FIGURE 15-19 Vision depends on the light-triggered isomerization of the retinal moiety of rhodopsin. Rhodopsin consists of the light-absorbing pigment 11-cis-retinal covalently attached to the amino group of lysine residue 296 in the opsin protein. Absorption of light causes rapid photoisomerization of the bound cis-retinal to the all-trans isomer. This change triggers a conformational change in rhodopsin, forming the unstable intermediate meta-rhodopsin II, or activated rhodopsin (see Figure 15-21), which activates G_t proteins. Within seconds, all-trans-retinal dissociates from opsin and is converted by a series of enzymes in the rod cell and pigmented epithelium back to the cis isomer, which then rebinds to another opsin molecule. See J. Nathans, 1992, Biochemistry 31:4923.