Protein Name

Photosystem I P700 chlorophyll a apoprotein A1


Synechococcus elongatus (cyanobacteria)

Biological Context

Life is a continuous destruction and creation of molecules. Creating new molecules requires energy. Adenosine triphosphate (ATP) is the primary fuel of the chemistry of life. Ultimately however all life on earth depends on the energy coming to us from the sun. The energy of the solar electromagnetic radiation, in particular the visible light part of the spectrum, drives the chemical processes of life. Means are needed to convert the energy contained in light into other more directly usable forms. This conversion is carried out in oxygen forming photosynthesis by cyanobacteria, green algae and plants. Photosynthesis, the conversion of electromagnetic into chemical energy, is accomplished with the help of two large protein complexes called photosystem I and II(see PDB:1FE1).

Structure Description


The structure shown here is that of photosystem I. PSI is a trimeric molecule with a molecular weight of about 1 million. Attached inside the protein itself are a total of more than 300 cofactor molecules, which are the energy-conversion machine. Since PSI is embedded inside a membrane, the large core of of PSI is formed by long trans-membrane helices. Each of the three identical building blocks of PSI contains about 18,000 protein atoms and almost 7,000 other atoms. Light is captured by an antenna system located on one side of the molecule. The energy of the captured light is used to help electrons flow along an electron transfer chain formed by cofactors. The flow of electrons from one side of the membrane to the other creates both a difference in hydrogen ion concentration and in electric potential. This difference is then used to synthesize ATP, the ultimate supply of energy. In a so-called dark reaction, the synthesized chemical fuel ATP is then used to convert carbon dioxide into carbohydrates.

Protein Data Bank (PDB)



Jordan, P. Fromme, P. Witt, H.T. Klukas, O. Saenger, W. Krauss, N.; "Three-dimensional Structure of Cyanobacterial Photosystem I at 2.5 angstroms Resolution"; NATURE; (2001) 411:909-917 PubMed:11418848.


UniProt:P25896 UniProt:P25897 UniProt:P18083 UniProt:P20452 UniProt:P25898 UniProt:P25899 UniProt:P25900 UniProt:P25901 UniProt:P20453 UniProt:P25902 UniProt:P25903

author: Arno Paehler

Japanese version:PDB:1JB0