Assignment of groups responsible for the "opsin shift" and light absorptions of rhodopsin and red, green, and blue iodopsins (cone pigments).

Abstract

A modified structural model of rhodopsin is presented. Seven (a-helical) segments of 24 largely hydrophobic amino acid residues are assembled with exobilayer connecting strands into an aligned set, using the sequences of human red, green, and blue iodopsins (cone pigments) and human and bovine rod rhodopsins. (Aligned set numbering is used in this article.) The inner region of the heptahelical hydrophobic domain includes one His-Glu (Asp) ion pair (red, green rod) near the retinylidene moiety in addition to an iminium ion Asp-99 pair. The negative charges posited in the "point-charge model" to cause the shift of the retinylidene iminium ion light absorption to longer wavelengths in the protein ("opsin shift") are Asp-" (red, green rod), green), and Glu-138 (rod). Blue iodopsin lacks both an ion pair and a counter charge to the iminium ion in the inner region, a fact that explains its absorption relative to rod rhodopsin. The spectroscopic difference between rod rhodopsin and the red/green iodopsins is due to the influence of Glu-102 in the latter. The red-green difference is due to the net effect of seven OH groups around the chromophore, all such groups being found within one helix turn of the retinylidene location. The tryptophan, which rotates as the retinylidene group isomerizes, may be Trp-142 or Trp-177. The geometric change (the rhodopsin "photoswitch") resulting from cis-trans isomerization in the first excited electronic state (S), ultimately leads to RX (photoactivated rhodopsin, metarhodopsin II) and changes the activity of exobilayer groups, possibly causing dissociation of Lys-83 and Arg-85 from the carboxylate groups at positions 263 and 265.Visible light (400-700 nm) is converted into a neural signal in animals via light absorption by rhodopsin (R) in retinal rod cells (iodopsin in cone cells) (1, 2). The 11-cis-retinylidene chromophore is converted to the 11-trans structure in the first excited electronic state (Si), which then decays to photorhodopsin (3). Photorhodopsin changes within 40 ps (3) to bathorhodopsin, which is transformed via lumirhodopsin and metarhodopsin I to metarhodopsin II. Metarhodopsin II [the phototransformed protein, RX (4)] initiates processes that temporarily decrease cyclic guanosine 3',5'-monophosphate (cGMP) within the cell (5, 6). The initial step is the phototransformed rhodopsin (iodopsin)-promoted displacement of guanosine diphosphate (GDP) by guanosine triphosphate (GTP) from the transducin [T; G-protein (5-7)] complex with GDP. The flow of information to the nervous system about a photon absorbed by a rod or cone may be expressed in the steps, photon + R --RX -* T-GTP -* (-cGMP) (-signal to nervous system). A "pointcharge" model (a negative charge stabilizing the retinylidene excited state more than the ground state) has been proposed to explain the shift of the retinylidene iminium ion light absorption to longer wavelengths in the protein ("opsin shift") (8-11).On the basis of the alignment of the amino acid sequences of human rod rhodopsin and red, gr...

The genetics of normal and defective color vision

“…It had been pretty well agreed that binding of the chromophore to opsin red-shifted the chromophore's absorption spectrum, and that amino acid sequence differences among the opsins were responsible for the spectral characteristics of each of the cone pigments (Chen, Nakamura, Ebrey, Ok, Konno, Derguini Nakanishi & Honig, 1989, Kosower, 1988, Wald, 1967). More recent technical innovations made it possible to measure spectral sensitivities of individual cone classes (Baylor, Nunn & Schnapf, 1987, Dartnall, Bowmaker & Mollon, 1983b, Kraft, Neitz & Neitz, 1998, Schnapf, Kraft & Baylor, 1987) and to evaluate the effects of amino acid sequence differences on spectral sensitivity (Asenjo, Rim & Oprian, 1994, Carroll, Neitz & Neitz, 2002, Merbs & Nathans, 1992, Merbs & Nathans, 1993, Merbs, 1992, Neitz et al, 1995b, Sharpe et al, 1998, Stockman, Sharpe, Merbs & Nathans, 2000).…”

Comparison of DNA Sequences of Exon-4 or Exon-5 in Visual Pigment Genes between Amphidromous and Landlocked Ayu-fish, Plecoglossus Altivelis.

“…It is unlikely that the change influences the spectral absorption of the rhodopsin pigment remarkably, because the amino acid at this position was reported to have no influence to the light absorption of visual pigments (Kosower, 1988;Hunt et al, 1993). One possibility is that we have found the polymorphic forms of rhodopsin pigment in the species.…”

Molecular genetics of human color vision

“…The presence of hydroxylbearing vs nonpolar residues constitutes the differ-ence at seven positions (amino acid residues 65, 180, 230, 233,277, 285, and 309), all of which are likely to interact with the chromophore and influence its absorption spectrum (Kosower, 1988). Two lines of evidence support the hypothesis that spectral tuning could result from the substitution of polar for nonpolar residues.…”