Pigment-Proteins of Antenna Complexes from Purple Non-Sulfur Bacteria: Localization in the Membrane, Alignments of Primary Structure and Structural Predictions

Abstract

Reaction center (RC) and light-harvesting (LH) or antenna complexes are the major pigment-proteins of the photosynthetic apparatus of non sulfur purple bacteria. They are localized on the intracytoplasmic membranes. The LH-complexes serve to gather light-energy and funnel it to the photochemical RC where the excitation energy is transduced into a charge separation state and a redox potential difference. Reaction centers are surrounded by a constant number of core antenna complexes (8870 or B1020). Most species have a second and variable light-harvesting (LH) complex (B800–850) which interconnects the core complexes (Drews 1985). The LH complexes are oligomers of basic subunits which consist of two different small pigment-binding polypeptides α and β, having Mr of about 5000 to 7000. These polypeptides are amphiphilic proteins and span the membrane only once by a central hydrophobic domain (Fig. 1). The N- and C-terminal domains consist of polar, charged and hydrophobic amino acid residues and are exposed on the membrane surface. Two or three bacteriochlorophyll (Bchl) and one or two carotenoid molecules are bound non-covalently to the α and β polypeptides.