Enzymatic Amplification Mechanism of Visual Transduction Signal in Retinal Rods

Abstract

The name “rhodopsin” is currently used for two classes of retinal proteins: the visual pigments of higher organisms, whose archetype is the rhodopsin of vertebrate retinal rod outer segments, and the retinal pigments of halophilic bacteria, whose archetype is the bacteriorhodopsin of Halobacterium rubrum. Much is known on the structure and function of these two proteins, and apparent similarities of their three dimensional structures have often led to speculation of a possible common origin and eventual functional analogies. Both pigment molecules are intrinsic membrane proteins whose hydrophobic core is constituted by a 7 α helices transmembrane barrel. There are, however, no significant analogies between their primary sequences, and their functions are totally different. Bacteriorhodopsin is a photoactivable ion pump, and rhodopsin a receptor molecule, which, in its photoexcited state, catalyses the activation of a biochemical cascade. Indeed, the two proteins may have evolved independently and converged towards a similar structure defined mainly by their intrinsic membrane protein character and their identical chromophore. The α helix is the predominant structure for the intrinsic region of membrane proteins, and helices usually cross membranes perpendicularly to their surface plane.