Structural comparison of allophycocyanin variants reveals the molecular basis for their spectral differences

Christopher J. Gisriel, Eduard Elias, Gaozhong Shen, Nathan T. Soulier, Gary W. Brudvig, Roberta Croce*, Donald A. Bryant*

*Corresponding author for this work

Research output: Contribution to JournalArticleAcademicpeer-review

Abstract

Allophycocyanins are phycobiliproteins that absorb red light and transfer the energy to the reaction centers of oxygenic photosynthesis in cyanobacteria and red algae. Recently, it was shown that some allophycocyanins absorb far-red light and that one subset of these allophycocyanins, comprising subunits from the ApcD4 and ApcB3 subfamilies (FRL-AP), form helical nanotubes. The lowest energy absorbance maximum of the oligomeric ApcD4-ApcB3 complexes occurs at 709 nm, which is unlike allophycocyanin (AP; ApcA-ApcB) and allophycocyanin B (AP-B; ApcD-ApcB) trimers that absorb maximally at ~ 650 nm and ~ 670 nm, respectively. The molecular bases of the different spectra of AP variants are presently unclear. To address this, we structurally compared FRL-AP with AP and AP-B, performed spectroscopic analyses on FRL-AP, and leveraged computational approaches. We show that among AP variants, the α-subunit constrains pyrrole ring A of its phycocyanobilin chromophore to different extents, and the coplanarity of ring A with rings B and C sets a baseline for the absorbance maximum of the chromophore. Upon oligomerization, the α-chromophores of all AP variants exhibit a red shift of the absorbance maximum of ~ 25 to 30 nm and band narrowing. We exclude excitonic coupling in FRL-AP as the basis for this red shift and extend the results to discuss AP and AP-B. Instead, we attribute these spectral changes to a conformational alteration of pyrrole ring D, which becomes more coplanar with rings B and C upon oligomerization. This study expands the molecular understanding of light-harvesting attributes of phycobiliproteins and will aid in designing phycobiliproteins for biotechnological applications.

Original languageEnglish
Pages (from-to)157-170
Number of pages14
JournalPhotosynthesis Research
Volume162
Issue number2
Early online date29 Sept 2023
DOIs
Publication statusPublished - Dec 2024

Bibliographical note

Publisher Copyright:
© The Author(s) 2023.

Keywords

  • Energy transfer
  • Exciton coupling
  • Far-red light photoacclimation
  • Low-light photoacclimation
  • Photosynthesis
  • Phycobiliprotein
  • Phycocyanobilin

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