Analogies between halorhodopsin and bacteriorhodopsin

Area of Science:

• Biochemistry
• Molecular Biology
• Biophysics

Background:

• Bacteriorhodopsin and halorhodopsin are light-activated ion pumps belonging to the retinal protein family.
• Both proteins feature seven transmembrane alpha helices surrounding a retinal binding pocket.

Purpose of the Study:

• To compare the structure, photocycle, and ion-transport mechanisms of bacteriorhodopsin and halorhodopsin.
• To elucidate the energetic and kinetic differences in their ion-pumping functions.

Main Methods:

• Time-resolved spectroscopy to determine intermediate spectra (K, L, N, O).
• Absorption kinetic measurements to analyze photocycle dynamics.
• Photoinduced transient electric signal measurements to characterize electrogenicity.

Main Results:

• Spectra of photocycle intermediates are highly similar between bacteriorhodopsin and halorhodopsin.
• Halorhodopsin's photocycle lacks the M intermediate and is dominated by L intermediate.
• Bacteriorhodopsin's photocycle is entropy-driven, while halorhodopsin's is enthalpy-driven.

Conclusions:

• The 'local access' model adequately describes the function of both bacteriorhodopsin and halorhodopsin.
• Ion specificity can be altered in these proteins, enabling conversion between proton and chloride pumps.