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 tec hniques and curve-fitting procedures have been used to determine the s econdary structural components from the amide I region, Bacterioopsin contains about 54% helicoidal structure (alpha(I) and alpha(II) helice s + 3(10) turns), 21% sheets, 16% reverse turns, and 9% unordered stru cture. Thus, after retinal removal, all of the secondary structural ty pes 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 bacteriorhod opsin. This gives further support to the idea of a more open tertiary structure of bacterioopsin, and to the consideration of the retinal mo lecule as an important element in complementing the interhelical inter actions in bacteriorhodopsin folding.