Receptors and Drug Action. Dr. Subasini Pharmacology Department Ishik University, Erbil

Transcription

1 Receptors and Drug Action Dr. Subasini Pharmacology Department Ishik University, Erbil

2 Receptors and Drug Action Receptor Receptor is defined as a macromolecule or binding site located on the surface or inside the effector cell that serves to recognize the signal molecule / drug and initiate the single response to it, but itself has no other function. Functions of receptors are Recognition and binding of the ligand. Propagation of the message. To amplify the signal.

3 Receptors

4 Type of Receptor 1. Ion-channel-linked receptors (Ionotropic receptors) 2. G-protein-linked receptors (Metabotropic receptors) 3. Enzyme-linked receptors (Kinase linked receptor) 4. Nuclear receptor (Cytosolic receptor)

5 Molecular mechanism of receptor /Signal transduction pathway The three stages of cell signaling are, o Reception o Transduction o Response The series of steps involved is 1. Reception of the signal depends on the receptor protein. 2. The binding signaling molecule activates the receptor, which in turn activates the one or more intracellular signaling pathways. 3. The target is effector proteins, which are activated when signaling path way is activated and implement the appropriate changes in cell behavior.

6 1. Ion channel receptor 1. Ion channel linked receptors are cell membrane bound receptors are also called ligand-gated ion channels. 2. A ligand-gated ion channel receptor acts as a gate when the receptor changes shape. 3. When a drug (agonist) molecule binds to an ion channel and the channel opens, allowing the ions to move in or out of the cell following their electrical gradients. 4. The subsequent flow of ions through these channels can elicit cellular response in the form of depolarization or hyperpolarization of the cell membrane.

7 Ion channel receptor cont. Ex: Nicotinic-cholinergic receptor, GABAA/B, Glutamate & Glycine receptors. Nicotinic cholinergic receptor channel permits passage of Na+ ions results in depolarisation. Benzodiazepines bind the GABA receptor chloride channel complex and facilitate the opening of the channel

8 Ion channel receptor- cont... Receptors: Specific areas of cell membranes (proteins, glycoproteins)* When bound to ligand, positive or negative biological responce Ligand cellmembrane with receptor Biological responce

9 Signal transduction mechanism

10 Signal transduction mechanism Ligand binds to ion channel Opening of ion channels Increase diffusion across membrane Change in electrical charge Cell response (i.e. Contraction/ conduction

11 2. G-protein-coupled receptors (Metabotropic receptors) G protein-coupled receptors (GPCRs) are the largest family of cellsurface receptors, also known as seven-trans membrane domain receptors, because they pass through the cell membrane seven times. A GPCR is a plasma membrane receptors which are coupled to the effector system (enzymes/channels) through GTP binding protein called as G-protein. G-proteins are heterotrimeric, with 3 subunits α, β, γ. There are three main varieties of G-protein (Gs, Gi and Gq), among these Gs and Gi produce stimulation or inhibition of adenyl cyclase, respectively, while Gq controls phospholipase C activity. Ex: Muscarinic-cholinergic receptor, adrenoceptors, dopaminergic receptor & 5-HT receptor.

12 G-Protein coupled receptor structure

13 G protein coupled receptor signal transduction pathway In resting state, GDP is bind to α subunit of α-β-γ trimer. When the receptor is activated by the binding of an agonist a conformational changes occurs results in a high affinity for the α-β & γ trimer of G-Protein. During this process GDP dissociates from α-β-γ subunit while GTP associates in its place. Binding of GTP activates the α-subunit and ultimately dissociates the remaining β & γ subunits The free α-gtp subunit then activates the effector/target cells. This is called activation state or switch on state.

14 G protein coupled receptor signal transduction pathway- cont

15 G protein coupled receptor signal transduction pathway The GTPase of the α-subunit quickly hydrolyses GTP to GDP and the resultant α-gdp unit then dissociate from effector and reunite with β-γ unit, complete the cycle. Resetting to switch back to the off state. Primarily there are three G-protein coupled effector system through which the G-Protein coupled receptor works a) The Adenylyl cyclase: camp system b) The Phospholipase-C: Inositol phosphate system c) Ion channels

16 a) The Adenylyl cyclase: camp effector signal transduction All receptors that act via cyclic AMP are coupled to a stimulatory G protein (Gs), which activates adenylyl cyclase and thereby increases cyclic AMP concentration. Another G protein, called inhibitory G protein (Gi), inhibits adenylyl cyclase, but it mainly acts by directly regulating ion channels rather than by decreasing cyclic AMP content.

17 b) The Phospholipase-C: Inositol phosphate signal transduction system In this signaling pathway signaling molecule (ligand) bind to the receptor linked to G-proteins (Gq) and activate the membrane enzyme phospholipase-c (PLC). Stimulation of PLC leads to the hydrolysis of Phosphotidyl inositol 4,5 diphosphate (PIP2) [phospholipid component of plasma membrane] On hydrolysis PIP2 splits into two second messengers: diacylglycerol (DAG) and inositol-1, 4, 5-tri-phosphate (IP3).

18 The Phospholipase-C: Inositol phosphate signal transduction system cont. IP3 being water soluble diffuse through cytoplasm where it trigger the release of Ca2+ from storage vesicles. The raised concentration of Ca2+ promotes the binding of Ca2+ to calmodulin which regulate the activity of various enzymes. DAG activates a phospholipid and the calcium sensitive protein kinase - C (PKC) which then phosphorylates specific protein leads to the response.

19 c) Ion channel regulation G-couple receptors can control the functioning of ion channel (e.g. K+ and Ca2+ channels) by the mechanisms that do not involve any role of secondary messengers. E.g.: In cardiac muscle, muscarinic Ach receptor. Opioid analgesics reduce neuronal excitability by opening K+ channel by this mechanism.

20 3. Enzyme-linked receptors (Kinase linked receptor) Enzyme-linked receptor, also known as a catalytic receptor, is a trans membrane receptor, where the binding of an extracellular ligand causes enzymatic activity on the intracellular side. The agonist binding site and inner face catalytic site are interconnected through a single trans membrane stretch of peptide chain. They are protein kinases and hence are also known as kinase linked receptors. There are two major subgroups of such receptors. a. Those that have intrinsic enzymatic activity. b. Those that lack intrinsic enzymatic activity, but bind a JAK- STAT kinase on activation.

21 a) Intrinsic enzyme receptor The intracellular domain is either a protein kinase or guanylyl cyclase. In most cases they are protein kinase and hence are also known as kinase linked receptor. Agonist binding induces dimerization of receptor molecules and activates the kinase to autophosphorylate tyrosine residues on each other, increasing their affinity for binding substrate proteins and carrying forward the cascade of tyrosine phosphorylations. Intracellular events are triggered by phosphorylation of relevant proteins, many of which carry a SH2 domain.

22 Intrinsic enzyme receptor cont..

23 b) JAK-STAT kinase binding receptor It is sub type of enzyme linked receptor. These consist of 2 identical single-pass trans membrane proteins embedded in the plasma membrane. Each of their cytoplasmic ends binds a molecule of a Janus kinase ("JAK"). The JAK-STAT system consists of three main components: (1) a receptor (2) Janus kinase (JAK) and (3) Signal Transducer and Activator of Transcription (STAT).

24 Enzyme-linked receptors signal transduction path way

25 4. Nuclear receptors (Cytosolic receptor) They are intercellular proteins which are in an inactive state. Binding of agonist activates the receptor. The agonist- receptor complex move to the nucleus where it interacts with DNA, regulate gene transcription and thereby directs the synthesis of specific protein to regulate the activity of target cells. E.g: Steroidal hormones, thyroid hormones, Vit-D and retinoids.

26 Nuclear receptors (Cytosolic receptor Drug

27 Drugs that do not act on receptors: Antacida: CaCO 3 + HCl Diuretica (osmotic) Akylating agents (cancer) Cl Cl Drugs that do act on receptors: Acetylcholin (Neurotransmittor) O O H ca. 5Å O N Acetylcholin Agonists Carbacholin H 2 N O O N O Pilocarpine O Me N N H Cl N Cl N Nu N Nu Acetylcholin Antagonists Atropin Cyclopentolat Psoralenes O N N O HN R NH 2 O N h O O O R' O O N N HN R O NH 2 N O O O R' N O O OH N O O OH Agonist: Binds to (have affinity for) receptor Binding leads to biolog. responce (Agonists have intrinsic activity / efficacy) Antagonist: Affinity for receptor No intrinsic activity

28 Types of receptors Super- Endogenous family ligands General structures 1 Fast neurotransmittors Ligand gated ion chanels ex. Acetylcholine 2 Slow neurotransm. ex. Noradrenalin Hormones G-Protein coupled receptors 3 Insuline Enzyme coupled receptors Growth factors Catalytic receptors 4 Steroid hormones Cytoplasmic receptors Thyroid hormones Vitamin A, D

29 Types, distribution and function

30 Stimulatory Receptors Inhibitory Receptors a 1 a 2 b 1 b 2 All the tissues to be stimulated have a 1 AR except Heart, JGA and Adipose tissue (β1) All the tissues to be inhibited should have b 2 except pre-synaptic nerve ending, platelets, beta cells (α2)

31 Heart Adrenaline (Epinephrine) Cardiac stimulant Increase in heart rate (+ Chronotropic effect) Increase in myocardial contractility ( + Inotropic effect) Increase in conduction velocity ( + Dromotropic effect) Increase in cardiac output Increase in automaticity

32 Suggested Reading Katzung BG. Basic & clinical pharmacology. 8 th ed., Katzung BG, Trevor AJ. Examination &board review pharmacology. 5 th ed Goodman&Gilman. Basic pharmacology. 9 th ed., Pharmacology, Lippincott s Illustrated Reviews 1992.

33 Thank you for your attention