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 λmax 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 λmax 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 λmax. 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 λmax 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.
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