All photosynthetic organisms contain a two-component photochemical apparatus comprised of an antenna complex and a reaction center [1, 2]. One can thus assume that useful biological conversion of light- energy requires the presence of a high enough input of energy by the antennas to sustain efficient electron transfer by the reaction centers. In the most primary sense, antenna complex proteins have been designed by evolution to perform three tasks: bind high densities of pigments (chlorophylls, bilins and carotenoids); create a functional funnel that efficiently transfers the absorbed energy into the reaction center; and self-assemble into units that allow the performance of the former tasks. These antenna structures must function correctly while preventing potentially deleterious side effects that could occur due to the proximity of excited molecules with other proteins. A variety of very different geometric solutions has evolved to attain these very same goals, to a much greater extent than the differences between reaction centers [3-5]. The four major antenna forms are: the circular transmembrane light-harvesting complexes (LH1 and LH2) of purple non - sulfur bacteria - the compact transmembrane light-harvesting complexes (LHC-I, LHC-II) of prochlorophytes, green algae, and plants -7, 8]- the membrane associated phycobilisome of cyanobacteria and red algae [9, 10]; and, the organelle-like chlorosome of green sulfur and green filamentous bacteria (Chlorobi and Chloroflexi) . As a result ofthe exceptionally different morphological nature of these antenna systems, each one is described in a separate chapter. This chapter describes in depth the knowledge obtained from the determination of near-atomic resolution crystal structures of isolated components of the phycobillisome. Models of the entire complex based on these structures will be presented along with a description of how the phycobillisome performs its tasks.
244 | 11 Structure of the Phycobilisome Antennae in Cyanobacteria and Red Algae 11.1.2
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