Fourier transform infrared spectroscopy indicates a major conformational rearrangement in the activation of rhodopsin

The study of the structural differences between rhodopsin and its active form (metarhodopsin II) has been carried out by means of deconvolution analysis of infrared spectra. Deconvolution techniques allow the direct identification of the spectral changes that have occurred, which results in a significantly different view of the conformational changes occurring after activation of the receptor as compared with previous difference spectroscopy analysis. Thus, a number of changes in the bands assigned to solvent-exposed domains of the receptor are detected, indicating significant decreases in extended (beta) sequences and in reverse turns, and increases in irregular/aperiodic sequences and in helices with a non-alpha geometry, whereas there is no decrease in alpha-helices. In addition to secondary structure conversions, qualitative alterations within a given secondary structure type are detected. These are seen to occur in both reverse turns and helices. The nature of this spectral change is of great importance, since a clear alteration in the helices bundle core is detected. All these changes indicate that the rhodopsin --> metarhodopsin II transition involves not a minor but a major conformational rearrangement, reconciling the infrared data with the energetics of the activation process. © 1995, The Biophysical Society. All rights reserved.