Present State of Knowledge of Chemistry of Our Vision: Photoreceptor Molecules and Vision Cycle

Abstract

Vision process involves the participation of two types of retinal photoreceptor cells: rod cells respond to dim light while cone cells respond to bright light and colours. The visual pigments in both types of photoreceptor cells contain the common chromophore, 11-cis-retinal, linked through a Schiff base linkage to the opsin protein, a member of G-protein coupled receptor (GPCR) family, composed of 7- transmembrane helices (a 7TM receptor). Rhodopsin is the visual pigment present in the rod cells while three distinct types of visual pigments known as photopsins (red cones, green cones and blue cones absorbing red, blue and green parts of the visible spectrum respectively) are present in the cone cells. Absorption of light by the visual pigment causes the photoexcitation followed by photoisomerization, 11-cis-retinal (Z) to all-trans-retinal (E) with a high quantum yield through a number of reactive intermediates characterized by low temperature and picosecond (ps) time resolved spectroscopies coupled with femtosecond spectroscopy. This photoisomerization leads to a change in the conformation of opsin GPCR and a signal transduction cascade by activating transducin, a heterotrimeric G-protein, to breakdown the cGMP to close the cGMP-gated cation channels resulting in hyperpolarization of the photoreceptor cell. This action potential creates a nerve signal that is transmitted to the brain to produce the sense of vision. The photoisomerized pigment undergoes rapidly hydrolysis to produce the opsin protein and all trans-retinal, which can be reconverted enzymatically to 11-cis-retinal for recharging opsin to generate the active visual pigment to maintain the vision cycle (Wald cycle). This brief review highlights the state of our understanding of the biology behind the art of vision in humans and other organisms.