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Human Glucagon Class B G Protein-Coupled Receptors (GPCRs)

spinqualitylabelsall models Class B 7TM Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image Contents 1 Introduction 2 Structure 3 Extracellular Tips 4 How These Structures Lead to Function Introduction , also known as secretin-like receptors, are a subfamily of the more well known class A (rhodopsin-like) glucagon receptor family[1] . Located in the liver, class B glucagon receptors (GCGRs) are activated by the binding of the hormonal peptide glucagon which leads to the release of glucose into the bloodstream and plays an essential role in glucose homeostasis. Class B GCGRs are composed of a seven transmembrane domain (7TM) and extracellular domain (ECD) that are of vital importance in glucagon binding. In comparison, class A vs. class B glucagon receptors share less than fifteen percent sequence homology, but both share this 7TM which is a primary area of comparison between the two [1]. Structure Secretin-like receptors are composed entirely of alpha helices in their 7TM and ECD. A distinct feature of class B receptors is the N-terminal end of helix one in the 7TM. It extends an extra three helical turns into the ECD, and is longer than any of the class A receptors. This region, referred to as the "stalk" of class B receptors, is highly involved in glucagon binding and also helps in defining the orientation of the ECD with respect to the 7TM domain [2]. Extracellular Tips The locations of the extracellular tips for class B glucagon receptors allow for a much wider and deeper binding cavity in the ligand-binding pocket, which is much more immense than any of the class A GCGRs [3]. These wide ranges specifically occur between alpha helices two and six (green) and three and seven (red). How These Structures Lead to Function Structurally, the N-terminal extracellular domain (ECD) and the 7TM comprise the signature seven helical structure that is involved in signaling via coupling to heterotrimeric proteins that activate adenylate cyclase to increase the levels of intracellular cyclic AMP, and also heterotrimeric G proteins that increase inositol phosphate and intracellular calcium levels [3]. The wider and deeper ligand-binding pocket of class B GPCRs allows for a vast array of receptors to be bound that allow for numerous functions activated by peptide receptors [4]. The conformation and orientation of the 7TM and the ECD regions dictate the functionality of the protein.There is an open and closed conformation of the GCGR. When glucagon binds to GCGR, the open conformation of GCGR is stabilized. There is no clear binding site location of the hormone peptide ligand. They do know the N-terminus of glucagon binds deep into the binding pocket.The amino acids at the N-terminus have the ability to form hydrogen bonds and ionic interactions.Here is the amino acid sequence of glucagon [5]. Image:Amino Acid Sequence.jpg Many of the residues that are in direct contact with the glucagon molecule are charged or are polar. There are also any bulk resides on glucagon that support the bulky resides on the GCGR. These residues are located within the binding pocket of the 7TM [4]. There are specific amino acid interactions that hold the helices of the 7TM in the conformation that maximizes affinity, this includes a disulfide bond between Cys 294 and Cys 224 that serves to hold the ECL1 and ECL2 in the proper orientation. Also, the salt bridges between Glu 406, Arg 173, and Arg 346 that hold the conformation of affinity. Alpha helical structure of the stalk is imperative to the affinity and binding of the glucagon [3].

Clinical Relevancy

Of the fifteen human class B GPCRs, eight have been identified as potential drug target^[6]. Therapeutic agents have been created from the peptides themselves within this protein, but overall pharmaceutical companies have had difficulty creating agents that act on family B GPCRS. There is an outward appearance and inherent flexibility in the class B GCGR 7TM because of conserved hydrogen bonds that flank a glycine residue, and this structure along with the ECD and its role of interactions on the extracellular side of receptors may provide evidence to how class B receptors adjust its conformational spectra for various receptors. Researchers hope to show how these conformations can be utilized in potential treatments of a wide array disorders. Research for class B GCGR inhibitors is primarily looking into allosteric inhibitors having the ability to target specific receptors in order to treat things like stress disorders, managing excess glucose in the bloodstream, and also alternative mechanisms for treating migraines ^[7]. Known inhibitors include monoclonal antibodies which inhibit GCGR through an allosteric mechanism. The monoclonal antibodies bind to two different sites, the ECD opposite of the binding region and then the helical portion of the ECD as well. But, these antibodies did not interact with the binding sites. This inhibitor shows further prove of importance that the ECD is to the function of GCGR ^[8].

References

1. ↑ ^{1.0 1.1} Hollenstein K, de Graaf C, Bortolato A, Wang MW, Marshall FH, Stevens RC. Insights into the structure of class B GPCRs. Trends Pharmacol Sci. 2014 Jan;35(1):12-22. doi: 10.1016/j.tips.2013.11.001. Epub, 2013 Dec 18. PMID:24359917 doi:http://dx.doi.org/10.1016/j.tips.2013.11.001
2. ↑ Yang L, Yang D, de Graaf C, Moeller A, West GM, Dharmarajan V, Wang C, Siu FY, Song G, Reedtz-Runge S, Pascal BD, Wu B, Potter CS, Zhou H, Griffin PR, Carragher B, Yang H, Wang MW, Stevens RC, Jiang H. Conformational states of the full-length glucagon receptor. Nat Commun. 2015 Jul 31;6:7859. doi: 10.1038/ncomms8859. PMID:26227798 doi:http://dx.doi.org/10.1038/ncomms8859
3. ↑ ^{3.0 3.1 3.2} Siu FY, He M, de Graaf C, Han GW, Yang D, Zhang Z, Zhou C, Xu Q, Wacker D, Joseph JS, Liu W, Lau J, Cherezov V, Katritch V, Wang MW, Stevens RC. Structure of the human glucagon class B G-protein-coupled receptor. Nature. 2013 Jul 25;499(7459):444-9. doi: 10.1038/nature12393. Epub 2013 Jul 17. PMID:23863937 doi:10.1038/nature12393
4. ↑ ^{4.0 4.1} Miller LJ, Dong M, Harikumar KG. Ligand binding and activation of the secretin receptor, a prototypic family B G protein-coupled receptor. Br J Pharmacol. 2012 May;166(1):18-26. doi: 10.1111/j.1476-5381.2011.01463.x. PMID:21542831 doi:http://dx.doi.org/10.1111/j.1476-5381.2011.01463.x
5. ↑ Thomsen J, Kristiansen K, Brunfeldt K, Sundby F. The amino acid sequence of human glucagon. FEBS Lett. 1972 Apr 1;21(3):315-319. PMID:11946536
6. ↑ Bortolato A, Dore AS, Hollenstein K, Tehan BG, Mason JS, Marshall FH. Structure of Class B GPCRs: new horizons for drug discovery. Br J Pharmacol. 2014 Jul;171(13):3132-45. doi: 10.1111/bph.12689. PMID:24628305 doi:http://dx.doi.org/10.1111/bph.12689
7. ↑ Mukund S, Shang Y, Clarke HJ, Madjidi A, Corn JE, Kates L, Kolumam G, Chiang V, Luis E, Murray J, Zhang Y, Hotzel I, Koth CM, Allan BB. Inhibitory mechanism of an allosteric antibody targeting the glucagon receptor. J Biol Chem. 2013 Nov 4. PMID:24189067 doi:http://dx.doi.org/10.1074/jbc.M113.496984
8. ↑ Hoare SR. Allosteric modulators of class B G-protein-coupled receptors. Curr Neuropharmacol. 2007 Sep;5(3):168-79. doi: 10.2174/157015907781695928. PMID:19305799 doi:http://dx.doi.org/10.2174/157015907781695928