Proton transport by bacteriorhodopsin through an interface film

Area of Science:

• Biophysics
• Membrane protein function
• Spectroscopy

Background:

• Bacteriorhodopsin in purple membranes serves as a model for studying protein function.
• Oriented bacteriorhodopsin at air-water interfaces allows investigation of its photochemical properties.

Purpose of the Study:

• To investigate the light-induced charge transport mechanism of bacteriorhodopsin.
• To characterize the photopotential generated by bacteriorhodopsin interface films.

Main Methods:

• Utilizing interface films of purple membrane and lipids with oriented bacteriorhodopsin.
• Measuring photopotentials at the air-water interface upon illumination.
• Employing decane layers and adding proton carriers (FCCP, DNP) or anions to modulate the interface.

Main Results:

• Illumination induced small positive photopotentials (<1 mV) at the air-water interface.
• Photopotentials were enhanced by lipid-soluble proton carriers and dependent on excitation fraction and proton acceptor concentration.
• Action spectrum of photopotential matched bacteriorhodopsin absorption spectrum, confirming its involvement.
• Polarity indicated positive charge transport from aqueous to organic phase, consistent with intracellular to extracellular proton translocation by bacteriorhodopsin.

Conclusions:

• Bacteriorhodopsin functions as a light-driven proton pump, translocating protons from its intracellular to extracellular surface.
• The observed photopotentials directly result from bacteriorhodopsin's proton translocation activity.
• Proton transport kinetics are influenced by light excitation and the availability of proton acceptors.