P-2 レチナール光異性化酵素レチノクロムにおける発色団の14位置換基のシッフ塩基に対する影響(ポスター発表の部)

抄録

Retinochrome is one of retinal proteins in squid retina, which has photoisomerase activity of all-trans-retinal to the 11-cis-isomer. In the protein the chromophore is linked to an amino group of lysin residue through the protonated Schiff base. To investigate protein environment surrounding the chromophore, which is considered to controll such a specific photoisomerization, 14-fluoro, 14-chloro, and 14-methyiretinals were incorporated into the protein. Highly electronegative fluorine substitution at 14-position of the chromophore exhibited mostly unprotonation of the Schiff base and less electronegative chlorine substituent gave less unprotonation than fluorine, whereas non-electronegative and bulky methyl substituent gave only a pigment having the protonated Schiff base same as natural retinochrome. Those results indicate that electrostatic effect of the 14-substituents causes of the unprotonation. PKa decreases of the Schiff bases due to 14-fluorination of retinal model Schiff bases, as such, were too small to cause the unprotonation in retinochrome, suggesting that, besides that effect, through space/medium electrostatic effect to protein moiety contributes to the unprotonation. Fully protonation of the Schiff base in the 14-Me-analog protein excluded another possibility of the steric effect for the unprotonation. The Schiff base counterion is most likely candidate for the protein moiety having the fluorine effect because both groups should be located in the vicinity of the Schiff base. It suggests 14,15-s-trans-C=N-anti conformation of the all-trans-chromophore in retinochrome.