Retinylidene protein

[2][3][4] All rhodopsins consist of two building blocks, a protein moiety and a reversibly covalently bound non-protein cofactor, retinal (retinaldehyde).

The protein structure of rhodopsin consists of a bundle of seven transmembrane helices that form an internal pocket binding the photoreactive chromophore.

Photoisomerization (light-dependent isomerization) of retinal from cis to trans or vice versa induces a conformational change in the receptor protein.

They can be divided into two distinct types based on their sequence as well as the retinal isomer they contain at the ground state and their signal transduction mechanisms.

Examples are bacterial sensory rhodopsins, channelrhodopsins, bacteriorhodopsins, halorhodopsins, proteorhodopsins, archaerhodopsins, heliorhodopsins and xanthorhodopsins to carry out phototrophy.

Animal opsins can also be found in the skin of amphibians, the pineal glands of lizards and birds, the hypothalamus of toads, and the human brain.

They can be categorized into several distinct classes including: The "visual purple" rhodopsin (opsin-2) of the rod cells in the vertebrate retina absorbs green-blue light.

The photopsins of the cone cells of the retina differ in a few amino acids resulting in a shift of their light absorption spectra.

Light signal transduction involves an enzyme cascade of G-proteins (transducin), cGMP phosphodiesterase, closure of a cation channel and ultimately hyperpolarization of the visual photoreceptor cell.