Opsins related proteins

Cleavage of P-carotene, once thought unique to the animal kingdom, has been observed in the bacterium Halobacterium halobium and in related halobacteria (Kushwaha et al., 1974 Oesterhelt, 1976). These bacteria subsequently utilize the retinaldehyde, coupled with an opsin-like protein, to form bacteriorhodopsin, an energy-generating light-dependent proton pump (Stoeckenius etal., 1979 see also Chapter 10). 

FIGURE 17 11 Imine formation between the aldehyde function of 11 as retinal and an ammo group of a protein (opsin) is involved in the chemistry of vision The numbering scheme in retinal is specifically developed for carotenes and related compounds... 

The evidence to-date shows that vertebrate photoreception is mediated by a closely related group of proteins termed opsins. These are G protein-coupled receptors characterized by their ability to bind a vitamin A based chromophore ( -cis-retinal) via a Schiff base linkage using a lysine residue in the 7th transmembrane a helix (Fig. 1). The primary events of image detection by the rods and cones occurs with the absorption of a photon of light by ll-r/r-retinal and its photoisomerization to the AUtrans state (Bums Baylor 2001, Menon et al 2001). Although photoreception is best understood in retinal rods and cones, photoreception is not confined to these structures. In non-mammalian... 

Section 4.3.5.4 addresses the question of the genetic code as it applies to the protein substrate, opsin. There is reason to consider merging these sections in the future. This section will present some fundamental background related to research into the genetic code and an overview of what work has been presented. 

Akhtar et al. [74] proposed that, in rhodopsin, an acceptor group on the protein forms a charge-transfer complex with the unprotonated Schiff base of retinal furthermore, upon ll-cis to trans isomerization, separation of donor and acceptor moieties would occur and the Schiff base linkage would be exposed to hydrolysis. This model can now be discarded as unrealistic the resonance Raman experiments have shown that it is not an unprotonated Schiff base, but a protonated base which is bound to opsin. Further, this and related models were examined by Komatsu and Suzuki [223] using theoretical calculations, who found that charge-transfer type models cannot satisfactorily explain the red shifts seen in visual pigments. 

Chemical processes that are triggered by photons are also understood using the kinetics concepts mentioned in this chapter. The chemistry of vision is one example. The currently accepted mechanism for vision involves a compound called rhodopsin, which is composed of a protein molecule (opsin) attached to a colored polyene molecule called c/s-retinal. (C/s-retinal is chemically related to a class of molecules called carotenes, which are highly colored compounds responsible for the colors of carrots and tomatoes. Eating carrots does help vision, specifically in low light.) The vision process begins when the c/s-retinal absorbs a photon and is isomerized about one of its double bonds to make frans-retinal ... 

---