title: Spectroscopic analysis of the effect of chloride on the active intermediates of the primate L group cone visual pigment.
abstract: Cone visual pigments responsible for color vision are classified into four groups; among these, the L(LWS) group contains the visual pigments having the most red-shifted λ(max) and a chloride-binding site in their protein moiety. Binding of chloride results in the so-called "chloride effect", e.g., the red shift of λ(max) and the faster decay of meta-I. These properties disappear upon replacement of chloride with nitrate. Because the amino acid residue primary responsible for the chloride effect is H197, we have replaced this residue with 19 other amino acids to gain insights into the mechanism creating these properties. Of the 19 single-site mutants, 13 were successfully expressed and bound 11-cis-retinal to form pigments. Eleven of the 13 mutants exhibited a red shift of λ(max) upon chloride binding, and histidine produced the most red-shifted λ(max). We classified H197 mutants into three groups according to their properties. The first group of mutants exhibited a chloride effect similar to that of the wild type, while the second group of mutants showed no chloride effect. The third group of mutants exhibited a small shift in λ(max) and enhanced decay rates of meta-I upon chloride binding. Furthermore, some of the mutants in this group showed meta-I decay faster than that of the wild type and extraordinarily fast decays of meta-I even in the absence of chloride. Interestingly, amino acid residues in the third group of mutants are characterized by their propensity to form β-sheets. These results suggest that the acquisition of H197 would be due to the most red-shifted absorption maximum, resulting in fast formation of the active state.
authors.affiliation: Department of Biophysics, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan.
authors.forename: Takefumi
authors.initials: T
authors.lastname: Morizumi
authors.forename: Keita
authors.initials: K
authors.lastname: Sato
authors.forename: Yoshinori
authors.initials: Y
authors.lastname: Shichida
mesh_terms: D000818:Animals; D002712:Chlorides / Q000494:pharmacology*; D057809:HEK293 Cells; D006639:Histidine / Q000737:chemistry; D006801:Humans; D008252:Macaca fascicularis; D016297:Mutagenesis, Site-Directed; D012168:Retinal Pigments / Q000737:chemistry*; D012172:Retinaldehyde / Q000737:chemistry; D012243:Rhodopsin / Q000737:chemistry; D013053:Spectrophotometry
chemical_list: D002712:Chlorides; D012168:Retinal Pigments; D006639:Histidine; D012243:Rhodopsin; D012172:Retinaldehyde