Vertebrate visual opsin

Vertebrate Visual Opsin

3D structure of rhodopsin

Vertebrate visual opsins are a group of light-sensitive proteins found in the photoreceptor cells of the retina in vertebrates. These proteins are crucial for the conversion of light into electrical signals, a process that enables vision. Opsins are a type of G protein-coupled receptor (GPCR) and are embedded in the membranes of photoreceptor cells.

Structure and Function

Opsins are composed of a protein part and a light-sensitive chromophore. The protein part is a seven-transmembrane domain structure typical of GPCRs. The chromophore, usually 11-cis-retinal, is covalently bound to the opsin and undergoes isomerization to all-trans-retinal upon absorption of a photon. This isomerization triggers a conformational change in the opsin, activating the associated G protein and initiating a phototransduction cascade.

Types of Visual Opsins

There are several types of visual opsins, each sensitive to different wavelengths of light:

• Rhodopsin: Found in rod cells, rhodopsin is sensitive to low light levels and is responsible for scotopic (night) vision.
• Cone opsins: Found in cone cells, these opsins are responsible for photopic (daylight) vision and color discrimination. There are typically three types of cone opsins, each sensitive to different parts of the light spectrum:

 * S-cone opsin: Sensitive to short wavelengths (blue light).
 * M-cone opsin: Sensitive to medium wavelengths (green light).
 * L-cone opsin: Sensitive to long wavelengths (red light).

Absorbance spectra of cone opsins

Phototransduction Cascade

The phototransduction cascade is the biochemical process by which photoreceptor cells convert light into electrical signals. When a photon is absorbed by the chromophore, it changes shape, activating the opsin. This activation leads to the activation of a G protein called transducin. Transducin then activates phosphodiesterase, which reduces the concentration of cyclic GMP (cGMP) in the cell. The decrease in cGMP causes the closure of cGMP-gated ion channels, leading to hyperpolarization of the photoreceptor cell and the transmission of an electrical signal to the brain.

Evolution of Opsins

Opsins have evolved to allow vertebrates to adapt to various light environments. The diversification of opsins is thought to have played a significant role in the evolution of vertebrate vision, enabling species to exploit different ecological niches. The evolution of color vision, in particular, has been a major adaptive advantage for many species, allowing for better detection of prey, predators, and mates.

Related Pages

• Photoreceptor cell
• Retina
• G protein-coupled receptor
• Color vision