Ultrafast, remote-controlled protonation reaction enables structural changes in a phytochrome.

Area of Science:

• Biochemistry
• Structural Biology
• Photochemistry

Background:

• Photoactive proteins rely on precise chromophore-protein interactions.
• Proton transfer reactions, like proton-coupled electron transfer, are known coupling mechanisms.
• Undiscovered proton dislocation mechanisms may exist.

Purpose of the Study:

• To investigate the mechanism of photoexcitation energy transfer in phytochrome.
• To elucidate novel proton transfer pathways in photoactive proteins.
• To understand how photoexcitation signals are transduced within protein matrices.

Main Methods:

• Femtosecond crystallography to capture dynamic structural changes.
• Molecular modeling to simulate and interpret observed rearrangements.
• Femtosecond infrared spectroscopy for independent confirmation of proton dynamics.

Main Results:

• A femtosecond movie revealed a space-conserving D-ring rotation in the excited state.
• Rearrangement of a hydrogen bond network occurred within 300 fs.
• A protonation shift of histidine-260 was identified as the key event preceding chromophore isomerization.

Conclusions:

• A 'remote-controlled' proton transfer mechanism involving histidine-260 was proposed.
• This mechanism rapidly relays photoexcitation to the protein matrix.
• This proton transfer pathway may be a widespread mechanism for signal transduction in enzymes.