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Publication Abstract from PubMed

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 A. 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 alpha-subunit (lambda(max) = 697 nm). On complexation with the beta-subunit, it is even further red-shifted (lambda(max, absorption) = 707 nm, lambda(max, emission) = 721 nm). The relevance of ring A for this shift is supported by mutagenesis data. A variant of the alpha-subunit, I123M, has been generated that shows an intense FR-band already in the absence of the beta-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.

Crystallographic and biochemical analyses of a far-red allophycocyanin to address the mechanism of the super-red-shift.,Zhou LJ, Hoppner A, Wang YQ, Hou JY, Scheer H, Zhao KH Photosynth Res. 2024 Jan 6. doi: 10.1007/s11120-023-01066-2. PMID:38182842^[1]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.