Crystallographic and biochemical analyses of a far-red allophycocyanin to address the mechanism of the super-red-shift

• Published in: Photosynthesis research 2024-12, Vol.162 (2), p.171-185
• Main Authors: Zhou, Li-Juan, Höppner, Astrid, Wang, Yi-Qing, Hou, Jian-Yun, Scheer, Hugo, Zhao, Kai-Hong
• Format: Article
• Language: English
• Subjects: Biochemistry; Biomedical and Life Sciences; Chromatography; Chromophores; Crystal structure; Crystallography, X-Ray; Cyanobacteria; Cyanobacteria - metabolism; E coli; Life Sciences; Light; Light-harvesting proteins; Models, Molecular; Mutagenesis; PCB; Photosynthesis; Phycocyanin - chemistry; Phycocyanin - metabolism; Plant Genetics and Genomics; Plant Physiology; Plant Sciences; Polychlorinated biphenyls; Protein purification; Proteins; Tautomerism
• ISSN: 0166-8595; 1573-5079; 1573-5079
• DOI: 10.1007/s11120-023-01066-2

Far-red absorbing allophycocyanins (APC), identified in cyanobacteria capable of FRL photoacclimation (FaRLiP) and low-light photoacclimation (LoLiP), absorb far-red light, functioning in energy transfer as light-harvesting proteins. We report an optimized method to obtain high purity far-red absorbing allophycocyanin B, AP-B2, of Chroococcidiopsis thermalis sp. PCC7203 by synthesis in Escherichia coli and an improved purification protocol. The crystal structure of the trimer, (PCB-ApcD5/PCB-ApcB2) 3 , has been resolved to 2.8 Å. The main difference to conventional APCs absorbing in the 650–670 nm range is a largely flat chromophore with the co-planarity extending, in particular, from rings BCD to ring A. This effectively extends the conjugation system of PCB and contributes to the super-red-shifted absorption of the α-subunit (λ max  = 697 nm). On complexation with the β-subunit, it is even further red-shifted (λ max, absorption  = 707 nm, λ max, emission  = 721 nm). The relevance of ring A for this shift is supported by mutagenesis data. A variant of the α-subunit, I123M, has been generated that shows an intense FR-band already in the absence of the β-subunit, a possible model is discussed. Two additional mechanisms are known to red-shift the chromophore spectrum: lactam-lactim tautomerism and deprotonation of the chromophore that both mechanisms appear inconsistent with our data, leaving this question unresolved.