GPCR

G Protein-Coupled Receptors

1. G PROTEIN COUPLED RECEPTOR (GPCR) Dr. Yash N. Panchal Pharmacology Department AMC MET Medical college Date – 28/07/2021

2. OUTLINE Mechanisms of drug action Types of receptors GPCR Structure G Protein GPCR Signaling Second messengers Types of G protein Effector pathways References 2 2

3. MECHANISM OF THE DRUG ACTION Receptor mediated mechanism – majority of drug acts by this mechanism Non-receptor mediated mechanism Targeting specific genetic changes 3 3

4. RECEPTOR MEDIATED MECHANISM RECEPTOR : - Is the chemical component of living cell, located on effector cell’s cell surface or intracellularly , serve to recognize specific ligand molecule, and bind to ligand selectively. Majority are macromolecular proteins . 4 4

5. TYPES OF RECEPTORS Based on, molecular structure and Signal transduction mechanism involved, receptors can be : 1- Inotropic receptor 2- Enzymatic receptor 3- GPCR 4- Intracellular receptors 5 5

6. GPCR - HISTORICAL BACKGROUND Robert Lefkowitz and Brian Kobilka: Won Nobel prize in 2012 in chemistry for discovery of inner workings of G protein coupled receptors 6 6

7. GPCR - SUPERFAMILY 7 7

8. GPCR - STRUCTURE These are cell membrane bound receptors with Segment of single polypeptide chain Of Hydrophobic amino-acids Spanning membrane 7 times Forms 3 extracellular and 3 intracellular loops Has 2 terminal – “n” and “c” 8 8

9. Continued.. Synonyms : “ 7 transmembrane receptor” : “ Heptahelical receptor” : “ Serpentine receptor” Are Metabotropic receptors Ligand binding domain – Extracellular site, Effector domain – Intracellular site 9 9

10. G Protein It is coupled with guanine nucleotide ( GDP or GTP ) so called G protein Is Hetero-trimeric protein, Consisting of 3 subunits α , β , γ 10 10

11. Continued.. α subunit is very important : 1- Has binding site for guanine nucleotide 2- Has intrinsic GTPase activity 3- Determines type of GPCR ( Excitatory/ Inhibitory ) At rest , all 3 subunits are linked together, and GDP is bound to α subunit 11 11

12. GPCR SIGNALLING Agonist binds to receptor and cause conformational change Activated receptors activate G-protein Exchange of GTP with GDP occurs α -GTP detaches from receptor and βγ subunits 12 12

13. Interact with the target ( Effector ) protein Effector proteins transfer signal to secondary messenger Secondary messenger based on type of g protein leads to response RGS – Rate of hydrolysis of GTP , and so time period for which receptor remains activated is determined by one protein- RGS ( Regulator of G protein signaling ) 13 13

14. SECOND MESSENGERS 1- Cyclic AMP 2- Cyclic GMP 3- Calcium ions 4- IP3/DAG 5- Nitric oxide 14 14

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16. TYPES OF G PROTEIN G PROTEIN SIGNALING PATHWAY Gs Stimulatory effects- Increase cAMP, Opening of Calcium channels Gi Inhibitory effects- Decrease Camp, Opening of Potassium channels Gq Increase cellular IP3/DAG by stimulating the phospholipase c , Go Closenning of Calcium channels , Opening of Potassium channels Gt Decrease cellular cGMP 16 16

17. EFFECTOR SYSTEMS Primarily there are 3 G-protein coupled effector systems : 1- Adenylate cyclase- cAMP system 2- Phospholipase C - Inositol phosphate system 3- Ion channel regulations 18 17

18. Adenylate cyclase- cAMP pathway Type of G-Protein involved – Gs Activated α -GTP complex activates Adenylyl Cyclase Intracellular cAMP level raises cAMP activates PKA ( Protein Kinase A ) 19 18

19. Continued… PKA Phosphorylates various proteins ( Enzymes, Ion channels, Transporters, Transcription factor) Biological response seen Protein kinase A may activate CREB Formation of Protein occurs 20 19

20. Continued.. Termination of action : By hydrolysis of cAMP with PDE ( Phosphodiesterase ) c.AMP may directly open membrane Voltage gated Calcium channels known as CNG ( Cyclic Nucleotide Gated channels ) Increase intracellular calcium concentration Increase force of the contraction in the Heart 21 20

21. cGMP Pathway Act as second messenger in limited locations like Vascular smooth muscles, Intestinal mucosal cell. 2 form of Guanylyl cyclase Membrane Cytosolic bound Results in Vascular smooth muscle relaxation 23 22

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23. Phospholipase C- Inositol phosphate system Type of G-Protein involved – Gq Activated α -GTP complex activates Phospholipase c It breakdown PIP2 into IP3 and DAG DAG remains within membrane , While IP3 being water soluble, diffuse into cytosol 25 24

24. IP3 Opens Calcium channel on Endoplasmic reticulum membrane Intracellular Calcium level raises Calcium bind to Calmodulin Calmodulin regulates activities of various enzymes Biological response seen 26 25

25. Continued… DAG ( Di acyl glycerol ) with the help of calcium activates PKC ( Protein Kinase C ) It phosphorylates various protein depending on type of effector cell Termination of action : 1- IP3 dephosphorylated to Inositol 2- DAG is converted to Phospholipids 3- Calcium is pumped out of cell by active pumps on membrane 27 26

26. Ion Channel Regulation Activated GPCR may open/close, Calcium/Potassium channels Is without involvement of Second messenger Carried out by βγ dimer Gs type Open Calcium channel in Myocardium and Skeletal muscle, e.g.- Beta adrenoceptors 28 27

27. Gi type Opens Potassium channel in Heart and smooth muscle Hyperpolarization of the cell inhibit electrical activity e.g- M2 receptors. 29 28

28. REFERENCES Goodman, Louis S., Alfred Gilman, Laurence L. Brunton, John S. Lazlo, and Keith L. Parker. 2017.  Goodman & Gilman's the pharmacological basis of therapeutics . New York: McGraw-Hill. Ritter, J. (2020).  Rang and Dale's pharmacology  (Ninth edition.). Edinburgh: Elsevier. Tripathi, K. D. (2018).  Essentials of medical pharmacology  (8th ed.). Jaypee Brothers Medical. Katzung, Betram G., and Bertram G Katzung.  Basic & Clinical Pharmacology / by Betram G. Katzung.  14th edition. New York: McGraw-Hill, 2018. Print. 30 29

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