Excitation of the M intermediates of bacteriorhodopsin

Area of Science:

• Biophysics
• Structural Biology
• Photochemistry

Background:

• Bacteriorhodopsin (bR) is a light-driven proton pump crucial for cellular energy generation.
• Understanding the dynamics of its intermediates, particularly the M state, is key to elucidating its mechanism.
• Previous studies suggested a single-phase electric response for M intermediates.

Purpose of the Study:

• To investigate the detailed kinetic and electric response signals of wild-type bacteriorhodopsin's M intermediates.
• To resolve discrepancies with prior findings regarding the M intermediate's response.
• To assign specific molecular events to observed electric signal components.

Main Methods:

• Recording of protein electric response signals (PERS) from M intermediates.
• Kinetic analysis of the recorded signals.
• Proton motion indicator experiments.
• Molecular dipole calculations.

Main Results:

• A three-component electric response was identified, contradicting previous single-phase reports.
• The fastest and slowest components were negative, while the middle component was positive.
• The fastest component corresponds to proton transfer from D85 to the Schiff base.
• The middle component is linked to protein core rearrangements.
• The slowest component reflects charge rearrangements during proton uptake from the bulk.

Conclusions:

• The M intermediate of bacteriorhodopsin exhibits a complex, multi-phase electric response.
• These phases are attributed to sequential proton transfer and conformational changes during the photocycle.
• The findings provide a more refined model for the M state dynamics and proton pumping mechanism in bR.