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Figure 3.3 Molecular structure of G-protein-coupled receptors. In (a) the electron density map of bovine rhodopsin is shown as obtained by cryoelectron microscopy of two-dimensional arrays of receptors embedded in lipid membrane. The electron densities show seven peaks reflecting the seven a-helices which are predicted to cross the cell membrane. In (b) is shown a helical-wheel diagram of the receptor orientated according to the electron density map shown in (a). The diagram is seen as the receptor would be viewed from outside the cell membrane. The agonist binding pocket is illustrated by the hatched region between TM3, TM5 and TM6. (From Schertler et al. 1993 and Baldwin 1993, reproduced from Schwartz 1996). Reprinted with permission from Textbook of Receptor Pharmacology. Eds Foreman, JC and Johansen, T. Copyright CRC Press, Boca Raton, Florida

Figure 3.3 Molecular structure of G-protein-coupled receptors. In (a) the electron density map of bovine rhodopsin is shown as obtained by cryoelectron microscopy of two-dimensional arrays of receptors embedded in lipid membrane. The electron densities show seven peaks reflecting the seven a-helices which are predicted to cross the cell membrane. In (b) is shown a helical-wheel diagram of the receptor orientated according to the electron density map shown in (a). The diagram is seen as the receptor would be viewed from outside the cell membrane. The agonist binding pocket is illustrated by the hatched region between TM3, TM5 and TM6. (From Schertler et al. 1993 and Baldwin 1993, reproduced from Schwartz 1996). Reprinted with permission from Textbook of Receptor Pharmacology. Eds Foreman, JC and Johansen, T. Copyright CRC Press, Boca Raton, Florida

RECEPTOR ACTIVATION

Most structure-function information for the G-protein-coupled receptors has been inferred from molecular genetic experiments where single amino acids or groups of amino acids in the protein have been changed to investigate their role. The ^-adrenoceptor was the first G-protein-coupled receptor to be cloned and a detailed picture of the receptor structure has emerged using the techniques of molecular biology combined with radioligand binding and classical pharmacology to study receptor function (see, for example, Lefkowitz et al. 1993). An outline structure of the ^-adrenoceptor is shown in Fig. 3.4. Two main structural domains are recognised in all G-protein-coupled receptors:

(1) Ligand-binding domain

(2) G-protein-binding domain on the third intracellular loop Ligand-binding domain

In the monoamine receptors the ligand-binding domain is located within the transmembrane helices. A pocket is formed between TM3, TM5 and TM6 where the agonist binds. A conserved aspartate residue in TM3 (Asp-113 in the ^-adrenoceptor) and a

Figure 3.4 Transmembrane topology of a 7-TM domain G-protein receptor such as the P-adrenoceptor. Agonist binding is predicted to be within the transmembrane domains. The extracellular structure is stabilised by the disulphide bond joining the first and second extracellular loop. The third intracellular loop is the main site of G-protein interaction while the third intracellular loop and carboxy tail are targets for phosphorylation by kinases responsible for initiating receptor desensitisation

Figure 3.4 Transmembrane topology of a 7-TM domain G-protein receptor such as the P-adrenoceptor. Agonist binding is predicted to be within the transmembrane domains. The extracellular structure is stabilised by the disulphide bond joining the first and second extracellular loop. The third intracellular loop is the main site of G-protein interaction while the third intracellular loop and carboxy tail are targets for phosphorylation by kinases responsible for initiating receptor desensitisation conserved phenylalanine in TM6 (Phe-290) and two serine residues in TM5 (Ser-204 and 207) are known to be crucial for agonist binding. Antagonists have been shown to have extra interaction points on TM4 and TM7 but are thought to largely share the same binding sites as the agonist and so can act by simple competition.

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