The structure of allophycocyanin B from Synechocystis PCC 6803 reveals the structural basis for the extreme redshift of the terminal emitter in phycobilisomes
Allophycocyanin B (AP‐B) is one of the two terminal emitters in phycobilisomes, the unique light‐harvesting complexes of cyanobacteria and red algae. Its low excitation‐energy level and the correspondingly redshifted absorption and fluorescence emission play an important role in funnelling excitatio...
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Published in | Acta crystallographica. Section D, Biological crystallography. Vol. 70; no. 10; pp. 2558 - 2569 |
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Main Authors | , , , , , , , |
Format | Journal Article |
Language | English |
Published |
5 Abbey Square, Chester, Cheshire CH1 2HU, England
International Union of Crystallography
01.10.2014
Wiley Subscription Services, Inc |
Subjects | |
Online Access | Get full text |
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Summary: | Allophycocyanin B (AP‐B) is one of the two terminal emitters in phycobilisomes, the unique light‐harvesting complexes of cyanobacteria and red algae. Its low excitation‐energy level and the correspondingly redshifted absorption and fluorescence emission play an important role in funnelling excitation energy from the hundreds of chromophores of the extramembraneous phycobilisome to the reaction centres within the photosynthetic membrane. In the absence of crystal structures of these low‐abundance terminal emitters, the molecular basis for the extreme redshift and directional energy transfer is largely unknown. Here, the crystal structure of trimeric AP‐B [(ApcD/ApcB)3] from Synechocystis sp. PCC 6803 at 1.75 Å resolution is reported. In the crystal lattice, eight trimers of AP‐B form a porous, spherical, 48‐subunit assembly of 193 Å in diameter with an internal cavity of 1.1 × 106 Å3. While the overall structure of trimeric AP‐B is similar to those reported for many other phycobiliprotein trimers, the chromophore pocket of the α‐subunit, ApcD, has more bulky residues that tightly pack the phycocyanobilin (PCB). Ring D of the chromophores is further stabilized by close interactions with ApcB from the adjacent monomer. The combined contributions from both subunits render the conjugated rings B, C and D of the PCB in ApcD almost perfectly coplanar. Together with mutagenesis data, it is proposed that the enhanced planarity effectively extends the conjugation system of PCB and leads to the redshifted absorption (λmax = 669 nm) and fluorescence emission (679 nm) of the ApcD chromophore in AP‐B, thereby enabling highly efficient energy transfer from the phycobilisome core to the reaction centres. |
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Bibliography: | istex:252FF55B43E35B6BC76631896847B87FA2B6EA1C ark:/67375/WNG-91FL79L0-R ArticleID:AYD2TZ5058 ObjectType-Article-1 SourceType-Scholarly Journals-1 ObjectType-Feature-2 content type line 23 FOREIGN |
ISSN: | 1399-0047 0907-4449 1399-0047 |
DOI: | 10.1107/S1399004714015776 |