Chloride- and pH-dependent proton transport by BR mutant D85N

Area of Science:

• Biophysics
• Photochemistry
• Membrane Protein Function

Background:

• Bacteriorhodopsin (BR) is a light-driven proton pump.
• Understanding ion translocation mechanisms is crucial for bioenergetics.
• The D85N BR mutant offers insights into proton transport pathways.

Purpose of the Study:

• To investigate the photocurrents and proton pumping activity of the D85N BR mutant.
• To determine the pH dependence of ion translocation.
• To explore the effects of azide and chloride on BR function.

Main Methods:

• Recording photocurrents from purple membrane suspensions adsorbed to black lipid membranes (BLM).
• Studying pH dependence (4.5-10.5) under different light conditions (yellow, blue, white).
• Analyzing the effects of azide and sodium chloride on stationary currents.

Main Results:

• Outward proton pumping observed in yellow and blue light, pH-dependent and similar to wild-type.
• Inward proton translocation (two-photon process) observed in white light, also pH-dependent.
• Azide significantly increased currents in blue and white light, indicating binding; chloride enhanced currents in yellow light, with azide potentially competing for binding sites.

Conclusions:

• The D85N BR mutant exhibits proton pumping activity and inverse proton translocation.
• Azide and chloride ions modulate proton transport, suggesting specific binding interactions.
• Results are discussed within the framework of the IST model for ion translocation by retinal proteins.