g- protein coupled receptors

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G-Protein-Coupled ReceptorsDr. Prashant ShuklaJunior ResidentDept of Pharmacology

2ContentsIntroductionHistorical Background GPCR - Basics GPCR as Targets for Drug DesigningGPCR associated DiseasesConcept of Orphan GPCRFuture Prospects & Conclusions

Introduction 3Receptors are the sensing elements in the system of chemical communications that coordinates the function of all the different cells in the body.

The chemical messengers can be hormones, transmitters and other mediators.


Types of receptors

The G protein-coupled receptor (GPCR) superfamily comprises the largest and most diverse group of proteins in mammals.

Synonym: seven-transmembrane (7-TM), serpentine receptors, heptahelical receptors,serpentine receptor, andG proteinlinked receptors(GPLR).

The human genome encodes >800 GPCRs.5GPCRs

GPCRsIt is involved in information transfer (signal transduction) from outside the cell to the cellular interior.

GPCRs are responsible for every aspect of human biology from vision, taste, sense of smell, sympathetic and parasympathetic nervous functions, metabolism, and immune regulation to reproduction.

~45% of all pharmaceutical drugs are known to target GPCRs.6

Receptors associated with GPCRs1. GABAB ReceptorsGABABR1 and GABABR2 2. Taste ReceptorsT1R3 and T1R23. Adrenergic ReceptorsThree subfamilies ( 1, 2 and ) Family A (rhodopsin-like) GPCRs4. Opioid ReceptorsThree cloned subtypes: , and



Receptors associated with GPCRs5. Somatostatin ReceptorsFive subtypes (SSTR1-5) 6. Purinergic Receptors Neurotransmitters in the CNS, CVS, immune system, and other tissues i.e. adenosine and ATP 7. Olfactory ReceptorsRepresent the largest family of GPCRs, with >300 members8. Vasopressin, Oxytocin and Other Receptors



The importance of GPCRsNumber (C.elegans 1100; H. sapiens, ~1000; D. melanogaster, 160; reflects number of olfactory receptor genes in worm [~1000] and mammal [several hundreds]), a few % of genome; 300-400 non-olfactory GPCRs)

Diversity (mostly small molecule ligands)

Evolutionarily conserved yeast to man (yeast Ga 45% identical to mammalian Gia)

Pharmaceutical importance: ~500 known molecular targets of drugs, 60% of these are cell surface receptors, 75% of these are GPCRs (GPCRs = ~45% of all known drug targets)


Historical background10

Robert Lefkowitzand Brian Kobilka: the 2012 Nobel Prize in Chemistry for groundbreaking discoveries that reveal the inner workings of an important family of receptors: G-proteincoupled receptors.

11Historical background

12Common Experimental Tools used to Study GPCRs

GPCR- basics13Structure ClassificationSignal molecules/ LigandsPhysiological roleG proteinsMechanism of action

GPCR- Basic structure


14Extracellular loopsIntracellular loopsPlasmamembrane

15GPCR- Basic structure

GPCR: Classification Based on Sequence homology and functional similarity Class A (or 1) (Rhodopsin-like)Class B (or 2) (Secretin receptor family)Class C(or 3) (Metabotropic glutamate/ pheromone)Class D (or 4) (Fungal mating pheromone receptors)Class E (or 5) (Cyclic AMP receptors)Class F (or 6) (Frizzled/Smoothened)16

Based on phylogenetic origin: TheGRAFSclassification system has been proposedGlutamateRhodopsin Adhesion Frizzled/TasteSecretin17GPCR: Classification



Signal molecules/ Ligands of GPCRsGPCRs interact with a number of ligands ranging from photons, ions, amino acids, odorants, pheromones, eicosanoids, neurotransmitters, peptides, proteins, and hormones.

Nevertheless, for the majority of GPCRs, the identity of their natural ligands is still unknown, hence remain orphan receptors.19

Signal molecules20Biogenic amines: Adrenaline, noradrenaline, dopamine, 5-HT, histamine, acetylcholine

Amino acids and ions: Glutamate, Ca2+, GABA

Lipids : PAF, prostaglandins, leukotrienes, anandamine

21Peptides / proteins : GnRH, angiotensin, bradykinin, thrombin, bombesin, glucagon, calcitonin, vasoactive intestinal peptides, PTH, FSH, LH, TSH

Nucleotides : Adenosine nucleotides, adenine nucleotides, uridine nucleotides

Others : Light, odorants, pheromones, opiatesSignal molecules

Physiological roles22Visual sense: RhodopsinSense of smell: Olfactory receptorBehavioral and mood regulation: Serotonin, dopamine, GABA and glutamateImmune system activity and inflammation: Chemokine receptors, histamine receptors ANS transmission: adrenergic receptorsApoptosis

Structure of G ProteinG proteins, also known asguanine nucleotide-binding proteins,involved in transmitting signals and function as molecular switches.

Their activity is regulated by factors that control their ability to bind to and hydrolyze GTP toGDP. When they bind GTP, they are 'on', and, when they bind GDP, they are 'off '.


G protein complexes are made up of20 alpha () 6 beta () 12 gamma ()subunits.

Beta and gamma subunits can form a stable dimeric complex referred to as thebeta-gamma complex.

subunit subunit subunit24

Types of G Proteins25

G protein cycle26

Basal stateActivated state

GPC ReceptorsG ProteinReceptorsSignaling Pathway GSBeta adrenergic receptors, glucagon, histamine, serotonin Increase CAMPExcitatory effectsGiAlpha2 adrenergic receptors, mAchR, opioid, serotonin Decrease CAMP Cardiac K+ channel open- decrease heart rateGqmAchR, H1, 1, Vasopressin type 1, 5HT1CPLC- IP3 , DAGIncrease Cytoplasmic CaGtRhodopsin and colour opsins in retinal rod and cone cellsIncrease cGMP phosphodiesterase.Decrease cGMP



G Protein Mediated PathwaysSecondary messenger Systems Involved In Signal Transduction:Adenylate cyclase cAMP mediated pathway Phospholipase mediated pathway

GPCR s can also directly activate the ion channels


cAMP Mediated Pathway

ThecAMP-dependent pathway, also known as theadenylyl cyclasepathway, is aG protein-coupled receptor triggeredsignaling cascade used incell communication.

Gs cAMP Dependent Pathway

Gi cAMP Dependent Pathway 29




Cell responseATProtein kinase


PInactive proteinActive protein

hormoneAdenylate cyclase Signaling SystemACRSInhibitorRi


Gi cAMP Dependent Pathway 31

32CYTOSOLEXTRACELLULARGq Protein Coupled Receptor


Gt PCR: involved in photo transduction.

Gt Protein Coupled Receptor

Signal Amplification through G proteins 34

Regulation of GPCRs Turning GPCRs Off A cell must also be able to stop responding to protect overstimulation

High activation of a receptor leads to a reduced ability to be stimulated in the future (desensitization)

Can also significantly limit therapeutic usefulness of many receptor agonists.



36Desensitization mechanisms includedown-regulation or reduction of receptor number

sequestration or apparent shielding of the receptors from interacting ligands

uncoupling from G-proteins.Regulation of GPCRs

Homologous desensitization: The activation dependent regulation of receptors.

Heterologous desensitization: Receptor activation-independent regulation of receptors.37Regulation of GPCRs


Homologous desensitizationThe activated state of GPCRs serves not only as an activator of G proteins, but also as the substrate for GPCR kinases (GRKs).



41Homologous desensitization

Based on feedback regulation of receptors by the second-messenger-regulated kinases.

Eg. Upon stimulation, - receptors leads to cAMP, which activates PKA. PKA can then phosphorylate the - receptors themselves, even those particular receptor proteins that were not activated by the current stimulation. These PKA-phosphorylated receptors are less able to mount a response. 42Heterologous desensitization

ReceptorDrugs and some key indicationsAT1 angiotensin II receptor Antagonists e.g. losartan in treatment of HT or CHF1A-c receptor Antagonists e.g. tamsulosin to treat disorders asso. with enlarged prostate1- receptorAntagonists e.g. propranolol, atenolol, metoprolol, carvedilol to treat essential HT or CHF2- receptorAgonists e.g. terbutaline, salbutamol, formoterol for treatment of COPD or Bronchial asthma D2 receptorAntagonists e.g. Haloperidol & clozapine to treat schizophreniaAgonists e.g. levodopa for Parkinsonism

43GPCR as drug targets


44ReceptorDrugs and some key indicationsD3 receptor Antagonists e.g. haloperidol in schizophrenia5-HT2A receptor Antagonists e.g. clozapine for schizophrenia.Indirect agonists e.g. fluvoxamine for depression5-HT2C receptor Antagonists e.g. clozapine for schizophreniaCCR5 Associated with progression of AIDS e.g. Aplaviroc and maraviroc M3Antagonists e.g. Atropine to dilate pupil; Scopolamine for motion sicknessNeuropeptide S receptor Asthma susceptibility e.g. Neuromedin and neurotensinP2Y12 Associated with bleeding diathesis e.g. Clopidogrel

GPCR as drug targets

45GPCR as drug targets

Diseases associated with G-proteinsAbnormal G protein signalling can result by

Bacterial toxins (Cholera and pertussis)

Gene mutationsLoss of function mutationsGain of function mutations

Altered GPCR folding



Mutations in GPCRMutations in genes encoding are an imp