Electrostatic Influence on Photoisomerization in Bacteriorhodopsin and Halorhodopsin

Area of Science:

• Structural biology and biophysics of membrane proteins.
• Photochemistry and photophysics of visual pigments.
• Computational chemistry and molecular dynamics simulations.

Background:

• Bacteriorhodopsin (bR) and halorhodopsin (hR) are halobacterial membrane proteins.
• Both proteins utilize photoactivated retinal protonated Schiff base (RPSB) isomerization for ion transport.
• bR and hR display distinct RPSB isomerization rates and quantum yields, with bR being more efficient.

Purpose of the Study:

• To elucidate the origin of differing photoisomerization efficiencies between bR and hR.
• To investigate the role of electrostatic interactions within the protein pocket surrounding the RPSB chromophore.
• To determine how protein electrostatics influence RPSB isomerization dynamics and quantum yields.

Main Methods:

• Utilized ab initio multiple spawning (AIMS) simulations combined with quantum mechanics/molecular mechanics (QM/MM).
• Simulated photoisomerization processes in bR and hR.
• Systematically modified the charge of the complex counterion in the protein pocket to alter electrostatic environment.

Main Results:

• Computational simulations successfully reproduced experimental observations of differing isomerization rates and quantum yields.
• Modifying the counterion charge in hR to be more negative induced bR-like isomerization behavior.
• Conversely, altering the counterion charge in bR to be less negative resulted in hR-like isomerization characteristics.

Conclusions:

• Electrostatic interactions within the protein pocket are critical determinants of RPSB photoisomerization outcomes.
• The observed differences in bR and hR function can be attributed to variations in their internal electrostatic environments.
• Protein electrostatics provide a key mechanism for tuning the efficiency and kinetics of chromophore photoisomerization.