Membrane potential stabilizes the O intermediate in liposomes containing bacteriorhodopsin.

Area of Science:

• Biophysics
• Photochemistry
• Membrane Protein Dynamics

Background:

• Bacteriorhodopsin functions as a light-driven proton pump, crucial for cellular energy generation.
• Understanding the photocycle intermediates of bacteriorhodopsin is key to elucidating its proton translocation mechanism.
• The role of specific amino acid residues and membrane potential in modulating these intermediates remains an active area of research.

Purpose of the Study:

• To investigate the formation and decay kinetics of a laser-induced bathointermediate in bacteriorhodopsin.
• To determine the influence of pH, uncouplers, lipophilic ions, and specific mutations (E204Q) on this intermediate.
• To explore the effect of membrane potential on the equilibrium between bacteriorhodopsin ground and O intermediate states.

Main Methods:

• Spectroscopic analysis of bacteriorhodopsin-containing proteoliposomes and purple membranes.
• Laser flash photolysis to induce and monitor transient intermediates.
• Utilized wild-type and E204Q bacteriorhodopsin mutants.
• Varied pH conditions and employed uncouplers and lipophilic ions.

Main Results:

• A laser flash induces a bathointermediate in bacteriorhodopsin proteoliposomes, with a decay time of several seconds.
• The formation of this intermediate is suppressed in the E204Q mutant and influenced by pH and membrane-perturbing agents.
• Membrane potential appears to stabilize the O intermediate by affecting the pK of Asp-85, particularly at low pH.

Conclusions:

• The bathointermediate is sensitive to the protonation state of key residues and the surrounding membrane environment.
• Membrane potential plays a significant role in stabilizing the O intermediate, likely through direct effects on residue pK values.
• Protonation of the outward proton pathway residues is essential for the O intermediate state, with membrane potential modulating this equilibrium.