Analysis of conformational changes in bacteriorhodopsin upon retinal removal

Area of Science:

• Biophysics
• Structural Biology
• Spectroscopy

Background:

• Bacteriorhodopsin is a light-driven proton pump found in Halobacterium salinarum.
• Understanding the structural changes upon retinal removal (bacterioopsin) is crucial for elucidating protein folding and function.
• Fourier transform infrared spectroscopy (FTIR) is a powerful tool for analyzing protein secondary structure.

Purpose of the Study:

• To quantitatively determine the secondary structural components of bacterioopsin using FTIR.
• To compare the secondary and tertiary structures of bacterioopsin with those of bacteriorhodopsin.
• To investigate the role of retinal in stabilizing the tertiary structure of bacteriorhodopsin.

Main Methods:

• Fourier transform infrared spectroscopy (FTIR) was employed to analyze bacterioopsin.
• Resolution enhancement techniques and curve-fitting procedures were applied to the amide I band for secondary structure analysis.
• Hydrogen-deuterium (H/D) exchange studies were conducted to probe tertiary structure differences.

Main Results:

• Bacterioopsin consists of approximately 54% helical structure, 21% sheets, 16% reverse turns, and 9% unordered structure.
• All secondary structural types present in bacteriorhodopsin are also found in bacterioopsin, with only minor quantitative variations.
• H/D exchange studies indicated a more open tertiary structure in bacterioopsin, with increased exchange in reverse turns and protonated carboxylic groups.

Conclusions:

• Bacterioopsin maintains a significant amount of secondary structure similar to bacteriorhodopsin.
• The retinal molecule plays a critical role in stabilizing the tertiary structure of bacteriorhodopsin through interhelical interactions.
• Bacterioopsin possesses a more flexible and open tertiary conformation compared to the retinal-bound bacteriorhodopsin.