The structure of the peripheral light harvesting complex from the photosynthetic bacterium Rps. acidophila (strain 10050) has been determined at 2.5Å resolution. The complex captures photon energy and passes it very rapidly and efficiently to the 'core' particle; comprising LH1, and Reaction Centre - where charge separation takes place. The electron density map is of a high quality and clearly shows 36 chromaphores and the protein structure apart from a few C-terminal residues.
The complex is formed from nine protomers each consisting of an alpha and beta polypeptide, three bacteriochlorophyll a (Bchl a) molecules, one rhodopin glucoside and one beta-octylglucoside molecule. The structure has precise 9 fold symmetry and comprises two concentric rings of trans-membrane helices. A continuous ring of 18 Bchl a molecules are situated between these helices. A further 9 Bchl a molecules are found between beta peptide helices at a distance of 18.0Å from the first ring. The two rings of molecules are linked through the intertwining of their phytol chains and the contacts of rhodopin glucoside molecules.
The structure explains many of observed spectral transfer processes between the various chromaphores within the complex. The peripheral antenna complexes aggregate into two dimensional arrays, incorporating the core complex comprising LH1 and the Reaction Centre. The homology of the LH1 subunit and that of LH2 suggests that the 850nm bacteriochlorin molecules from LH2 and the 870nm absorbers in LH1 are at the same point in the membrane. Thus efficient energy transfer within these arrays occurs without special relative orientations of the light harvesting molecules.
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