Figure 2. Two-step reaction scheme of thermal activation and deactivation of visual pigments.
R and R* indicate visual pigments in the inactive and active state, respectively. The kth indicates a thermal activation rate constant of R. The kd indicates a thermal decay rate constant of R*. An opsin regenerated by normal 11-cis retinal (Opsin-n) spontaneously becomes R* by thermal cis-trans isomerization of retinal in the complete darkness. After the first reaction, R* is degraded into opsin and retinal. In contrast, an opsin regenerated by 11-cis-locked-7-membered-ring-retinal (Opsin-7mr) cannot become R* due to the inhibition of cis-trans isomerization.