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Proton-Coupled Chromophore and Protein Structural Changes Control Phytochrome Activation.

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Phytochromes use proton transfer to switch between states, triggering structural changes. This intramolecular proton transfer is essential for phytochrome function and secondary structure transitions in general.

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Area of Science:

  • Biochemistry
  • Molecular Biology
  • Spectroscopy

Background:

  • Phytochromes are light sensors controlling physiological processes.
  • Photoisomerization initiates phytochrome activation, involving intermediate states like Meta-Rc.
  • The Meta-Rc state is crucial for Pfr formation and phytochrome's tongue structure transition.

Purpose of the Study:

  • Investigate the structure and reactions of the Meta-Rc state in bacterial phytochrome Agp1.
  • Elucidate the role of proton migration in phytochrome signaling.
  • Determine the mechanism of secondary structure transitions in phytochromes.

Main Methods:

  • Infrared (IR) difference spectroscopy.
  • Resonance Raman spectroscopy.
  • Studied Agp1 phytochrome at varying temperatures and pH.

Main Results:

  • Meta-Rc formation involves chromophore enolization and deprotonation; decay involves reprotonation.
  • Proton migration triggers the tongue's secondary structure transition (β-sheet/α-helix interconversion).
  • pH-dependent conformational equilibrium observed in Meta-Rc and Pfr states.

Conclusions:

  • Secondary structure transitions are induced by chromophore-linked proton transfer, not chromophore relaxation.
  • Intramolecular proton transfer is a prerequisite for secondary structure transitions in phytochromes.
  • Findings support a general mechanism for phytochrome signaling across different species.