Rhodopsin, the photoreceptor molecule of the vertebrate rod cell, is a G protein-coupled
receptor. Rhodopsin consists of the opsin apoprotein and its 11-cis-retinal chromophore,
which is covalently bound to a specific lysine residue by a stable protonated Schiff base
linkage. Rhodopsin activation occurs when light causes photoisomerization of the 11-
cischromophore to its all-trans form. The all-trans chromophore is the receptor agonist. The
11-cis-retinylidene chromophore is analogous pharmacologically to a potent inverse agonist …