Abstract
More than 1,000 rhodopsins and their related pigments (rhodopsins) have been identified thus far. The counterion, a negatively-charged amino acid residue which stabilizes a proton on the retinylidene Schiff base of the chromophore in rhodopsin is crucial for receiving a visible light. The mutational analyses by using the vertebrate and invertebrate rhodopsins revealed that counterion displacement from Glu181 to Glu113 had occurred during the molecular evolution of rhodopsins. Further biochemical analyses suggested that acquiring a new counterion Glu113 could be an essential replacement for not only emergence of efficient activation of G protein but also emergence of red-sensitive visual pigments.
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