The retinylidene Schiff base counterion in bacteriorhodopsin

Area of Science:

• Biochemistry
• Structural Biology
• Photochemistry

Background:

• Bacteriorhodopsin is a light-driven proton pump with a retinylidene Schiff base chromophore.
• Interactions between amino acid residues (Asp-85, Asp-212, Arg-82) and the Schiff base are crucial for function.
• The precise role of these residues as counterions to the Schiff base requires further elucidation.

Purpose of the Study:

• To investigate the counterion environment of the retinylidene Schiff base in bacteriorhodopsin.
• To determine the specific roles of Asp-85 and Asp-212 in Schiff base stabilization and protonation.
• To analyze the impact of mutations on chromophore regeneration, pKa, and spectral properties.

Main Methods:

• Construction and characterization of single and double mutants of Asp-85 and Asp-212 in bacteriorhodopsin.
• Spectroscopic analysis, including absorption spectroscopy, to monitor chromophore formation and spectral shifts.
• pKa measurements of the Schiff base in wild-type and mutant proteins.

Main Results:

• A single carboxylate group at either position 85 or 212 is necessary for retinal binding and protonated Schiff base formation.
• Mutations at Asp-85 significantly lower Schiff base pKa and cause large red shifts, partially reversible by anions.
• Asp-85 is identified as the primary counterion, while Asp-212 plays a less critical role; exogenous anions can act as surrogate counterions.

Conclusions:

• Aspartic acid-85 serves as the essential counterion for the retinylidene Schiff base in wild-type bacteriorhodopsin.
• Aspartic acid-212 contributes to Schiff base stabilization but is not the primary counterion.
• The study elucidates the specific roles of Asp-85 and Asp-212 in bacteriorhodopsin's photocycle and provides insights into anion binding.