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M. Kono, M.R. Lewis; Interactions between retinal and the second intradiscal loop of rhodopsin modulates activation and deactivation rates . Invest. Ophthalmol. Vis. Sci. 2004;45(13):3632.
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© ARVO (1962-2015); The Authors (2016-present)
The 9–methyl group of 11–cis retinal is important in forming the active Meta II intermediate of rhodopsin. Without this methyl group, activation is impaired. Purpose: to determine if changing the steric bulk of Y191 of rhodopsin will effect rates of formation and/or decay of Meta II and retinal hydrolysis. Methods: Wild–type and Y191 mutant rhodopsins were transiently expressed in COS cells. Pigments were generated with 11–cis retinal and immunopurified. Pigments were studied by absorption spectrophotometry. Results: A Y191I mutation formed MetaII but relaxed quickly with a time constant of about 3 min to a Meta III–like intermediate which slowly decayed to opsin and free retinal. Retinal release from the protein occurred with a time constant of about 1 hr. Under the same conditions, wild–type rhodopsin does not form Meta III and the decay of Meta II is simply chromophore hydrolysis, which occurs with a time constant of about 15–20 min. Y191A also greatly delays the release of retinal into solution. Conclusions: Photoisomerization of 11–cis retinal results in specific interactions between its 9–methyl group and intradiscal loop connecting helices 4 and 5 that facilitates the efficient formation of the active Meta II intermediate and also the timely release of the chromophore.
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