Rhodopsin-like 7TM receptors

Rhodopsin-Like 7TM Receptors Are the Quantitatively Dominant Family.88... 

RHODOPSIN-LIKE 7TM RECEPTORS ARE THE QUANTITATIVELY DOMINANT FAMILY... 

FURTHER READING Rhodopsin-like 7TM receptor structure... 

FIGURE 2.3 The three main families of mammalian G-protein-coupled 7TM receptors in mammals. No obvious sequence identity is found between the rhodopsin-like family A, the glucagon/VIP/calcitonin family B, and the metabotropic glutamate/chemosensor family C of G-protein-coupled 7TM receptors, with the exception of the disulfide bridge between the top of TM-III and the middle of extracellular loop-2 (see Figure 2.2). Similarly, no apparent sequence identity exists among members of these three families and, for example the 7TM bitter taste receptors, the V1R pheromone receptors, and the 7TM frizzled proteins, which all are either known or believed to be G-protein-coupled receptors. Bacteriorhodopsins, which are not G-protein-coupled proteins but proton pumps, are totally different in respect to amino-acid sequence but have a seven-helical bundle arranged rather similarly to that for the G-protein-coupled receptors. 

Cone cells, like rod cells, contain visual pigments. Like rhodopsin, these photoreceptor proteins are members of the 7TM receptor family and utilize 11-cA-retinal as their chromophore. In human cone cells, there are three distinct photoreceptor proteins with absorption maxima at 426, 530, and 560 nm (Figure 32.26). These absorbances correspond to (in fact, define) the blue, green, and red regions of the spectrum. Recall that the absorption maximum for rhodopsin is 500 nm. 

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