User:R. Jeremy Johnson/GPR40: Difference between revisions
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=== Binding Sites === | === Binding Sites === | ||
GPR40’s natural substrate are FFAs in which a free [https://en.wikipedia.org/wiki/Carboxylic_acid carboxyl group] is required to bind. However, GPR40 can be activated by a wide variety of fatty acids with chain lengths ranging from [https://en.wikipedia.org/wiki/Saturated_fat saturated fatty acids] with 8 carbons to 23 carbons. In addition, various [https://en.wikipedia.org/wiki/Monounsaturated_fat mono] (i.e. [https://en.wikipedia.org/wiki/Palmitoleic_acid palmitoleic] (C16:1) and [https://en.wikipedia.org/wiki/Oleic_acid oleic] (C18:1) acids) and [https://en.wikipedia.org/wiki/Polyunsaturated_fatty_acid poly-unsaturated fatty acids] (i.e.[https://en.wikipedia.org/wiki/Linoleic_acid linoleic] (C18:2) and [https://en.wikipedia.org/wiki/List_of_unsaturated_fatty_acids#Eicosatrienoic_acid eicosatrienoic] (C20:3) acids) can activate GPR40 <ref name= " | GPR40’s natural substrate are FFAs in which a free [https://en.wikipedia.org/wiki/Carboxylic_acid carboxyl group] is required to bind. However, GPR40 can be activated by a wide variety of fatty acids with chain lengths ranging from [https://en.wikipedia.org/wiki/Saturated_fat saturated fatty acids] with 8 carbons to 23 carbons. In addition, various [https://en.wikipedia.org/wiki/Monounsaturated_fat mono] (i.e. [https://en.wikipedia.org/wiki/Palmitoleic_acid palmitoleic] (C16:1) and [https://en.wikipedia.org/wiki/Oleic_acid oleic] (C18:1) acids) and [https://en.wikipedia.org/wiki/Polyunsaturated_fatty_acid poly-unsaturated fatty acids] (i.e.[https://en.wikipedia.org/wiki/Linoleic_acid linoleic] (C18:2) and [https://en.wikipedia.org/wiki/List_of_unsaturated_fatty_acids#Eicosatrienoic_acid eicosatrienoic] (C20:3) acids) can activate GPR40.<ref name="Morgan"/> The agonists potency varies according to the carbon-chain length however. The activity of GPR40 increases when the chain is increased from C6 to C15 but then decreased when the chain was extended beyond C15. One explanation for this is that as [https://en.wikipedia.org/wiki/Alkyl alkyl] chain increased, so did the [https://en.wikipedia.org/wiki/Hydrophobe hydrophobic] interactions with the protein within the binding pocket. However, for FFAs with carbon chains longer than C15, the molecular size is too large for the binding pocket. This causes the alkyl chain to extend beyond the binding pocket and destabilize the binding.<ref name=”REN”/> | ||
FFAs bind to hGPR40 by coordinating its free carboxyl group to three amino acids, <scene name='72/727085/Ffa_binding/1'>Arg183, Tyr2240, and Arg258</scene>, which are located close to the <scene name='72/727085/Hgpr40_transmane_active/1'>extracellular domain</scene> of hGPR40 on TM5, 6 and 7. Because of the close proximity of these residues to the extracellular domain and the dominantly hydrophobic nature of FFA’s, it is likely that ligand binding occurs close to the plane of the membrane <ref name=" | FFAs bind to hGPR40 by coordinating its free carboxyl group to three amino acids, <scene name='72/727085/Ffa_binding/1'>Arg183, Tyr2240, and Arg258</scene>, which are located close to the <scene name='72/727085/Hgpr40_transmane_active/1'>extracellular domain</scene> of hGPR40 on TM5, 6 and 7. Because of the close proximity of these residues to the extracellular domain and the dominantly hydrophobic nature of FFA’s, it is likely that ligand binding occurs close to the plane of the membrane.<ref name="Morgan"/> | ||
[http://metislabs.com/radioligand-binding-assays Radioligand binding studies] identified multiple [https://en.wikipedia.org/wiki/Binding_site binding sites] in hGPR40.<ref name="Srivastava"/> [https://en.wikipedia.org/wiki/Agonist Full agonists] and [https://en.wikipedia.org/wiki/Partial_agonist partial agonists] were shown to bind in separate sites with positive [http://www.britannica.com/science/cooperativity cooperativity].<ref name="Lin">PMID:22859723</ref> The <scene name='72/721541/Tak_binding_site/4'>binding site for the partial agonist TAK-875</scene> has been identified, but other binding sites were hypothesized. TAK-875 binds between transmembrane helices 3, 4, and 5 and underneath ECL2. By visual inspection, a second possible binding site was proposed between transmembrane helices 3, 4, and 5 on the intracellular side of the transmembrane helices. The location of this binding site with respect to the membrane proposes that substrates would gain entry to the membrane by binding in this site. Also by visual inspection, a third possible binding site was proposed between transmembrane helices 1, 2, and 7 on the extracellular side of hGPR40, close to the TAK-875 binding site.<ref name="Srivastava"/> These binding sites could potentially serve as regulation points for hGPR40. Many proteins that exhibit cooperativity are regulated by the binding of inhibitors. | [http://metislabs.com/radioligand-binding-assays Radioligand binding studies] identified multiple [https://en.wikipedia.org/wiki/Binding_site binding sites] in hGPR40.<ref name="Srivastava"/> [https://en.wikipedia.org/wiki/Agonist Full agonists] and [https://en.wikipedia.org/wiki/Partial_agonist partial agonists] were shown to bind in separate sites with positive [http://www.britannica.com/science/cooperativity cooperativity].<ref name="Lin">PMID:22859723</ref> The <scene name='72/721541/Tak_binding_site/4'>binding site for the partial agonist TAK-875</scene> has been identified, but other binding sites were hypothesized. TAK-875 binds between transmembrane helices 3, 4, and 5 and underneath ECL2. By visual inspection, a second possible binding site was proposed between transmembrane helices 3, 4, and 5 on the intracellular side of the transmembrane helices. The location of this binding site with respect to the membrane proposes that substrates would gain entry to the membrane by binding in this site. Also by visual inspection, a third possible binding site was proposed between transmembrane helices 1, 2, and 7 on the extracellular side of hGPR40, close to the TAK-875 binding site.<ref name="Srivastava"/> These binding sites could potentially serve as regulation points for hGPR40. Many proteins that exhibit cooperativity are regulated by the binding of inhibitors. | ||
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[[Image:Gpr40 insulin pathway.png|400 px|center|thumb|'''Figure 3:''' Signaling pathway mediated by GPR40 in pancreatic β-cells. The 𝝰q subunit represents the Gq unit that is coupled to hGPR40. This subunit breaks off to activate phospholipase C (PLC), resulting in the hydrolysis of phosphatidylinositol-4,5-biphosphate (PIP<sub>2</sub>). PIP<sub>2</sub> goes on to create diacylglycerol (DAG) and inositol 1,4,5-triphosphate (IP3). The DAG activates protein kinase c (PKC), triggering secretion of insulin. Activated IP3 diffuses to the endoplasmic reticulum (ER), releasing the stored Ca<sup>2+</sup>, which aids in insulin secretion. ]] | [[Image:Gpr40 insulin pathway.png|400 px|center|thumb|'''Figure 3:''' Signaling pathway mediated by GPR40 in pancreatic β-cells. The 𝝰q subunit represents the Gq unit that is coupled to hGPR40. This subunit breaks off to activate phospholipase C (PLC), resulting in the hydrolysis of phosphatidylinositol-4,5-biphosphate (PIP<sub>2</sub>). PIP<sub>2</sub> goes on to create diacylglycerol (DAG) and inositol 1,4,5-triphosphate (IP3). The DAG activates protein kinase c (PKC), triggering secretion of insulin. Activated IP3 diffuses to the endoplasmic reticulum (ER), releasing the stored Ca<sup>2+</sup>, which aids in insulin secretion. ]] | ||
The natural substrate of hGPR40 is free fatty acids (FFAs) which bind to the G-protein and enhance glucose-stimulated insulin secretion <ref name=" | The natural substrate of hGPR40 is free fatty acids (FFAs) which bind to the G-protein and enhance glucose-stimulated insulin secretion.<ref name="Morgan"/> FFAs bind to GPR40 which then couples with the G-protein Gq leading to increased [https://en.wikipedia.org/wiki/Phospholipase_C phospholipase C] (PLC) activity.<ref name="Morgan"/> PLC catalyzes the hydrolysis of the phospholipid [https://en.wikipedia.org/wiki/Phosphatidylinositol_4,5-bisphosphate phosphatidylinositol-4,5-biphosphate ] (PIP<sub>2</sub>) resulting in the formation of [https://en.wikipedia.org/wiki/Diglyceride diacylglycerol] (DAG) and [https://en.wikipedia.org/wiki/Inositol_trisphosphate inositol 1,4,5-triphosphate] (IP3).<ref name="Morgan"/> DAG can then activate [https://en.wikipedia.org/wiki/Protein_kinase_C protein kinase C] (PKC) to enhance insulin secretion.<ref name="Morgan"/> IP3 on the other hand is soluble and diffuses to the [https://en.wikipedia.org/wiki/Endoplasmic_reticulum endoplasmic reticulum] where it binds a ligand-gated Ca<sup>2+</sup> channel.<ref name="Morgan"/> This binding triggers the opening of the channel causing stored Ca<sup>2+</sup> to be released into the cytoplasm.<ref name="Morgan"/> This large increase in intracellular free Ca<sup>2+</sup> produces a proportional increase in glucose-dependent insulin secretion, suggesting that insulin release can be contributed in part to the changes in Ca<sup>2+</sup> concentration resulting from activated GPR40.<ref name="Morgan"/> | ||
== Clinical Relevance == | == Clinical Relevance == | ||