Complex active site structures influence absorption spectrum of Chrimson wild type and mutants

Area of Science:

• Optogenetics
• Biophysics
• Computational Biology

Background:

• Chrimson is a red light-activated channelrhodopsin crucial for optogenetics.
• Understanding its spectral shift is key for applications like vision restoration.
• Previous studies provided structural data but lacked molecular details of the spectral shift.

Purpose of the Study:

• To computationally investigate the molecular determinants of Chrimson's red-shifted absorption spectrum.
• To analyze the role of active site flexibility and specific residues in spectral tuning.

Main Methods:

• Quantum Mechanics/Molecular Mechanics (QM/MM) computational approach.
• Molecular Dynamics (MD) simulations to sample conformational ensembles.
• Comparison of computed spectra with experimental data.

Main Results:

• Identified significant active site flexibility with multiple conformations on the nano-second timescale.
• Highlighted the role of residue S169 in modulating the flexibility of counterion E165.
• Found a direct hydrogen bond between counterions as a key factor for red-shifted absorption, more prevalent in the S169A mutant.

Conclusions:

• Active site dynamics and specific residue interactions are critical for Chrimson's spectral properties.
• The S169A mutation enhances red-shifted absorption through altered conformational dynamics and counterion interactions.
• Computational models accurately reflect experimental observations, providing insights into channelrhodopsin function.