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From Proteopedia
Glucagon G protein-coupled receptor
Class B GPCRsG protein coupled receptors (GPCRs) are the largest class of integral membrane proteins. GPCRs are divided into five families; the rhodopsin family (class A), the secretin family (class B), the glutamate family (class C), the frizzled/taste family (class F), and the adhesion family.[1] Roughly 5% of the human genome encodes g protein-coupled receptors, which are responsible for the transduction of endogenous signals and the instigation of cellular responses. All GPCRs contain a similar seven α-helical transmembrane domain that once bound to its ligand, undergoes a conformational change and tranduces a signal to coupled, heterotrimeric G proteins. The initiation of intracellular signal pathways occur in response to stimuli such as light, Ca2+, amino acids, nucleotides, odorants, peptides, and other proteins, and accomplishes many interesting physiological roles. [1] Class B GPCRs contain 15 distinct receptors for peptide hormones and generate their signal pathway through the activation of adenylate cyclase (AC) which increases the intracellular concentration of cAMP, inositol phosphate, and calcium levels. [2] These secondary messengers are essential elements of intracellular signal cascades for human diseases including type II diabetes mellitus, osteoporosis, obesity, cancer, neurological degeneration, cardiovascular diseases, headaches, and psychiatric disorders; making their regulation through drug targeting of particular interest as disease targets. [3] Structural approaches to the development of agonists and antagonists have however been hampered by the lack of accurate Class B TMD visualizations. Recent crystal structure images of corticoptropin-releasing factor receptor 1 (PDB: 4K5Y) and human glucagon receptor (PDB: 4L6R) were accomplished through x-ray crystallography. [4] [5]
Glucagon Receptor (GCGR)The glucagon class B GPCR (GCGR) is involved in glucose homeostasis through the binding of the signal peptide glucagon. See also PSI Structural Biology Database Glucagon is released from pancreatic α-cells when blood glucose levels fall after a period of fasting or several hours following intake of dietary carbohydrates.[6] Once the peptide hormone is released, it binds to GCGR, a 485 amino acid protein found in the liver, kidney, intestinal smooth muscle, brain, and adipose tissues. [7] Upon binding, signaling is initiated to heterotrimeric G-proteins containing Gαs. [8] GCGR can regulate additional signal pathways, including G-proteins of the Gαi family through the adoption of differing receptor conformations. [9] Glucagon's main role is the regulation of blood glucose levels. Glucagon lowers the concentration of fructose 2,6-bisphosphate which is an allosteric inhibitor of the gluconeogenic enzyme fructose 1,6-bisphosphotase and activates phosphofructose kinase 1, which increases glucose levels via glycolysis. Glucagon is also a regulator of the production of cholesterol, which is an energetically intensive process. When energy resources are low, downregulation of cholesterol production begins with glucagon binding to GCGR, which stimulates the phosphorylation of HMG-CoA.[6] HMG-CoA is inactivated by phosphorylation and moderates cholesterol production to conserve energy. Glucagon also takes part in fatty acid mobilization by affecting levels of adipose tissue in the organism. Activation of GCGR by glucagon initiates triacylglycerol breakdown and the phosphorylation of perilipin and lipases via cAMP signal pathways. This allows the body to export fatty acids to the liver and other crucial tissues for energy use and makes more glucose available for use in brain functioning. [6]
StructureThe class B GPCRs, including GCGR, are different from other GPCRs in several ways. The first is that class B GPCRs contain a protrusion known as a 'stalk,' a three α-helical turn elongation of the N-terminus that protrudes past the extracellular (EC) membrane. Structural integrity of this domain in GCGR is . A135P mutations impact stalk stability by removing an important salt bridge between Glu133 - Lys136. A second difference between class B and other GPCRs is that the extracellular loop 1 (ECL1) is 3-4 times longer than comparable loops in class A GPCRs, and also affects ligand binding affinity.[5] Most notably, class B GPCRs contain a which is solvent filled and accessible from the extracellular side. This central splay is notably absent , and represents a tantalizing target for agonists/antagonists.[3] Because of the difficulty in stabilizing and crystallizing Class B TMDs, very little is known about the conformational changes that transduce cell signals endogenously. GCGR is known to regulate additional signal pathways through the adoption of differing receptor conformations and to interact with receptor activity-modifying proteins (RAMPs) altering the signaling bias of the receptor.[9]
Glucagon BindingThe large, soluble N-terminal extracellular domains (ECD) of GCGR provide initial ligand selectivity with the deep, ligand pocket (Fig. 2) of the TMD providing secondary recognition.[7]    Mutagenesis and photo cross-linking studies determined essential, conserved residues in glucagon and have been in red.[5] Glucagon residues His 1, Gln 3, Phe 6, and Tyr 10 are critical to successful binding interaction with the GCGR while others are important for structural rigidity. The n-terminus of glucagon (Fig. 3) leads to a protuberance that fits into the deep, interior cavity of the GCGR 7TMD (Fig. 2) where four residues reside that play strong roles in ligand binding affinity. There is a to the entrance of the cavity, providing a firm anchor during peptide docking.
Clinical relevanceClass B secretin-like receptors have gained relevance in therapeutics and drug targets. Maintaining information about the class B GPCRs conformational flexibility, allows for a better understanding of the receptor-ligand binding and its pharmaceutical relevance. The 7TM structure offers a direct connect between the extracellular and intracellular region, which offers a mechanism for signal transduction within the cell. GPCRs regulate cellular processes as required by the organs in which they are located. GPCR’s are used in the functioning of neuron synapses, ion transport regulation, homeostasis, cell division, and cell morphology. Mutations in the GPCR have been linked with retinitis pigmentosa, female infertility, nephrogenic diabetes insipidus, and familial exudative vitreoretinopathy. [10] A variety of small molecule modulators have been developed over the past several years providing the promise of enhanced pharmaceutical regulation of GCGR. [7](Fig's. 12 and 13)   Utilizing the visualizations of the GCGR 7TMD and glucagon peptide ligand, dimensional/structural analyses can be performed to develop models for novel molecules of increasing specificity for GCGR binding/regulation. Performing a dimensional analysis between the binding pocket and the base of the EC stalk, a large pseudopeptide molecule of 17-24 angstroms in size could be utilized to mimic the characteristics of GCGR's natural ligand, glucagon. (Fig's. 14 and 15) See AlsoPSI Structural Biology Database G protein-coupled receptors Wikipedia page
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