Ultraviolet resonance Raman examination of the light-induced protein structural changes in rhodopsin activation

Ultraviolet resonance Raman (UVRR) spectra of rhodopsin and its metarhodopsin I and metarhodopsin II photointermediates have been obtained to examine the molecular mechanism of G-protein-coupled receptor activation. Spectra were acquired using a single-pass capillary flow technique in combination with a Littrow prism UV prefilter detection system. The UVRR difference spectra between rhodopsin and mearhodopsin I exhibit small differences assignalbe to tyrosine residues and no differences due to tryptophan. The UVRR difference spectra between rhodopsin and metarhodopsin II exhibit significant differences for vibrations of both tryptophan and tyrosine residues. Most importantly, there is and intensity decrease of the totally symmetric tryptophan modes at 759, 1008, and 1545 cm-1, an intensity decrease of the tryptophan W7 band at 1357 cm-1, and a frequency shift of the tryptophan W17 ban from 885 to 892 cm-1. These difference features are assigned to one or more tryptophan residues that reside in a hydrophobic, weakly hydrogen-bonding environment in rhodopsin and that are transferred to a less hydrophobic, non-hydrogen-bonding environment during rhodopsin activation. The available evidence suggests that Trp265 makes a dominant contribution to the tryptophan features in this difference spectrum. These results are interpreted with a model for rhodopsin activation in which retinal isomerization alters the interaction of Trp265 with the ionone ring of the retinal chromophore.