Synechococcus elongatus (cyanobacteria)
The source of energy needed to support life on earth and all associated chemical complexes is the light coming to us from the sun. Certain types of bacteria and plants have developed energy conversion systems to turn this light energy into chemical energy. The energy needed for chemical reactions is predominantly stored in the energy-rich phosphate bonds of the molecule adenosine triphosphate (ATP). Splitting this molecule into adenosine diphosphate (ADP) and a phosphate group releases energy that can be used for chemical reactions. But first ATP has to be created, before it becomes available to provide energy. Plants and bacteria employ a system of two proteins for this purpose, photo system I (PSI) and photo system II (PSII). PSI's role (see PDB:1JB0) is to create a flow of electrons across the membrane to drive the ATP synthesis. These electrons (and the corresponding protons) have to be produced somewhere and the producer of the electrons and protons is PSII.
The structure of PSII is smaller than that of PSI. It is made of two identical building blocks, each composed of 17 subunits. 14 of them are used to embed PSII into the membrane. PSII, in an involved reaction, oxidizes water molecules and generates, as a product, electrons, hydrogen ions (protons) and in a final step, diatomic oxygen which is released into the atmosphere. The electrons and protons are used by its partner molecule PSI to create ATP for chemical reactions. Together they form a small chemical factory whose purpose is to support life on earth as we know it.
Protein Data Bank (PDB)
Zouni, A. Witt, H.T. Kern, J. Fromme, P. Krauss, N. Saenger, W. Orth, P.; "Crystal structure of photosystem II from Synechococcus elongatus at 3.8 angstroms resolution."; Nature; (2001) 409:739-743 PubMed:11217865.
author: Arno Paehler