Photosynthetic Reaction Center
Rhodopseudomonas viridis (phototrophic bacteria)
The photosynthetic reaction center shown here stands out in several respects. When the structure was first published, it was one of the largest protein structures determined at that time and it was also the first structure of a protein that has its place inside the cell membrane. For this result Deisenhofer, Huber and Michel received the 1988 Nobel prize in chemistry. The protein however is not buried inside the cell membrane, it has parts that extend to either side of it, both the intracellular and the extracellular domain.
The membrane-bound part shows up as eleven tilted helices, with more globular part to either side of it. Altogether the molecule consists of 4 protein polypeptide chains. In addition the structure contains 14 cofactors, small chemical groups associated with the protein. These cofactors constitute the chemical factory where the work of the protein is done, namely the conversion of light energy into the chemical energy of ATP. Some of them have the job to absorb the light photons. Other then generate and transport electrons down a transport chain. In the process of doing so they create a difference in proton concentration between the inside and the outside of the cell. This proton gradient will then be used to synthesize adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and phosphate. As far as photosynthesis systems go, this molecule is actually quite modest. Some other photosystems are substantially larger and more complex. This is partly because they integrate more functions into one structure. Reaction centers like the one here delegate some light catching functions to other molecules, light-harvesting complexes (see PDB:1KZU), in their neighborhood.
Protein Data Bank (PDB)
Deisenhofer, J. Epp, O. Sinning, I. Michel, H.; ";Crystallographic refinement at 2.3 Angstroms resolution and refined model of the photosynthetic reaction centre from Rhodopseudomonas viridis.;"; J. Mol. Biol.; (1995) 246:429-457 PubMed:7877166.
author: Arno Paehler