||
| Photosystem I is a ] ] structure composed of several proteins and embedded with ] ]<ref>Taiz, Lincoln and Eduardo Zeiger. “Photosystem I.” A Companion to Plant Physiology, Fourth Edition http://4e.plantphys.net/article.php?ch=3&id=73 | |||
| </ref>. This structure is located inside ]s and secured within the ] membrane with exposure to the thylakoid ] on one side and to the chloroplast ] on the other side<ref>Bukman, Yana, et al. “Structure and Function of Photosystem I.”</ref>. PS I acts as an energy converter for various photosynthetic organisms<ref>Bukman, Yana, et al. “Structure and Function of Photosystem I.”</ref>. | |||
| ====How Does Photosystem I Work==== | |||
| Light energy in the form of ]s is converted into electrons to power the generation of ] or the reduction of ] to ]<ref>“The Photosynthetic Process” http://kentsimmons.uwinnipeg.ca/cm1504/lightreact.htm | |||
| </ref>. Photons are received by an antenna complex of pigment molecules. *Antenna molecules become photoexcited and pass the energy as resonance energy (text). The resonance energy is transferred to the reaction center pigment ]<ref>Raven, Peter H., Ray F. Evert, Susan E. Eichhorn. "Photosynthesis, Light, and Life." Biology of Plants, Seventh Edition. New York: W.H. Freeman and Company, 2005. 121-127</ref>. The reaction center in turn transfers electrons to a primary ] acceptor and subsequent electron acceptors and carriers<ref>Raven, Peter H., Ray F. Evert, Susan E. Eichhorn. "Photosynthesis, Light, and Life." Biology of Plants, Seventh Edition. New York: W.H. Freeman and Company, 2005. 121-127</ref>. Finally, the electrons reduce NADP<sup>+</sup> or help generate ATP<ref>“The Photosynthetic Process” http://kentsimmons.uwinnipeg.ca/cm1504/lightreact.htm | |||
| </ref>. Electrons may be recycled to increase the proton concentration in the thylakoid lumen in a process called ]<ref>“The Photosynthetic Process” http://kentsimmons.uwinnipeg.ca/cm1504/lightreact.htm | |||
| </ref><ref>Raven, Peter H., Ray F. Evert, Susan E. Eichhorn. "Photosynthesis, Light, and Life." Biology of Plants, Seventh Edition. New York: W.H. Freeman and Company, 2005. 121-127</ref>. In cyclic electron flow electrons are passed from the PS I ] and then carried to a ] complex where they help transport ]s into the thylakoid lumen thus creating ATP<ref>Raven, Peter H., Ray F. Evert, Susan E. Eichhorn. "Photosynthesis, Light, and Life." Biology of Plants, Seventh Edition. New York: W.H. Freeman and Company, 2005. 121-127</ref>. | |||
| ===Components and Action of Photosystem I=== | |||
| The PS I system comprises more than 110 ]s, significantly more than photosystem II<ref>Bukman, Yana, et al. “Structure and Function of Photosystem I.” | |||
| </ref>. These cofactors include several different types of molecules ranging from pigments to ]s<ref>Bukman, Yana, et al. “Structure and Function of Photosystem I.” | |||
| </ref>. These various components have a wide range of functions. | |||
| ====Plastocyanin==== | |||
| ] is a metallic protein containing a ] ] and with patches of ]<ref>Frazão, Carlos, et al. “Ab Initio Structure Solution of a Dimeric Cytochrome C 3 from Desulfovibrio gigas Containing Disulfide Bridges.” Journal of Biological Inorganic Chemistry 4.2 (1999): 162-165</ref>. After an electron is carried to a cytochrome complex, it is passed on to plastocyanin<ref>Frazão, Carlos, et al. “Ab Initio Structure Solution of a Dimeric Cytochrome C 3 from Desulfovibrio gigas Containing Disulfide Bridges.” Journal of Biological Inorganic Chemistry 4.2 (1999): 162-165</ref>. Plastocyanin binds to cytochrome though little is known about the mechanism of this binding<ref>Hope, A.B. “Electron Transfers Amongst Cytochrome F1, Plastocyanin and Photosystem I: Kinetics and Mechanisms.” Biochimica et Biophysica Acta (BBA)/Bioenergetics 1456.1 (2000): 5-26 | |||
| </ref>. Plastocyanin then transfers the electron directly to the P700 reaction center in the PS I ]<ref>Raven, Peter H., Ray F. Evert, Susan E. Eichhorn. "Photosynthesis, Light, and Life." Biology of Plants, Seventh Edition. New York: W.H. Freeman and Company, 2005. 121-127</ref>. | |||
| ====Photon==== | |||
| Photons of light photoexcite pigment molecules in the antenna complex<ref>Raven, Peter H., Ray F. Evert, Susan E. Eichhorn. "Photosynthesis, Light, and Life." Biology of Plants, Seventh Edition. New York: W.H. Freeman and Company, 2005. 121-127</ref>. Each photon is converted into one electron<ref>Raven, Peter H., Ray F. Evert, Susan E. Eichhorn. "Photosynthesis, Light, and Life." Biology of Plants, Seventh Edition. New York: W.H. Freeman and Company, 2005. 121-127</ref>. | |||
| ====Antenna Complex==== | |||
| The antenna complex is composed of molecules of chlorophyll and ]s mounted on two proteins<ref>Taiz, Lincoln and Eduardo Zeiger. “Photosystem I.” A Companion to Plant Physiology, Fourth Edition http://4e.plantphys.net/article.php?ch=3&id=73</ref>. These pigment molecules transmit the ] from photons when they become photoexcited. Antenna molecules can absorb all ]s of light within the ]<ref>“The Photosynthetic Process” http://kentsimmons.uwinnipeg.ca/cm1504/lightreact.htm | |||
| </ref>. The number of these pigment molecules varies from organism to organism. For instance, the ] ''Synechococcus elongatus'' (''Thermosynechococcus elongatus'') has about 100 chlorophylls and 20 carotenoids while ] chloroplasts have around 200 chlorophylls and 50 carotenoids<ref>“The Photosynthetic Process” http://kentsimmons.uwinnipeg.ca/cm1504/lightreact.htm</ref><ref>Bukman, Yana, et al. “Structure and Function of Photosystem I.”</ref>. *Located within the antenna complex of PS I are molecules of chlorophyll called ] 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>Shubin, V.V., N.V. Karapetyan, A.A. Krasnovsky. “Molecular Arrangemnet of Pigment-Protein Complex of Photosystem I.” Photosynthesis Research Vol. 9 issues 1-2 (1986): 3-12</ref>. | |||
| ====P700 Reaction Center==== | |||
| *The P700 reaction center is composed of modified chlorophyll a that best absorbs light at a wavelength of 700] with higher wavelengths causing bleaching<ref>Rutherford, A.W., P. Heathcote. “Primary Photochemistry in Photosystem-I.” Photosynthesis Research 6.4 (1985): 295-316</ref>. *P700 transmits energy from antenna molecules and converts the energy from each photon into an electron through ]. P700 has an ] of about -1.2 ]s. The reaction center is made of two chlorophyll molecules and is therefore referred to as a ]<ref>Taiz, Lincoln and Eduardo Zeiger. “Photosystem I.” A Companion to Plant Physiology, Fourth Edition http://4e.plantphys.net/article.php?ch=3&id=73 | |||
| </ref>. The dimer is thought to be composed of one chlorophyll a molecule and one chlorophyll a’ molecule (p700, webber). However, if P700 forms a complex with other antenna molecules it can no longer be a dimer<ref>Shubin, V.V., N.V. Karapetyan, A.A. Krasnovsky. “Molecular Arrangemnet of Pigment-Protein Complex of Photosystem I.” Photosynthesis Research Vol. 9 issues 1-2 (1986): 3-12</ref>. | |||
| ====Modified Chlorophyll A<sub>0</sub>==== | |||
| *Modified chlorophyll A<sub>0</sub> is an early electron acceptor in PS I. *Chlorophyll A<sub>0</sub> accepts electrons from P700 before passing them along to another early electron acceptor<ref>Rutherford, A.W., P. Heathcote. “Primary Photochemistry in Photosystem-I.” Photosynthesis Research 6.4 (1985): 295-316 | |||
| </ref>. | |||
| ====Phylloquinone A<sub>1</sub>==== | |||
| *Phylloquinone A<sub>1</sub> is the next early electron acceptor in PS I. Phylloquinone is a polypeptide comprised of ]<ref>Itoh, Shigeru, Msayo Iwaki. “Vitamin K1 (Phylloquinone) Restores the Turnover of FeS centers of Ether-extracted Spinach PS I Particles.” FEBS Letters 243.1 (1989): 47-52 | |||
| </ref>. Phylloquinone A<sub>1</sub> oxidizes A<sub>0</sub> in order to receive the electron and in turn reduces F<sub>x</sub> in order to pass the electron to F<sub>b</sub> and F<sub>a</sub><ref>Itoh, Shigeru, Msayo Iwaki. “Vitamin K1 (Phylloquinone) Restores the Turnover of FeS centers of Ether-extracted Spinach PS I Particles.” FEBS Letters 243.1 (1989): 47-52 | |||
| </ref>. A<sub>1</sub> transfers electrons from A<sub>0</sub> to the iron-sulfur complex yet it seems that this molecule is not required for electron transport from chlorophyll A<sub>0</sub> to the iron-sulfur centers F<sub>x</sub>, F<sub>b</sub>, and F<sub>a</sub> (A<sub>2</sub>)<ref>Palace, Gerard P., James E. Franke, Joseph T. Warden. “Is Phylloquinone an Obligate Electron Carrier in Photosystem I?” FEBS Letters 215.1 (1987): 58-62</ref>. However, A<sub>1</sub> may function in ]<ref>Palace, Gerard P., James E. Franke, Joseph T. Warden. “Is Phylloquinone an Obligate Electron Carrier in Photosystem I?” FEBS Letters 215.1 (1987): 58-62</ref>. | |||
| ====The Iron-sulfur Complex==== | |||
| Three proteinaceous iron-sulfur reaction centers exist in this ]<ref>Vassiliev, Ilya R. "Iron Sulfur Clusters in Type I Reaction Centers." Biochimica et Biophysica Acta (BBA)/Bioenergetics Vol. 1507 issues 1-3 (2001): 139-160</ref>. The structure of iron-sulfur proteins is ]-like with four ] atoms and four ] atoms making eight points of the cube<ref>Vassiliev, Ilya R. "Iron Sulfur Clusters in Type I Reaction Centers." Biochimica et Biophysica Acta (BBA)/Bioenergetics Vol. 1507 issues 1-3 (2001): 139-160</ref>. The reaction centers in this complex are secondary electron acceptors<ref>Reilly, Patricia, Nathan Nelson. “Photosystem I Complex.” Photosystem Research Vol. 19 issues 1-2 (1988): 73-84 | |||
| </ref>. The three centers named F<sub>x</sub>, F<sub>a</sub>, and F<sub>b</sub> direct electrons to ferredoxin<ref>Vassiliev, Ilya R. "Iron Sulfur Clusters in Type I Reaction Centers." Biochimica et Biophysica Acta (BBA)/Bioenergetics Vol. 1507 issues 1-3 (2001): 139-160</ref>. F<sub>a</sub> and F<sub>b</sub> are bound to ]s of the PS I complex and F<sub>x</sub> is tied to the PS I complex by ]s<ref>Vassiliev, Ilya R. "Iron Sulfur Clusters in Type I Reaction Centers." Biochimica et Biophysica Acta (BBA)/Bioenergetics Vol. 1507 issues 1-3 (2001): 139-160</ref>. Various experiments have shown some disparity between theories of iron-sulfur co-factor orientation and operation order<ref>Vassiliev, Ilya R. "Iron Sulfur Clusters in Type I Reaction Centers." Biochimica et Biophysica Acta (BBA)/Bioenergetics Vol. 1507 issues 1-3 (2001): 139-160</ref>. However, most of the results of these experiments point to three conclusions. First, the placement of F<sub>x</sub>, F<sub>a</sub>, and F<sub>b</sub> form a ] with F<sub>a</sub> placed closer to F<sub>x</sub> than F<sub>b</sub><ref>Vassiliev, Ilya R. "Iron Sulfur Clusters in Type I Reaction Centers." Biochimica et Biophysica Acta (BBA)/Bioenergetics Vol. 1507 issues 1-3 (2001): 139-160</ref>. Second, the order of ] within the iron-sulfur complex is from F<sub>x</sub> to F<sub>a</sub> to F<sub>b</sub> wherein F<sub>a</sub> and F<sub>b</sub> form a terminal for electron receipt from F<sub>x</sub><ref>Vassiliev, Ilya R. "Iron Sulfur Clusters in Type I Reaction Centers." Biochimica et Biophysica Acta (BBA)/Bioenergetics Vol. 1507 issues 1-3 (2001): 139-160</ref>. Finally, F<sub>b</sub> is the component that reduces ] in order to pass on the electron<ref>Vassiliev, Ilya R. "Iron Sulfur Clusters in Type I Reaction Centers." Biochimica et Biophysica Acta (BBA)/Bioenergetics Vol. 1507 issues 1-3 (2001): 139-160</ref>. | |||
| ====Ferredoxin==== | |||
| Ferredoxin (Fd) is a ] protein that facilitates reduction of NADP<sup>+</sup> to NADPH<ref>Forti, Georgio, Paola Maria Giovanna Grubas. “Two Sites of Interaction of Ferredoxin with thylakoids. FEBS Letters 186.2 (1985): 149-152</ref>. Fd moves to carry an electron either to a lone thylakoid or to an ] that reduces NADP<sup>+</sup><ref>Forti, Georgio, Paola Maria Giovanna Grubas. “Two Sites of Interaction of Ferredoxin with thylakoids. FEBS Letters 186.2 (1985): 149-152</ref>. Thylakoid membranes have one binding site for each function of Fd<ref>Forti, Georgio, Paola Maria Giovanna Grubas. “Two Sites of Interaction of Ferredoxin with thylakoids. FEBS Letters 186.2 (1985): 149-152</ref>. The main function of Fd is to carry an electron from the iron-sulfur complex to the enzyme <ref>Forti, Georgio, Paola Maria Giovanna Grubas. “Two Sites of Interaction of Ferredoxin with thylakoids. FEBS Letters 186.2 (1985): 149-152</ref>. | |||
| ====Ferredoxin-NADP<sup>+</sup> Reductase (FNR)==== | |||
| This enzyme transfers the electron from reduced ferredoxin to NADP<sup>+</sup> to complete the reduction to NADPH<ref>Madoz, Juan, et al. “Ivestigation of the Diaphorase Reaction of Ferredoxin-NADP+ Reductase by Electrochemical Methods.” Bioelectrochemistry and Bioenergetics 47.1 (1998): 179-183</ref>. FNR may also accept an electron from NADPH by binding to it<ref>Madoz, Juan, et al. “Ivestigation of the Diaphorase Reaction of Ferredoxin-NADP+ Reductase by Electrochemical Methods.” Bioelectrochemistry and Bioenergetics 47.1 (1998): 179-183</ref>. | |||
| ===Green Sulfur Bacteria and the Evolution of PS I=== | |||
| Molecular data show that PS I likely evolved from the photosystems of green-sulfur bacteria. The photosystems of ] and those of cyanobacteria, algae, and higher plants are not the same however there are many analogous functions and similar structures. Three main features are similar between the different photosystems<ref>Lockau, Wolfgang, Wolfgang Nitschke. “Photosystem I and its Bacterial Counterparts.” Physiologia Plantarum 88.2 (1993): 372-381 </ref>. First, ferredoxin is able to be reduced due to a suitably high ] concentration<ref>Lockau, Wolfgang, Wolfgang Nitschke. “Photosystem I and its Bacterial Counterparts.” Physiologia Plantarum 88.2 (1993): 372-381 </ref>. Next, the electron-accepting reaction centers include iron-sulfur proteins<ref>Lockau, Wolfgang, Wolfgang Nitschke. “Photosystem I and its Bacterial Counterparts.” Physiologia Plantarum 88.2 (1993): 372-381 </ref>. Lastly, the antenna complexes of both photosystems are constructed upon a protein subunit dimer<ref>Lockau, Wolfgang, Wolfgang Nitschke. “Photosystem I and its Bacterial Counterparts.” Physiologia Plantarum 88.2 (1993): 372-381</ref>. The photosystem of green sulfur bacteria even contains all of the same co-factors of the ] in PS I<ref>Lockau, Wolfgang, Wolfgang Nitschke. “Photosystem I and its Bacterial Counterparts.” Physiologia Plantarum 88.2 (1993): 372-381 | |||
| /</ref>. The number and degree of similarities between the two photosystems strongly indicates that PS I is derived from the analgous photosystem of green-sulfur bacteria. | |||
| ===References=== | |||
| {{reflist}} | |||
| ===External Links=== | |||
| http://www.rcsb.org/pdb/static.do?p=education_discussion/molecule_of_the_month/pdb22_1.html | |||
| http://kentsimmons.uwinnipeg.ca/cm1504/lightreact.htm | |||
| http://4e.plantphys.net/article.php?ch=3&id=73 | |||
| http://www.bio.ic.ac.uk/research/barber/ | |||
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