Vertebrate rod and cone photoreceptors detect light via a specialized organelle called the outer segment. This structure is packed with light-sensitive molecules known as visual pigments that consist of a G-protein-coupled, seven-transmembrane protein known as opsin, and a chromophore prosthetic group, either 11-cis retinal ('A1') or 11-cis 3,4-didehydroretinal ('A2'). The enzyme cyp27c1 converts A1 into A2 in the retinal pigment epithelium. Replacing A1 with A2 in a visual pigment red-shifts its spectral sensitivity and broadens its bandwidth of absorption at the expense of decreased photosensitivity and increased thermal noise. The use of vitamin A2-based visual pigments is strongly associated with the occupation of aquatic habitats in which the ambient light is red-shifted. By modulating the A1/A2 ratio in the retina, an organism can dynamically tune the spectral sensitivity of the visual system to better match the predominant wavelengths of light in its environment. As many as a quarter of all vertebrate species utilize A2, at least during a part of their life cycle or under certain environmental conditions. A2 utilization therefore represents an important and widespread mechanism of sensory plasticity. This review provides an up-to-date account of the A1/A2 chromophore exchange system.
Keywords: Chromophore; Cones; Opsins; Photoreceptors; Porphyropsin; Retina; Rods; Sensory plasticity; Spectral tuning; Visual ecology; Visual pigments; Visual plasticity; Vitamin A(1); Vitamin A(2).
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