double bond where the kink is and straightens out, into a form called
the all-trans isomer of retinal. This process is called light-induced iso-
merization, or photoisomerization.
Ww sy11-cis-retinal ~light > N —protein.all-trans-retinal SOA OR Ua a
N —proteinIn straightening, the retinal pushes on the amino acids surround-
ing it and, in so doing, shifts the shape of the entire opsin protein. It
so happens that opsin proteins are familiar friends—G-protein cou-
pled receptors (GPCRs). Thus, when they change shape, a cascade of
intracellular events is initiated. In the case of neuronal signaling at
synapses, the binding of a neurotransmitter molecule shape-shifted
the GPCR; here, the light-induced isomerization of the retinal changes
the shape. Opsin GPCRs are light detectors.
Here’s what happens in the rhodopsin GPCR cascade: When a
photon of light is absorbed by 11-cis retinal in rhodopsin, the retinal
isomerizes to the all-trans form, shape-shifting the opsin protein
and thereby “activating” it. The activated opsin is available to bind
an intracellular G-protein, and the G-protein becomes activated. The
activated G-protein then interacts with an enzyme called cGMP phos-