Opsin

A general term for the protein portion of visual substances (including bacteriorhodopsin). Although it varies depending on the species and the type of photoreceptor, there are two types: rod opsin, which works in mesopic vision (distinguishing between light and dark), and cone opsin, which works in diurnal vision (distinguishing between colors). The protein portion of rhodopsin belongs to the former. The protein portion has a wide absorption spectrum (350-620 nm) due to Schiff bonds between retinal and lysine residues in the protein portion. The molecular weight of rod opsin is 4 x 10 ⁴ , with about 50% being hydrophobic amino acids, and it has oligosaccharide chains but is insoluble in water. In humans, 348 amino acid residues form seven α-helices. Cone opsins are divided into opsins that distinguish red, blue, and green, and it is known that red and green are made up of 346 amino acid residues, and blue is made up of 348 amino acid residues. There is considerable homology in the structures of these opsins.

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

視覚物質(バクテリオロドプシンを含む)中のタンパク質部分の総称．生物種や視細胞の種類により異なるが，薄明視(明暗の区別)にはたらく桿(かん)体オプシンと，昼間視(色の区別)にはたらく錐体オプシンとの2種類がある．ロドプシンのタンパク質部分は前者に属する．レチナールがタンパク質部分のリシン残基とシッフ結合することにより，広い吸収スペクトル(350～620 nm)をもつようになる．桿体オプシンの分子量は4×10⁴ で，疎水性アミノ酸が約50％ を占め，オリゴ糖鎖をもつが水に不溶である．ヒトでは348個のアミノ酸残基が7本のαへリックスを形成している．錐体オプシンは，赤，青，緑の各色を区別するオプシンに分かれており，赤と緑346個，青は348個のアミノ酸残基からなることが知られている．これらオプシンの構造には，かなりの相同性がみられる．

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