Introduction to Receptors Tim Bloom, Ph.D. Room 206 Maddox Hall

2 Lecture Overview History of receptors Receptor theory Biochemistry of receptors Examples of common receptor types

3 Pharmacology Pharmacokinetics –Absorption –Distribution –Metabolism –Excretion Pharmacodynamics –Receptors –Signaling

4 Berthold and the Roosters –Effects of castration Secondary sex characteristics Behavior –Effects of transplant –Observation and hypothesis

5 Isolated Muscle Setup flow in flow out nicotine rise in tension

6 Langley and the Frogs –In vitro study with leg muscle strips –Muscle stimulation by Electricity Nerve Nicotine –Effect of curare on animals –Effect of curare on in vitro muscle stimulation Electricity into nerve Nicotine Electricity into muscle

7 Ehrlich and the Parasites Organic chemist making clothing dyes Saw dyeing of cells with selective stains –Staining a cell type is dye-dependent –Small changes in chemical alter staining “Receptive substance” on cells –Use as target for selective drugs –Attach toxin to selective dye

8 Sum of History Chemicals affect tissues Some chemicals interfere with others Chemical structure impacts action Cells produce chemicals that affect other cells Therefore, cells can detect chemicals

9 Receptor Theory Core of pharmacodynamics Cells have “receptors” –Act as targets for “ligands” (drugs, hormones, neurotransmitters, etc.) –Required for biological effect of above agents –Sensitive to small changes in ligand structure –Mediate action of ligand NO RECEPTOR = NO RESPONSE

10 Biochemistry of Receptors Chemical receptors –Non-specific –Bind via one or two chemical bonds –Examples are stomach acid, heavy metals Macromolecular receptors –Detect specific molecules –Require multiple chemical bonds –Rely on 3-D shape of ligand for recognition –These are of interest to pharmacologists

11 Molecules as Receptors DNA –Alkylating chemicals as cancer chemotherapy –DNA damage gives therapeutic result Structural proteins –Colchicine –Interferes with tubulin polymerization Enzymes –NSAIDs –Inhibit cyclo-oxygenase

12 Molecules as Receptors Ion channels –Nicotine –Allow ions to cross cell membranes Transcription factors –Steroid hormones –Alter rates of gene expression up or down Plasma membrane signaling proteins –Insulin or adrenaline –Binding results in a signal detected inside cell

13 Importance of Bonds Chemical bonds are formed between a receptor and its ligand(s) –Hydrogen bonds –Hydrophobic interactions –Van der Waals forces –Ionic bonds –(covalent bonds)

14 Bonds for Activity Ability to create proper bonds is vital Proper bonds possible with proper shape Bonds allow “proper” interactions Small modifications can have large effect Weak bonds = temporary binding

15 Receptor Functions Most common is to generate a “signal” –Alters some facet of cell balance –“Signal” results in some cellular change Basal cell at rest has certain features: –Stable pH and electrical charge (ion concentrations) –Stable transcription rates –Stable levels of signaling molecules –Stable levels of protein modifications –Stable metabolic rate

16 Signaling Receptor Classes Four major classes of signaling receptors –Cytoplasmic transcription factors –Ion channels –Transmembrane signaling enzymes –G-protein coupled receptors

17 Cytoplasmic Transcription Factors Inactive at rest Bound to inhibitor protein Ligand removes inhibitor L-R complex moves to nucleus Transcription is altered

18 Ion Channels Made up of subunits Group forms a pore Gate blocks ions Ligand binding affects gate behavior Some ligands activate channel, let ions flow

19 Transmembrane Enzymes Receptor is single protein Dimerization required for activity Inactive at rest, activated by ligand Most common type is tyrosine kinase

20 Tyrosine Kinases Kinases transfer phosphates (phosphorylation) –From ATP to proteins –Addition to serine, threonine or tyrosine –Modifies substrate protein activity Activate Inactivate Alters interactions with other proteins

21 Kinase Receptors These receptors have tyrosine kinase activity –Phosphorylate substrate proteins, including other receptor in dimer –Receptor phosphorylation makes it active: even without ligand!

22 Kinase receptors Phosphorylated receptors also act as ligands –“SH2 proteins” recognize Tyr-P and bind –Binding activates SH2 proteins –Creation of signaling complex

23 Example kinase P0 4 SH2 protein PLC-  substrate product

24 G protein-coupled Receptors Largest class of receptors Wide range of ligands Wide range of binding “methods”

25 G protein-coupled Receptors Differ from other classes Seven transmembrane domains All act through combination of G protein and effector protein R G E A B

26 G Proteins Made of three subunits:   has three functions: –Detects ligand-bound receptor (gets active) –Activates effector –Turns itself off    GDP

27 Cycle of the  Subunit Activated by ligand- bound receptor Swaps GDP for GTP Loses  subunits Activates effector Hydrolyzes GTP to GDP- is inactivated Rebinds  subunits  GDP  GTP R*   GTP  GDP E

28 Effectors Enzymes –Synthesize product when activated –Modify proteins when active Ion channels –Ions flow down gradient –Change in electrical state Activated effectors produce effects

29 Review Cells respond to substances via receptors Receptors provide two functions –Detect ligand presence –Generate a signal in response to ligand –(signal = change) Signaling through receptor modifies cell Most cellular molecules can be a receptor

30 Review Many receptors are in one of four classes –Ion channels –Transcription factors –Membrane-associated enzymes –G-protein coupled receptors Normal function of a receptor determines the nature of its signal