Photosystem I (PS I) (or plastocyanin: ferredoxin oxidoreductase) is the second photosystem in the photosynthetic light reactions of algae, plants, and some bacteria. Photosystem I is 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.
Plant photosystem I, PDB 2001
|PDB structures||RCSB PDB PDBe PDBsum|
crystal structure of photosystem i: a photosynthetic reaction center and core antenna system from cyanobacteria
- 1 Components and action of photosystem I
- 2 Ycf4 protein domain
- 3 Green sulfur bacteria and the evolution of PS I
- 4 See also
- 5 References
- 6 External links
Components and action of photosystem I
Photosystem I (PSI)  is an integral membrane protein complex that uses light energy to mediate electron transfer from plastocyanin to ferredoxin. The PS I system comprises more than 110 co-factors, significantly more than photosystem II. These various components have a wide range of functions. The electron transfer components of the reaction center of PSI are a primary electron donor P-700 (chlorophyll dimer) and five electron acceptors: A0 (chlorophyll), A1 (a phylloquinone) and three 4Fe-4S iron-sulphur centres: Fx, Fa, and Fb.
Two main subunits of PS I, PsaA and PsaB, are closely related proteins involved in the binding of P700, A0, A1, and Fx. PsaA and PsaB are both integral membrane proteins of 730 to 750 amino acids that seem to contain 11 transmembrane segments. The Fx 4Fe-4S iron-sulphur centre is bound by four cysteines; two of these cysteines are provided by the PsaA protein and the two others by PsaB. The two cysteines in both proteins are proximal and located in a loop between the ninth and tenth transmembrane segments. A leucine zipper motif seems to be present  downstream of the cysteines and could contribute to dimerisation of psaA/psaB. The terminal electron acceptors, FA and FB, are located in a 9 kDa protein called PsaC.
|Protein Subunits||proton turn into form of ATP|
|Cytochrome b6f complex||Soluble protein|
|Fa||In electron transport chain (ETC)|
|Ferredoxin||Electron carrier in ETC|
|MGDG II||Monogalactosyldiglyceride lipid|
|PG I||Phosphatidylglycerol phospholipid|
|PG III||Phosphatidylglycerol phospholipid|
|PG IV||Phosphatidylglycerol phospholipid|
|Chlorophyll a||90 pigment molecules in antenna system|
|Chlorophyll a||5 pigment molecules in ETC|
|Chlorophyll a0||Early electron acceptor of modified chlorophyll in ETC|
|Chlorophyll a'||1 pigment molecule in ETC|
|β-Carotene||22 carotenoid pigment molecules|
|QK-A||Early electron acceptor vitamin K1 phylloquinone in ETC|
|QK-B||Early electron acceptor vitamin K1 phylloquinone in ETC|
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, whereas 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 receives energy from antenna molecules and uses the energy from each photon to raise an electron to a higher energy level. These electrons are moved in pairs in an oxidation/reduction process from P700 to electron acceptors. 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 is the next early electron acceptor in PS I. Phylloquinone is also sometimes called 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. 
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 (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+
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 of the PS I.
Ycf4 protein domain
The Ycf4 protein domain is found on the thylakoid membrane and is vital to photosystem I. This thylakoid transmembrane protein helps assemble the components of photosystem I, without it, photosynthesis would be inefficient.
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, redox potential is negative enough to reduce ferredoxin. Next, the electron-accepting reaction centers include iron-sulfur proteins. Last, redox centres in 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 analogous photosystem of green-sulfur bacteria.
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- 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