Fig. 3.

Conformational change of Opn5L1-bound retinal to trigger the photocyclic reaction. The dark state binds all-trans retinal to activate G protein. Light irradiation causes retinal isomerization to the 11-cis form to suppress G protein activation. In this state, Cys188 and C11 of retinal form an adduct, thereby converting the double bond of C11=C12 to a single bond. Thermal rotation of the C11–C12 single bond is facile and permits conversion to the all-trans form. In this state, Cys188 and C11 of retinal dissociate to regenerate the original dark state containing all-trans retinal.