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Photosystem 1

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Photosystem I

Photosystem I (PS I) is the second photosystem in the photosynthetic light reactions of algae, plants, and some bacteria. Photosystem I is so named because it was discovered before photosystem II. Aspects of PS I were discovered in the 1950s but the significances of these discoveries was not yet known. 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 photosynthetic reactions. Hill and Bendall’s hypothesis was later justified in experiments conducted in 1961 by Duysens and Witt groups.

General overview

What is Photosystem I

Photosystem I is a proteinaceous transmembrane structure composed of several proteins and embedded with pigment molecules. This structure is located inside chloroplasts and secured within the thylakoid membrane with exposure to the thylakoid lumen on one side and to the chloroplast stroma on the other side. PS I acts as an energy converter for various photosynthetic organisms.

How Does Photosystem I Work

Light energy in the form of photons is converted into electrons to power the generation of ATP or the reduction of NADP to NADPH. 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 chlorophyll a. The reaction center in turn transfers electrons to a primary electron acceptor and subsequent electron acceptors and carriers. Finally, the electrons reduce NADP or help generate ATP. Electrons may be recycled to increase the proton concentration in the thylakoid lumen in a process called cyclic electron flow. In cyclic electron flow electrons are passed from the PS I reaction center and then carried to a cytochrome complex where they help transport protons into the thylakoid lumen thus creating ATP.

Components and Action of Photosystem I

The PS I system comprises more than 110 co-factors, significantly more than photosystem II. These cofactors include several different types of molecules ranging from pigments to iron-sulfur proteins. These various components have a wide range of functions.

Plastocyanin

Plastocyanin is a metallic protein containing a copper atom and with patches of negative charge. After an electron is carried to a cytochrome complex, it is passed on to plastocyanin. Plastocyanin binds to cytochrome though little is known about the mechanism of this binding. Plastocyanin then transfers the electron directly to the P700 reaction center in the PS I antenna complex.

Photon

Photons of light photoexcite pigment molecules in the antenna complex. Each photon is converted into one electron.

Antenna Complex

The antenna complex is composed of molecules of chlorophyll and carotenoids mounted on two proteins. 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. 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 while spinach chloroplasts have around 200 chlorophylls and 50 carotenoids. *Located within the antenna complex of PS I 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.

P700 Reaction Center

  • The P700 reaction center is composed of modified chlorophyll a that best absorbs light at a wavelength of 700nm with higher wavelengths causing bleaching. *P700 transmits energy from antenna molecules and converts the energy from each photon into an electron through oxidation. 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. 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.

Modified Chlorophyll A0

  • Modified chlorophyll A0 is an early electron acceptor in PS I. *Chlorophyll A0 accepts electrons from P700 before passing them along to another early electron acceptor.

Phylloquinone A1

  • Phylloquinone A1 is the next early electron acceptor in PS I. Phylloquinone is a polypeptide comprised of vitamin K1. Phylloquinone A1 oxidizes A0 in order to receive the electron and in turn reduces Fx in order to pass the electron to Fb and Fa. A1 transfers electrons from A0 to the iron-sulfur complex yet it seems that this molecule is not required for electron transport from chlorophyll A0 to the iron-sulfur centers Fx, Fb, and Fa (A2). However, A1 may function in non-cyclic transfer.

The Iron-sulfur Complex

Three proteinaceous iron-sulfur reaction centers exist in this complex. The structure of iron-sulfur proteins is cube-like with four iron atoms and four sulfur atoms making eight points of the cube. The reaction centers in this complex are secondary electron acceptors. The three centers named Fx, Fa, and Fb direct electrons to ferredoxin. Fa and Fb are bound to protein subunits of the PS I complex and Fx is tied to the PS I complex by cysteines. Various experiments have shown some disparity between theories of iron-sulfur co-factor orientation and operation order. However, most of the results of these experiments point to three conclusions. First, the placement of Fx, Fa, and Fb form a triangle with Fa placed closer to Fx than Fb. Second, the order of electron transport within the iron-sulfur complex is from Fx to Fa to Fb wherein Fa and Fb form a terminal for electron receipt from Fx. Finally, Fb is the component that reduces ferredoxin in order to pass on the electron.

Ferredoxin

Ferredoxin (Fd) is a soluble protein that facilitates reduction of NADP to NADPH. Fd moves to carry an electron either to a lone thylakoid or to an enzyme that reduces NADP. Thylakoid membranes have one binding site for each function of Fd. The main function of Fd is to carry an electron from the iron-sulfur complex to the enzyme .

Ferredoxin-NADP Reductase (FNR)

This enzyme transfers the electron from reduced ferredoxin to NADP to complete the reduction to NADPH. FNR may also accept an electron from NADPH by binding to it.

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 green sulfur bacteria 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. First, ferredoxin is able to be reduced due to a suitably high ionic concentration. Next, the electron-accepting reaction centers include iron-sulfur proteins. Lastly, the antenna complexes of both photosystems are constructed upon a protein subunit dimer. The photosystem of green sulfur bacteria even contains all of the same co-factors of the electron transport chain in PS I. 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

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  45. Forti, Georgio, Paola Maria Giovanna Grubas. “Two Sites of Interaction of Ferredoxin with thylakoids. FEBS Letters 186.2 (1985): 149-152
  46. Forti, Georgio, Paola Maria Giovanna Grubas. “Two Sites of Interaction of Ferredoxin with thylakoids. FEBS Letters 186.2 (1985): 149-152
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  51. Lockau, Wolfgang, Wolfgang Nitschke. “Photosystem I and its Bacterial Counterparts.” Physiologia Plantarum 88.2 (1993): 372-381
  52. Lockau, Wolfgang, Wolfgang Nitschke. “Photosystem I and its Bacterial Counterparts.” Physiologia Plantarum 88.2 (1993): 372-381
  53. Lockau, Wolfgang, Wolfgang Nitschke. “Photosystem I and its Bacterial Counterparts.” Physiologia Plantarum 88.2 (1993): 372-381
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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/