They are arranged so that PE (absorption maximum A max 540–570 nm) is on the outside of the structure, PC (A max 610–620 nm) is in the middle and APC (A max 655 nm) is nearest to the photosynthetic membrane. The phycobilisomes are usually made up of phycoerythrin (PE), phycocyanin (PC) and allophycocyanin (APC). These large, water-soluble pigment–protein complexes bind to the surface of the photosynthetic membranes and funnel absorbed light energy into the chlorophyll-containing photosystems. The major light-harvesting complexes of cyanobacteria and red algae are the phycobilisomes (Zhao et al. This explains some of the differences seen in previous lower resolution structures determined at two different pH values (Kumar et al. We show that some subtle differences in one of these PEB binding sites in two of the 12 subunits are caused by crystal contacts between neighboring hexamers in the crystal lattice. It was possible, however, to suggest a model for which chromophores contribute to the different regions of absorption spectrum and propose a tentative scheme for energy transfer. These precise structural details will facilitate theoretical calculations of each PEB’s spectroscopic properties. This improved structure has allowed us to define in great detail the structure of the PEBs and their binding sites. In order for the pigment present, phycoerythrobilin (PEB), to function as an efficient light-harvesting molecule it must be held rigidly (Kupka and Scheer in Biochim Biophys Acta 1777:94–103, 2008) and, moreover, the different PEB molecules in PE must be arranged, relative to each other, so as to promote efficient energy transfer between them. A09DM has been crystallized using different conditions and its structure determined to atomic resolution (1.14 Å).
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