Analysis of conformational changes in bacteriorhodopsin upon retinal removal

The conformation of bacterioopsin in the apomembrane has been studied by Fourier transform infrared spectroscopy. Resolution enhancement techniques and curve-fitting procedures have been used to determine the secondary structural components from the amide I region. Bacterioopsin contains about 54% helicoidal structure (α(I) and α(II) helices + 3₁₀ turns), 21% sheets, 16% reverse turns, and 9% unordered structure. Thus, after retinal removal, all of the secondary structural types of bacteriorhodopsin remain present, and only slight quantitative differences appear. On the other hand, H/D exchange studies show that there is a higher degree of exchange for reverse turns and protonated carboxylic lateral chains in bacterioopsin as compared to bacteriorhodopsin. This gives further support to the idea of a more open tertiary structure of bacterioopsin, and to the consideration of the retinal molecule as an important element in complementing the interhelical interactions in bacteriorhodopsin folding.