Receptors Structure and Function

1. Receptors Structure and Function Chapter 4

2. The role of the receptor • Globular proteins • Located mostly in the cell membrane • Receive messages from chemical messengers coming from other cells (CNS) • Transmit a message into the cell leading to a cellular effect • Different receptors specific for different chemical messengers • Each cell has a range of receptors in the cell membrane making it responsive to different chemical messengers

3. Nerve Nerve Signal Messenger Receptor Response Nucleus Cell Cell The Role of the receptor

4. The Role of the receptor • Neurotransmitters: Chemicals released from nerve endings which travel across a nerve synapse to bind with receptors on target cells, such as muscle cells or another nerve. Usually short lived and responsible for messages between individual cells • Hormones:Chemicals released from cells or glands and which travel some distance to bind with receptors on target cells throughout the body • Note:Chemical messengers ‘switch on’ receptors without undergoing a reaction

5. The role of the receptor • Receptors contain a binding site (hollow or cleft on the receptor surface) that is recognised by the chemical messenger • Binding of the messenger involves intermolecular bonds • Binding results in an induced fit of the receptor protein • Change in receptor shape results in a ‘domino’ effect • Domino effect is known as signal transduction, leading to a chemical signal being received inside the cell • Chemical messenger does not enter the cell. It departs the receptor unchanged and is not permanently bound

6. Messenger Messenger Messenger Induced fit Receptor Receptor Cell Membrane Cell Cell message Message The Binding Site Receptor Cell

7. Binding site ENZYME The Binding Site • A hydrophobic hollow or cleft on the receptor surface - equivalent to the active site of an enzyme • Accepts and binds a chemical messenger • Contains amino acids which bind the messenger • No reaction or catalysis takes place

8. Messenger M Induced fit The Binding Site • Binding site is nearly the correct shape for the messenger • Binding alters the shape of the receptor (induced fit) • Altered receptor shape leads to further effects - signal transduction

9. Phe Phe H O H O Ser Ser CO2 Induced Fit CO2 Asp Asp How does the Binding Site Change Shape? • Before – • Intermolecular bonds not optimum length for maximum binding strength • After – • Intermolecular bond lengths optimised

10. M M M R E R E R Signal transduction Induced Fit • Binding interactions must be strong enough to hold the messenger sufficiently long for signal transduction to take place • Interactions must be weak enough to allow the messenger to depart • Implies a fine balance • Designing molecules with stronger binding interactions results in drugs that block the binding site - antagonists

11. Main Types of Receptors • ION CHANNEL RECEPTORS • G-PROTEIN-COUPLED RECEPTORS • KINASE-LINKED RECEPTORS • INTRACELLULAR RECEPTORS

12. Ion Channel Receptors • Receptor protein is part of an ion channel protein complex • Receptor binds a messenger leading to an induced fit • Ion channel is opened or closed • Ion channels are specific for specific ions (Na+, Ca2+, Cl-, K+) • Ions flow across cell membrane down concentration gradient • Polarises or depolarises nerve membranes • Activates or deactivates enzyme catalysed reactions within cell

13. Hydrophilic tunnel MESSENGER ION CHANNEL (open) RECEPTOR BINDING SITE MESSENGER Induced fit and opening of ion channel Lock Cell membrane Cell membrane Cell membrane Cell membrane Gate Ion channel Ion channel Ion channel Ion channel Cell membrane Cell Cell Ion Channel Receptors

14. Protein subunits TM4 TM1 TM3 TM2 TM1 TM3 TM2 TM2 TM4 TM4 TM3 TM1 TM3 TM2 TM2 TM1 TM4 TM1 TM3 TM4 Ion Channel Receptors Transmembrane Proteins TM2 of each protein subunit ‘lines’ the central pore

15. Binding site Receptor Messenger Cell Induced fit ‘Gating’ (ion channel opens) Cell membrane membrane Gating Five glycoprotein subunits traversing cell membrane

16. TM2 TM2 Cell membrane TM2 TM2 TM2 TM2 TM2 TM2 Transverse view Transverse view TM2 TM2 TM2 TM2 Closed Open gating • Chemical messenger binds to receptor binding site • Induced fit results in further conformational changes • TM2 segments rotate to open central pore

17. Gating • Fast response measured in msec • Ideal for transmission between nerves • Binding of messenger leads directly to ion flows across cell membrane • Ion flow = secondary effect (signal transduction) • Ion concentration within cell alters • Leads to variation in cell chemistry

18. messenger induced fit closed open G-protein split G-PROTEIN-COUPLED RECEPTORS • Receptor binds a messenger leading to an induced fit • Opens a binding site for a signal protein (G-protein) • G-protein binds, is destabilised then split

19. Enzyme Enzyme active site (open) active site (closed) Intracellular reaction G-PROTEIN-COUPLEDRECEPTORS • G-protein subunit activates membrane bound enzyme • Binds to allosteric binding site • Induced fit results in opening of active site • Intracellular reaction catalysed

20. C N -Terminal chain -Terminal chain NH2 Extracellular loops HO2C Transmembrane Membrane VII VI V IV III II I helix G-Protein binding region Variable Intracellular loops intracellular loop G-PROTEIN-COUPLED RECEPTORS

21. Ligand A B C D LIGAND BINDING SITE - varies depending on receptor type A) Monoamines: pocket in TM helices B) Peptide hormones: top of TM helices + extracellular loops + N-terminal chain C) Hormones: extracellular loops + N-terminal chain D) Glutamate: N-terminal chain

22. Bacteriorhodopsin & Rhodopsin Family • Rhodopsin = visual receptor • Many common receptors belong to this same family • Implications for drug selectivity depending on similarity (evolution) • Membrane bound receptors difficult to crystallise • X-Ray structure of bacteriorhodopsin solved - bacterial protein similar to rhodopsin • Bacteriorhodopsin structure used as ‘template’ for other receptors • Construct model receptors based on template and amino acid sequence • Leads to model binding sites for drug design • Crystal structures for rhodopsin and b2-adrenergic receptors now solved - better templates

23. Bacteriorhodopsin & Rhodopsin Family

24. RECEPTOR TYPES AND SUBTYPES • Receptor types and subtypes not equally distributed amongst tissues. • Target selectivity leads to tissue selectivity Heart muscle b1 adrenergic receptors Fat cells b3 adrenergic receptors Bronchial muscle a1& b2 adrenergic receptors GI-tract a1 a2 & b2 adrenergic receptors

25. Tyrosine kinase - linked receptors • Bifunctional receptor / enzyme • Activated by hormones • Overexpression can result in cancer

26. messenger messenger closed closed induced fit intracellular reaction Tyrosine kinase-linked receptors • Protein serves dual role - receptor plus enzyme • Receptor binds messenger leading to an induced fit • Protein changes shape and opens active site • Reaction catalysed within cell • Overexpression related to several cancers active site open

27. Ligand binding region N H 2 Hydrophilic transmembrane region (a-helix) Cell membrane C O H 2 Catalytic binding region (closed in resting state) Tyrosine kinase-linked receptors Extracellular N-terminal chain Intracellular C-terminal chain

28. Tyrosine kinase O O C C N N Protein Protein Protein Protein Mg++ ATP ADP OH O P Tyrosine residue Phosphorylated tyrosine residue Reaction catalysed by tyrosine kinase

29. EGF Ligand binding Phosphorylation and dimerisation Cell membrane OH HO OP PO OH OH OP OP ATP ADP Inactive EGF-R monomers Binding site for EGF EGF - protein hormone - bivalent ligand Active site of tyrosine kinase Epidermal growth factor receptor (EGF- R) Induced fit opens tyrosine kinase active sites

30. Epidermal growth factor receptor (EGF- R) • Active site on one half of dimer catalyses phosphorylation of Tyr residues on other half • Dimerisation of receptor is crucial • Phosphorylated regions act as binding sites for further proteins and enzymes • Results in activation of signalling proteins and enzymes • Message carried into cell

31. Insulin Phosphorylation Cell membrane HO OH PO OP OP ADP ATP OP OH OH Insulin binding site Kinase active site Insulin receptor (tetrameric complex) Kinase active site opened by induced fit

32. GH GH binding & dimerisation Binding Activation and of kinases phosphorylation GH receptors (no kinase activity) ATP ADP PO HO OH OP OP OH OP OH kinases HO OH OH OH Growth hormone binding site Kinase active site Growth hormone receptor Tetrameric complex constructed in presence of growth hormone Kinase active site opened by induced fit

33. CO2H Steroid binding region Zinc DNA binding region (‘zinc fingers’) H2N Intracellular receptors • Chemical messengers must cross cell membrane • Chemical messengers must be hydrophobic • Example-steroids and • steroid receptors Zinc fingers contain Cys residues (SH) Allow S-Zn interactions

34. Co-activator protein Receptor DNA Messenger Receptor-ligand complex Dimerisation Cell membrane 1. Messenger crosses membrane 5. Complex binds to DNA 2. Binds to receptor 6. Transcription switched on or off 3. Receptor dimerisation Intracellular receptor Mechanism 7. Protein synthesis activated or inhibited 4. Binds co-activator protein