Light induced perturbation of aromatic residues in bovine rhod opsin and bacterio rhod opsin
Photochemistry and Photobiology 29(1): 109-120
Light-induced changes in the UV absorption spectrum of bovine rod outer segment membranes were measured by conventional difference spectroscopy and by flash photolysis methods. Different thermal intermediates of rhodopsin (lumirhodopsin and metarhodopsins I, II and III) have absorption spectra in the UV which differ from the rhodopsin spectrum and from each other. The spectra associated with metarhodopsins I, II and metarhodopsin III are characteristic of perturbation of a small number of Tyr and/or Trp residues, most likely 1 Trp residue. These aromatic residues are in the neighborhood of the retinal Schiff base and undergo coordinated changes of interaction with retinal during the bleaching sequence. At the metarhodopsin II stage, the magnitude of the UV spectral changes is consistent with the exposure of a previously shielded trp residue to an aqueous environment. The present results are consistent with previous spectral studies which limit the extent of light-induced conformational changes to regions of the protein in the neighborhood of the retinal Schiff base. An analogous study was made on light-adapted purple membranes of Halobacterium halobium. The UV absorption spectrum associated with the deprotonated Schiff base intermediate of the trans-bacteriorhodopsin cycle is indicative, in part, of aromatic residue perturbation. However, significant changes in the secondary and tertiary structures of the bacteriorhodopsin protein characteristic of a delocalized conformational change are unlikely at this intermediate stage.