Structural changes in the peptide backbone in complex formation between activated rhodopsin and transducin studied by FTIR spectroscopy.

Structural changes in the complex formation between transducin and metarhodopsin II, the activated form of photolyzed rhodopsin, in visual transduction processes were analyzed by Fourier transform infrared spectroscopy. The spectrum of the complex was obtained by subtracting the contribution of metarhodopsin I and uncomplexed metarhodopsin II. The averaged spectrum upon the complex formation was then compared with that in the conversion of rhodopsin-to-metarhodopsin II. Frequency shifts of the peptide carbonyl vibrations at 1686, 1674, and 1661 cm-1 to 1640 cm-1 were observed upon complex formation from metarhodopsin II plus transducin. These changes must have resulted from the strengthening of H-bonding of one or a few peptide groups but is not ascribable to global conformation change. Changes in the frequencies of the peptide amides were also detected. With regard to intramembrane carboxylic acid residues, no further changes were noticed in the carboxyl vibrations of Asp83, Glu122, and Glu113. Only a small change possibly due to Glu134 was detected.