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.^[1] 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.^[1] Hill and Bendall’s hypothesis was later justified in experiments conducted in 1961 by Duysens and Witt groups.^[1]

[edit] Components and Action of Photosystem I

^[2]

The PS I system comprises more than 110 co-factors, significantly more than photosystem II.^[3] These various components have a wide range of functions.

[edit] Photon

Photons of light photoexcite pigment molecules in the antenna complex. Energy from each photon is transferred to an electron causing an excited state.^[4]

[edit] Antenna Complex

The antenna complex is composed of molecules of chlorophyll and carotenoids mounted on two proteins.^[5] 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.^[6] 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.^[3][6] 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.^[7]

[edit] 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^[8]. P700 transmits energy from antenna molecules and converts the energy from each photon into an electron through oxidation. Electrons are moved in pairs to modified chlorophyll a. 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.^[5] 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.^[7]

[edit] 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.^[8]

[edit] Phylloquinone A₁

Phylloquinone A₁ is the next early electron acceptor in PS I. Phylloquinone is a polypeptide made up of vitamin K1.^[9] Phylloquinone A₁ oxidizes A₀ in order to receive the electron and in turn reduces F_x in order to pass the electron to F_b and F_a.^[9] 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 F_x, F_b, and F_a (A₂). However, A₁ may function in non-cyclic transfer^[10].

[edit] The Iron-sulfur Complex

Three proteinaceous iron-sulfur reaction centers exist in this complex^[11]. The structure of iron-sulfur proteins is cube-like with four iron atoms and four sulfur atoms making eight points of the cube.^[11] The reaction centers in this complex are secondary electron acceptors^[12]. The three centers named F_x, F_a, and F_b direct electrons to ferredoxin.^[11] F_a and F_b are bound to protein subunits of the PS I complex and F_x is tied to the PS I complex by cysteines.^[11] Various experiments have shown some disparity between theories of iron-sulfur co-factor orientation and operation order.^[11] However, most of the results of these experiments point to three conclusions. First, the placement of F_x, F_a, and F_b form a triangle with F_a placed closer to F_x than F_b.^[11] Second, the order of electron transport within the iron-sulfur complex is from F_x to F_a to F_b wherein F_a and F_b form a terminal for electron receipt from F_x.^[11] Finally, F_b is the component that reduces ferredoxin in order to pass on the electron.^[11]

Schematic drawing of photosystem I from higher plants

[edit] Ferredoxin

Ferredoxin (Fd) is a soluble protein that facilitates reduction of NADP⁺ to NADPH.^[13] Fd moves to carry an electron either to a lone thylakoid or to an enzyme that reduces NADP⁺.^[13] Thylakoid membranes have one binding site for each function of Fd.^[13] The main function of Fd is to carry an electron from the iron-sulfur complex to the enzyme [[ferredoxin-NADP⁺ reductase]].^[13]

[edit] Ferredoxin-NADP⁺ Reductase (FNR)

This enzyme transfers the electron from reduced ferredoxin to NADP⁺ to complete the reduction to NADPH^[14]. FNR may also accept an electron from NADPH by binding to it.^[14]

[edit] Plastocyanin

Plastocyanin is a metallic protein containing a copper atom and with patches of negative charge.^[15] After an electron is carried to a cytochrome complex, it is passed on to plastocyanin.^[15] Plastocyanin binds to cytochrome though little is known about the mechanism of this binding^[16]. Plastocyanin then transfers the electron directly to the P700 reaction center in the PS I antenna complex.^[4]

[edit] 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^[17]. First, ferredoxin is able to be reduced due to a suitably high ionic concentration.^[17] Next, the electron-accepting reaction centers include iron-sulfur proteins.^[17] Lastly, the antenna complexes of both photosystems are constructed upon a protein subunit dimer.^[17] The photosystem of green sulfur bacteria even contains all of the same co-factors of the electron transport chain in PS I.^[17] The number and degree of similarities between the two photosystems strongly indicates that PS I is derived from the analogous photosystem of green-sulfur bacteria.

[edit] References

[edit] External links

• Photosystem I: Molecule of the Month in the Protein Data Bank
• Photosystem I in A Companion to Plant Physiology
• James Barber FRS Photosystems I & II