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Structure of the human M2 muscarinic acetylcholine receptor bound to an antagonist.

Haga K, Kruse AC, Asada H, Yurugi-Kobayashi T, Shiroishi M, Zhang C, Weis WI, Okada T, Kobilka BK, Haga T, Kobayashi T - Nature (2012)

Bottom Line: The orthosteric binding pocket is formed by amino acids that are identical in all five muscarinic receptor subtypes, and shares structural homology with other functionally unrelated acetylcholine binding proteins from different species.A binding site for allosteric ligands has been mapped to residues at the entrance to the binding pocket near this aromatic cap.The structure of the M2 receptor provides insights into the challenges of developing subtype-selective ligands for muscarinic receptors and their propensity for allosteric regulation.

View Article: PubMed Central - PubMed

Affiliation: Department of Life Science, Faculty of Science, Gakushuin University, Mejiro 1-5-1, Tokyo 171-8588, Japan.

ABSTRACT
The parasympathetic branch of the autonomic nervous system regulates the activity of multiple organ systems. Muscarinic receptors are G-protein-coupled receptors that mediate the response to acetylcholine released from parasympathetic nerves. Their role in the unconscious regulation of organ and central nervous system function makes them potential therapeutic targets for a broad spectrum of diseases. The M2 muscarinic acetylcholine receptor (M2 receptor) is essential for the physiological control of cardiovascular function through activation of G-protein-coupled inwardly rectifying potassium channels, and is of particular interest because of its extensive pharmacological characterization with both orthosteric and allosteric ligands. Here we report the structure of the antagonist-bound human M2 receptor, the first human acetylcholine receptor to be characterized structurally, to our knowledge. The antagonist 3-quinuclidinyl-benzilate binds in the middle of a long aqueous channel extending approximately two-thirds through the membrane. The orthosteric binding pocket is formed by amino acids that are identical in all five muscarinic receptor subtypes, and shares structural homology with other functionally unrelated acetylcholine binding proteins from different species. A layer of tyrosine residues forms an aromatic cap restricting dissociation of the bound ligand. A binding site for allosteric ligands has been mapped to residues at the entrance to the binding pocket near this aromatic cap. The structure of the M2 receptor provides insights into the challenges of developing subtype-selective ligands for muscarinic receptors and their propensity for allosteric regulation.

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Convergent evolution of acetylcholine binding sites. a, Acetylcholine is modeled into the crystal structure of the M2 receptor. b, Acetylcholine binding pocket in the crystal structure of the acetylcholine binding protein from the snail Aplysia californica (PDB ID: 2XZ5). c, Acetylcholine binding pocket in the acetylcholine binding protein ChoX from the gram negative bacterium Sinorhizobium meliloti (PDB ID: 2RIN). d, Binding site for thio-acetylcholine in the enzyme acetylcholine esterase from the electric ray Torpedo californica (PDB ID: 2C4H).
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Figure 3: Convergent evolution of acetylcholine binding sites. a, Acetylcholine is modeled into the crystal structure of the M2 receptor. b, Acetylcholine binding pocket in the crystal structure of the acetylcholine binding protein from the snail Aplysia californica (PDB ID: 2XZ5). c, Acetylcholine binding pocket in the acetylcholine binding protein ChoX from the gram negative bacterium Sinorhizobium meliloti (PDB ID: 2RIN). d, Binding site for thio-acetylcholine in the enzyme acetylcholine esterase from the electric ray Torpedo californica (PDB ID: 2C4H).

Mentions: The M2 and other muscarinic receptors represent one of four families of acetylcholine binding proteins to be structurally characterized thus far. Figure 3a shows the orthosteric binding site of the M2 receptor with acetylcholine docked with the gauche form of the O-C2-C1-N dihedral angle, which places the choline group in the aromatic cage interacting with Asp1033.32, while the carbonyl oxygen is tentatively bound to Asn4046.52 (Fig 3a). The natural agonist acetylcholine is much smaller than the bulky antagonist QNB. As described in agonist-bound structure of the β2 adrenergic receptor, the contraction of ligand binding pocket is expected as a result of an inward shift of TM525. This result is consistent with the previous mutation studies showing that Thr1875.39 and Thr1905.42 in TM5 (Fig. 2) alter binding of most agonists but not of antagonists20. Bulky compounds capable of blocking activation-related contraction of the pocket would be very efficient in locking M2 receptor in an inactive conformation as is exemplified here by the antagonist QNB. It has been proposed that the conformational change of M2 receptor upon activation might be accompanied by conformational change of acetylcholine from the gauche to trans form of the O-C2-C1-N dihedral angle26. It remains to be determined in which pose acetylcholine binds to the M2 receptor or to the M2 receptor-G protein complex, and if acetylcholine hydrogen bonds with Asn4046.52 or other residues.


Structure of the human M2 muscarinic acetylcholine receptor bound to an antagonist.

Haga K, Kruse AC, Asada H, Yurugi-Kobayashi T, Shiroishi M, Zhang C, Weis WI, Okada T, Kobilka BK, Haga T, Kobayashi T - Nature (2012)

Convergent evolution of acetylcholine binding sites. a, Acetylcholine is modeled into the crystal structure of the M2 receptor. b, Acetylcholine binding pocket in the crystal structure of the acetylcholine binding protein from the snail Aplysia californica (PDB ID: 2XZ5). c, Acetylcholine binding pocket in the acetylcholine binding protein ChoX from the gram negative bacterium Sinorhizobium meliloti (PDB ID: 2RIN). d, Binding site for thio-acetylcholine in the enzyme acetylcholine esterase from the electric ray Torpedo californica (PDB ID: 2C4H).
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3345277&req=5

Figure 3: Convergent evolution of acetylcholine binding sites. a, Acetylcholine is modeled into the crystal structure of the M2 receptor. b, Acetylcholine binding pocket in the crystal structure of the acetylcholine binding protein from the snail Aplysia californica (PDB ID: 2XZ5). c, Acetylcholine binding pocket in the acetylcholine binding protein ChoX from the gram negative bacterium Sinorhizobium meliloti (PDB ID: 2RIN). d, Binding site for thio-acetylcholine in the enzyme acetylcholine esterase from the electric ray Torpedo californica (PDB ID: 2C4H).
Mentions: The M2 and other muscarinic receptors represent one of four families of acetylcholine binding proteins to be structurally characterized thus far. Figure 3a shows the orthosteric binding site of the M2 receptor with acetylcholine docked with the gauche form of the O-C2-C1-N dihedral angle, which places the choline group in the aromatic cage interacting with Asp1033.32, while the carbonyl oxygen is tentatively bound to Asn4046.52 (Fig 3a). The natural agonist acetylcholine is much smaller than the bulky antagonist QNB. As described in agonist-bound structure of the β2 adrenergic receptor, the contraction of ligand binding pocket is expected as a result of an inward shift of TM525. This result is consistent with the previous mutation studies showing that Thr1875.39 and Thr1905.42 in TM5 (Fig. 2) alter binding of most agonists but not of antagonists20. Bulky compounds capable of blocking activation-related contraction of the pocket would be very efficient in locking M2 receptor in an inactive conformation as is exemplified here by the antagonist QNB. It has been proposed that the conformational change of M2 receptor upon activation might be accompanied by conformational change of acetylcholine from the gauche to trans form of the O-C2-C1-N dihedral angle26. It remains to be determined in which pose acetylcholine binds to the M2 receptor or to the M2 receptor-G protein complex, and if acetylcholine hydrogen bonds with Asn4046.52 or other residues.

Bottom Line: The orthosteric binding pocket is formed by amino acids that are identical in all five muscarinic receptor subtypes, and shares structural homology with other functionally unrelated acetylcholine binding proteins from different species.A binding site for allosteric ligands has been mapped to residues at the entrance to the binding pocket near this aromatic cap.The structure of the M2 receptor provides insights into the challenges of developing subtype-selective ligands for muscarinic receptors and their propensity for allosteric regulation.

View Article: PubMed Central - PubMed

Affiliation: Department of Life Science, Faculty of Science, Gakushuin University, Mejiro 1-5-1, Tokyo 171-8588, Japan.

ABSTRACT
The parasympathetic branch of the autonomic nervous system regulates the activity of multiple organ systems. Muscarinic receptors are G-protein-coupled receptors that mediate the response to acetylcholine released from parasympathetic nerves. Their role in the unconscious regulation of organ and central nervous system function makes them potential therapeutic targets for a broad spectrum of diseases. The M2 muscarinic acetylcholine receptor (M2 receptor) is essential for the physiological control of cardiovascular function through activation of G-protein-coupled inwardly rectifying potassium channels, and is of particular interest because of its extensive pharmacological characterization with both orthosteric and allosteric ligands. Here we report the structure of the antagonist-bound human M2 receptor, the first human acetylcholine receptor to be characterized structurally, to our knowledge. The antagonist 3-quinuclidinyl-benzilate binds in the middle of a long aqueous channel extending approximately two-thirds through the membrane. The orthosteric binding pocket is formed by amino acids that are identical in all five muscarinic receptor subtypes, and shares structural homology with other functionally unrelated acetylcholine binding proteins from different species. A layer of tyrosine residues forms an aromatic cap restricting dissociation of the bound ligand. A binding site for allosteric ligands has been mapped to residues at the entrance to the binding pocket near this aromatic cap. The structure of the M2 receptor provides insights into the challenges of developing subtype-selective ligands for muscarinic receptors and their propensity for allosteric regulation.

Show MeSH
Related in: MedlinePlus