Many integral membrane glycoprotein membranes share a seven transmembrane alpha-helix motif (Figure II-23). The 6-adrenergic receptor, whose natural ligands are epinephrine and norepinephrine, is an example of such receptors. Similarly in the opiate receptor, binding of a ligand presumably initiates a conformational change in the membrane protein that is transmitted to the cell interior. This physical reaction can then facilitate other physical or chemical reactions, which are conveyed to ion channels, resulting in a change of transmembrane ion flow. The trans-duction of the signals from external messengers, including opiate ligands involves intracellular heterotrimeric G-proteins, which are bound to the inner cell (plasma) membrane, a secondary messenger system, involving cyclic AMP, and a target response.
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