Photosystem I

**Photosystem I** (**PSI**, or **plastocyanin-ferredoxin oxidoreductase**) is one of two photosystems in the photosynthetic light reactions of algae, plants, and cyanobacteria. Photosystem I is an integral membrane protein complex that uses light energy to catalyze the transfer of electrons across the thylakoid membrane from plastocyanin to ferredoxin. Ultimately, the electrons that are transferred by Photosystem I are used to produce the high energy carrier NADPH. The combined action of the entire photosynthetic electron transport chain also produces a proton-motive force that is used to generate ATP. PSI is composed of more than 110 cofactors, significantly more than Photosystem II.

## History

This photosystem is known as PSI because it was discovered before Photosystem II, although future experiments showed that Photosystem II is actually the first enzyme of the photosynthetic electron transport chain. Aspects of PSI 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 the Duysens and Witt groups.

## Components and action

Two main subunits of PSI, PsaA and PsaB, are closely related proteins involved in the binding of the vital electron transfer cofactors P700, Acc, A0, A1, and Fx. PsaA and PsaB are both integral membrane proteins of 730 to 750 amino acids that contain 11 transmembrane segments. A [4Fe-4S] iron-sulfur cluster called Fx is coordinated by four cysteines; two cysteines are provided each by PsaA and PsaB. The two cysteines in each 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, also [4Fe-4S] iron-sulfur clusters, are located in a 9-kDa protein called PsaC that binds to the PsaA/PsaB core near FX.

### Photon

Photoexcitation of the pigment molecules in the antenna complex induces electron transfer.

### 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, whereas spinach chloroplasts have around 200 chlorophylls and 50 carotenoids. Located within the antenna complex of PSI 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

Main articles: P700
The P700 reaction center is composed of modified chlorophyll a that best absorbs light at a wavelength of 700 nm, 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 PSI. 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 PSI. 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.

### Iron–sulfur complex

Three proteinaceous iron–sulfur reaction centers are found in PSI. Labeled Fx, Fa, and Fb, they serve as electron relays. Fa and Fb are bound to protein subunits of the PSI complex and Fx is tied to the PSI complex. Various experiments have shown some disparity between theories of iron–sulfur cofactor orientation and operation order.

### 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.

### 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.

### Plastocyanin

Plastocyanin is an electron carrier that transfers the electron from cytochrome b6f to the P700 cofactor of PSI.

## 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.

## Evolution

Molecular data show that PSI 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 cofactors of the electron transport chain in PSI. The number and degree of similarities between the two photosystems strongly indicates that PSI is derived from the analogous photosystem of green sulfur bacteria.