Light-harvesting complexes chlorosome model

Chapter 8 The Green Bacteria I. The Light-Harvesting Complex, the Chlorosomes 149 I. Models for the Reaction Center-Antenna Complex of Green Bacteria... 

Fig. 3 Schematic model of light-harvesting compartments in photosynthetic organisms and their position with respect to the membrane and the reaction centers. RC1(2) Photosystem I(II) reaction centre. Peripheral membrane antennas Chlorosome/FMO in green sulfur and nonsulfur bacteria, phycobilisome (PBS) in cyanobacteria and rhodophytes and peridinin-chlorophyll proteins (PCP) in dyno-phytes. Integral membrane accessory antennas LH2 in purple bacteria, LHC family in all eukaryotes. Integral membrane core antennas B808-867 complex in green nonsulfur bacteria, LH1 in purple bacteria, CP43/CP47 (not shown) in cyanobacteria and all eukaryotes.

Fig. 8. Amino acid sequence of the BChl c-binding (antenna) polypeptide of the chlorosomes from Chloroflexus aurantiacus (A) and the proposed a-helix model of this antenna polypeptide (B) with possible BChl c binding sites (Gin 12,15,22,26,33, Asn 30,41) via the central Mg atom. (C) Chlorosome subunit ( globular subunit ) (light-harvesting BChl c-protein complex of C. aurantiacus) composed of 12 polypeptide chains (a-helices) of the BChl c-binding polypeptide. Pairs of a-helices (dimeric basic units) are tilted to expose the areas for BChl c binding (hatched areas). (D) Chlorosome model (cross-section, C. aurantiacus) containing the chlorosome subunits of the rod-shaped elements in the chlorosome core region (taken from Refs. 69 and 72).

Chlorosomes, the major light-harvesting antenna complex of the green bacteria, form a natural model system for the study of energy transfer in large ordered aggregates of chromophores. 

The light-harvesting system of green bacteria consists of BChl c containing chlorosomes attached to the cell membrane by a baseplate containing BChl a. The cell membrane contains the B 808-866 antenna complex and the reaction center. We report here a comparison of the properties of chlorosome preparations obtained by different treatments. Energy transfer in these preparations was studied by picosecond absorbance recovery and steady-state fluorescence measurements. The results are used to propose a new model for the structure of chlorosomes. 

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