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The Antenna Complex01:15

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Plants and other photosynthetic organisms comprise pigments capable of absorption of direct sunlight. These pigments are present in the reaction center - the main site of photochemical reactions as well as in the antenna complex. Under average light conditions, the rate at which reaction center pigments absorb light is far below the electron transport chain's capacity. As a result, the reaction center alone cannot provide enough energy to drive photosynthesis. The photosynthetic efficiency can...
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Although structurally similar to photosystem II (PSII), photosystem I (PSI) is has a different electron supplier and electron acceptor.
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Anoxygenic photosynthesis is a phototrophic process that captures light energy to drive carbon fixation without producing molecular oxygen. Unlike oxygenic photosynthesis, which utilizes water as an electron donor and releases oxygen, anoxygenic phototrophs use alternative electron donors such as hydrogen sulfide (H₂S), elemental sulfur (S⁰), or thiosulfate (S₂O₃²⁻). This process is carried out by diverse groups of bacteria, including purple bacteria, green...
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Photosystems are multiprotein complexes that form the functional units of photosynthesis in plants, algae, and cyanobacteria. They are found embedded in the membrane of tiny sac-like structures called thylakoids placed inside the chloroplast.
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Reaction centers are pigment-protein complexes that initiate energy conversion from photons to chemical entities. Therefore, photochemical reaction center is a more appropriate term that describes these complexes. The Nobel laureates Robert Emerson and William Arnold provided the first experimental evidence of photochemical reaction centers by demonstrating the participation of nearly 2,500 chlorophyll molecules for the release of just one molecule of oxygen. Despite thousands of photosynthetic...
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Light- and pH-dependent structural changes in cyanobacteriochrome AnPixJg2.

Susanne Altmayer1, Lisa Köhler1, Pavlo Bielytskyi1

  • 1Institut Für Analytische Chemie, Universität Leipzig, Linnéstraße 3, 04103, Leipzig, Germany.

Photochemical & Photobiological Sciences : Official Journal of the European Photochemistry Association and the European Society for Photobiology
|April 8, 2022
PubMed
Summary

Cyanobacteriochromes (CBCRs) are key sensors in cyanobacteria. Solid-state NMR reveals bilin chromophore changes and histidine protonation in the AnPixJ protein photocycle, impacting light and pH responses.

Keywords:
Bilin chromophorePhotosensorProtein–chromophore interactionProtonation stateSolid-state NMRStructural modeling

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Area of Science:

  • Biochemistry
  • Spectroscopy
  • Structural Biology

Background:

  • Cyanobacteriochromes (CBCRs) are vital photosensory proteins in cyanobacteria, regulating critical functions like phototaxis and chromatic acclimation.
  • The GAF2 domain of AnPixJ, a specific CBCR, is crucial for these light-dependent processes.

Purpose of the Study:

  • To investigate the structural and electronic changes of the bilin chromophore within the AnPixJ GAF2 domain during its photocycle using solid-state NMR.
  • To elucidate the role of protein-chromophore interactions, hydration, and pH in CBCR function.

Main Methods:

  • Solid-state NMR spectroscopy on uniformly 13C- and 15N-labeled bilin chromophore within the AnPixJ GAF2 domain.
  • Quantum mechanics/molecular mechanics (QM/MM) modeling to understand structural and electronic properties.
  • pH titration studies to assess environmental effects.

Main Results:

  • Confirmed a twisted bilin ring D in both dark and photoproduct states, with increased chromophore heterogeneity in the photoproduct.
  • Identified increased pocket hydration and suggested a biprotonated histidine residue (His322) interacting with the photoproduct chromophore.
  • Demonstrated significant alterations in chromophore electronic structure and protonation upon a small pH drop.

Conclusions:

  • The study provides detailed insights into light- and pH-induced structural and electronic rearrangements of the bilin chromophore in AnPixJ.
  • Findings highlight the correlation between chromophore structural heterogeneity, histidine protonation, and pocket hydration in CBCR photocycling.
  • Results generalize recent theoretical findings and offer a deeper understanding of CBCR mechanisms.