title: Glutamic acid-113 serves as the retinylidene Schiff base counterion in bovine rhodopsin.
abstract: The characteristic wavelength at which a visual pigment absorbs light is regulated by interactions between protein (opsin) and retinylidene Schiff base chromophore. By using site-directed mutagenesis, charged amino acids in bovine rhodopsin transmembrane helix C were systematically replaced. Substitution of glutamic acid-134 or arginine-135 did not affect spectral properties. However, substitution of glutamic acid-122 by glutamine or by aspartic acid formed pigments that were blue-shifted in light absorption (lambda max = 480 nm and 475 nm, respectively). While the substitution of glutamic acid-113 by aspartic acid gave a slightly red-shifted pigment (lambda max = 505 nm), replacement by glutamine formed a pigment that was strikingly blue-shifted in light absorption (lambda max = 380 nm). The 380-nm species existed in a pH-dependent equilibrium with a 490-nm species such that at acidic pH all of the pigment was converted to lambda max = 490 nm. We conclude that glutamic acid-113 serves as the retinylidene Schiff base counterion in rhodopsin. We believe that this opsin-chromophore interaction is an example of a general mechanism of color regulation in the visual pigments.
authors.affiliation: Department of Biology, Massachusetts Institute of Technology, Cambridge 01239.
authors.forename: T P
authors.initials: TP
authors.lastname: Sakmar
authors.forename: R R
authors.initials: RR
authors.lastname: Franke
authors.forename: H G
authors.initials: HG
authors.lastname: Khorana
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001224:Aspartic Acid; D002417:Cattle; D020558:GTP Phosphohydrolases / Q000378:metabolism; D005971:Glutamates; D018698:Glutamic Acid; D007700:Kinetics; D008565:Membrane Proteins / Q000378:metabolism; D008969:Molecular Sequence Data; D009154:Mutation; D011487:Protein Conformation; D012168:Retinal Pigments / Q000378:metabolism*; D012176:Retinoids / Q000378:metabolism*; D012243:Rhodopsin / Q000235:genetics / Q000378:metabolism*; D012545:Schiff Bases; D012689:Sequence Homology, Nucleic Acid; D015293:Transducin / Q000378:metabolism
chemical_list: D005971:Glutamates; D008565:Membrane Proteins; D012168:Retinal Pigments; D012176:Retinoids; D012545:Schiff Bases; D001224:Aspartic Acid; D018698:Glutamic Acid; D012243:Rhodopsin; D020558:GTP Phosphohydrolases; D015293:Transducin