Crystallographic and biochemical analyses of a far-red allophycocyanin to address the mechanism of the super-red-shift.
Photosynth Res
; 2024 Jan 06.
Article
in En
| MEDLINE
| ID: mdl-38182842
ABSTRACT
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 (λmax = 697 nm). On complexation with the ß-subunit, it is even further red-shifted (λmax, absorption = 707 nm, λmax, 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 ß-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.
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Collection:
01-internacional
Database:
MEDLINE
Language:
En
Journal:
Photosynth Res
/
Photosynth. res
/
Photosynthesis research
Journal subject:
METABOLISMO
Year:
2024
Document type:
Article
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