User:R. Jeremy Johnson/GPR40: Difference between revisions
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== Background == | == Background == | ||
Human G-protein coupled receptor 40 (hGPR40), also known as free fatty acid 1 receptor (FFAR1), is a seven helical transmembrane domain receptor for long-chain free [https://en.wikipedia.org/wiki/Fatty_acid fatty acids] that induces insulin secretion.<ref name="Srivastava">PMID:25043059</ref> Some known fatty acid substrates of hGPR40 include [http://www.news-medical.net/health/What-is-Linoleic-Acid.aspx linoleic acid], [http://www.livestrong.com/article/438717-what-is-oleic-acid/ oleic acid], [http://www.hmdb.ca/metabolites/hmdb02925 eicosatrienoic acid], and [https://en.wikipedia.org/wiki/Palmitoleic_acid palmitoleic acid]<ref name="Morgan">PMID:19660440</ref>. hGPR40 is highly expressed in human pancreatic [https://en.wikipedia.org/wiki/Beta_cell β cells], brain, and endocrine cells of the [https://en.wikipedia.org/wiki/Gastrointestinal_tract gastrointestinal tract] <ref name=”REN”>PMID:26974599</ref>. hGPR40 is of particular interest because the triggering of insulin secrection is [https://en.wikipedia.org/wiki/Glucose glucose] dependent.This glucose-dependence for hGPR40 signaling makes it a target for the treatment of [https://en.wikipedia.org/wiki/Diabetes_mellitus_type_2 type-2 diabetes] as agonists could increase glycemic control and lower the risk of hypoglycemia<ref name=" | Human G-protein coupled receptor 40 (hGPR40), also known as free fatty acid 1 receptor (FFAR1), is a seven helical transmembrane domain receptor for long-chain free [https://en.wikipedia.org/wiki/Fatty_acid fatty acids] that induces insulin secretion.<ref name="Srivastava">PMID:25043059</ref> Some known fatty acid substrates of hGPR40 include [http://www.news-medical.net/health/What-is-Linoleic-Acid.aspx linoleic acid], [http://www.livestrong.com/article/438717-what-is-oleic-acid/ oleic acid], [http://www.hmdb.ca/metabolites/hmdb02925 eicosatrienoic acid], and [https://en.wikipedia.org/wiki/Palmitoleic_acid palmitoleic acid]<ref name="Morgan">PMID:19660440</ref>. hGPR40 is highly expressed in human pancreatic [https://en.wikipedia.org/wiki/Beta_cell β cells], brain, and endocrine cells of the [https://en.wikipedia.org/wiki/Gastrointestinal_tract gastrointestinal tract] <ref name=”REN”>PMID:26974599</ref>. hGPR40 is of particular interest because the triggering of insulin secrection is [https://en.wikipedia.org/wiki/Glucose glucose] dependent.This glucose-dependence for hGPR40 signaling makes it a target for the treatment of [https://en.wikipedia.org/wiki/Diabetes_mellitus_type_2 type-2 diabetes] as agonists could increase glycemic control and lower the risk of hypoglycemia.<ref name="Srivastava"/> GPR40 is a member of a group of homologous [[GPCRs]] all located on chromosome 19q13.1 including GPCR41, 42, and 43.<ref name="Burant">PMID:23882043</ref> Evidence exists that shows GPCR43 is involved in adipogenesis. | ||
== Function == | == Function == | ||
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Like most G-protein coupled receptors, hGPR40 contains <scene name='72/721541/Top_view_transmembrane_helices/2'>seven transmembrane helices</scene> (<scene name='72/721541/Top_view_transmembrane_helices/1'>top view of TM helices</scene>). To obtain a [https://en.wikipedia.org/wiki/Protein_crystallization crystallized structure] of the protein, four <scene name='72/721541/Stabilizing_mutations/4'>stabilizing mutations</scene> (<scene name='72/721541/L42a/3'>L42A</scene>, <scene name='72/721541/F88a/4'>F88A</scene>, <scene name='72/721541/G103a/3'>G103A</scene>, <scene name='72/721541/Y202f/3'>Y202F</scene>) were made to increase expression levels and thermal stability of the protein. These mutations did not significantly impact the enzyme's binding affinity with a known agonist, TAK-875.<ref name="Srivastava"/> A <scene name='72/721541/Lysozyme_crimson/2'>T4 Lysozyme</scene> (shown in <FONT COLOR="#DC143C">'''crimson'''</FONT>) was also added to intracellular loop 3 to aid in the formation of crystals. T4 Lysozyme had little effect on TAK-875 binding.<ref name="Srivastava"/> For clarity, lysozyme is removed in all further renderings of hGPR40. hGPR40 also contains an extracellular loop that is conserved among most G-protein coupled receptors (ECL2). This loop has two subsections and is involved in the permeability of the binding site. | Like most G-protein coupled receptors, hGPR40 contains <scene name='72/721541/Top_view_transmembrane_helices/2'>seven transmembrane helices</scene> (<scene name='72/721541/Top_view_transmembrane_helices/1'>top view of TM helices</scene>). To obtain a [https://en.wikipedia.org/wiki/Protein_crystallization crystallized structure] of the protein, four <scene name='72/721541/Stabilizing_mutations/4'>stabilizing mutations</scene> (<scene name='72/721541/L42a/3'>L42A</scene>, <scene name='72/721541/F88a/4'>F88A</scene>, <scene name='72/721541/G103a/3'>G103A</scene>, <scene name='72/721541/Y202f/3'>Y202F</scene>) were made to increase expression levels and thermal stability of the protein. These mutations did not significantly impact the enzyme's binding affinity with a known agonist, TAK-875.<ref name="Srivastava"/> A <scene name='72/721541/Lysozyme_crimson/2'>T4 Lysozyme</scene> (shown in <FONT COLOR="#DC143C">'''crimson'''</FONT>) was also added to intracellular loop 3 to aid in the formation of crystals. T4 Lysozyme had little effect on TAK-875 binding.<ref name="Srivastava"/> For clarity, lysozyme is removed in all further renderings of hGPR40. hGPR40 also contains an extracellular loop that is conserved among most G-protein coupled receptors (ECL2). This loop has two subsections and is involved in the permeability of the binding site. | ||
While there is relatively low sequence identity between hGPR40 and peptide-binding and [https://en.wikipedia.org/wiki/Opioid_receptor opioid GPCRs], they do share structural similarities such as a conserved <scene name='72/727085/Hairpin_loop/4'>hairpin loop</scene> motif on <scene name='72/727085/Ecl2/4'>extracellular loop 2 </scene>(ECL2)<ref name=" | While there is relatively low sequence identity between hGPR40 and peptide-binding and [https://en.wikipedia.org/wiki/Opioid_receptor opioid GPCRs], they do share structural similarities such as a conserved <scene name='72/727085/Hairpin_loop/4'>hairpin loop</scene> motif on <scene name='72/727085/Ecl2/4'>extracellular loop 2 </scene>(ECL2).<ref name="Srivastava"/> In addition, there is a conserved <scene name='72/727085/Disulfide/3'>disulphide bond</scene> that is formed between transmembrane helix 3 (Cys 79) and the C-terminus of ECL2 (Cys170).<ref name="Srivastava"/> Compared to peptide-binding and opioid GPCRs which have distinctive [https://en.wikipedia.org/wiki/Beta_sheet β-sheets] spanning from transmembrane helix 4 to 5, hGPR40 possesses a shorter B-sheet-like region which has [http://proteopedia.org/wiki/index.php/Image:Beta-like_factors_of_hGPR40_ECL2.png low B-factors].<ref name="Srivastava"/> This reflects the low mobility of the region that limits the overall flexibility of the adjacent portion of ECL2 between Leu171 and Asp175.<ref name="Srivastava"/> A unique feature of hGPR40 is the presence of an additional 13 residues (Pro147 to Gly159) on ECL2 which is absent on all the other peptide/opioid receptors.<ref name="Srivastava"/> These extra residues form a separate <scene name='72/727085/Auxiliary_loop/3'>auxiliary loop</scene> between the B-sheet-like region and transmembrane 4. Together, the auxiliary loop and ECL2 of hGPR40 function as a <scene name='72/727085/Ecl2_cap/3'>roof </scene> over the canonical binding site covering it from the central extracellular region.<ref name="Srivastava"/> | ||
The canonical binding pocket for many other GPCRs is solvent exposed and centrally located between the transmembrane helices allowing ligands to directly bind from the extracellular space<ref name=" | The canonical binding pocket for many other GPCRs is solvent exposed and centrally located between the transmembrane helices allowing ligands to directly bind from the extracellular space.<ref name="Srivastava"/> However, because the ECL2 acts as a roof to this canonical binding site, it inhibits ligands from entering directly from the extracellular region. Instead, the highly lipophilic nature of hGPRC40’s ligands allow it to enter a <scene name='72/727085/Hgpr40_entry/2'>noncanonical binding pocket </scene> between TM3 and TM4 by moving through the lipid bilayer.<ref name="Srivastava"/> | ||
[[Image:Binding site comparison.png|380 px|thumb|center|'''Figure 2:''' Comparison of the canonical binding site represented in pink of most opioid/peptide binding GPCRs (left) compared to the noncanonical binding site of ligands with hGPR40 (right). ]] | [[Image:Binding site comparison.png|380 px|thumb|center|'''Figure 2:''' Comparison of the canonical binding site represented in pink of most opioid/peptide binding GPCRs (left) compared to the noncanonical binding site of ligands with hGPR40 (right). ]] | ||