The Photosynthetic Reaction Center from the Purple Bacterium Rhodopseudomonas viridis: Aspects of Membrane Protein Structure

This chapter focuses on the photosynthetic reaction centers from the purple photosynthetic bacteria that are easy to isolate and relatively stable. Membrane proteins can be crystallized, and an X-ray crystallographic analysis can lead to results of the same quality as with water soluble proteins. Some important conclusions on the structure and biosynthesis of membrane proteins could be drawn as well as on the function of the photosynthetic reaction center. Most of the reaction centers from purple bacteria contain three protein subunits that are called heavy (H), medium (M), and light (L) subunits according to their apparent molecular weights as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The L and M subunits form the membrane-embedded core of the photosynthetic reaction center. Both possess five long membrane-spanning helices. Many reaction centers, including that from Rhodopseudomonas (Rps.) viridis, contain a tightly bound cytochrome subunit, which is involved in the rereduction of the photo-oxidized primary electron donors. But most of the reaction centers from the other purple bacteria contain bacteriochlorophyll a instead of bacteriochlorophyll b, bacteriopheophytin a instead of bacteriopheophytin b, and the primary acceptor is formed by another ubiquinone. Therefore, they are the model membrane proteins that can be used to test the various methods of predicting the topology and the secondary structure of membrane proteins. This chapter describes and discusses pigment arrangement, protein structure, and the membrane protein aspects of the reaction-center structure with special emphasis on the membrane-protein structure.