Introduction to Receptors

Tim Bloom, Ph.D.

Room 206 Maddox Hall

893-1712

bloom@campbell.edu

www.campbell.edu/faculty/bloom

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Lecture Overview

• History of receptors

• Receptor theory

• Biochemistry of receptors

• Examples of common receptor types

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Pharmacology

• Pharmacokinetics– Absorption

– Distribution

– Metabolism

– Excretion

• Pharmacodynamics– Receptors

– Signaling

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Berthold and the Roosters

– Effects of castration• Secondary sex characteristics

• Behavior

– Effects of transplant– Observation and hypothesis

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Isolated Muscle Setup

flow in flow outnicotine

rise in tensionrise in tensionrise in tension

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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

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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

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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

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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

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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

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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

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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

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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)

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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

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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

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Signaling Receptor Classes

• Four major classes of signaling receptors– Cytoplasmic transcription factors

– Ion channels

– Transmembrane signaling enzymes

– G-protein coupled receptors

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Cytoplasmic Transcription Factors

• Inactive at rest• Bound to

inhibitor protein• Ligand removes

inhibitor• L-R complex

moves to nucleus

• Transcription is altered

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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

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Transmembrane Enzymes

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

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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

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Kinase Receptors

• These receptors have tyrosine kinase activity– Phosphorylate substrate proteins, including other

receptor in dimer– Receptor phosphorylation makes it active:

even without ligand!

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Kinase receptors

• Phosphorylated receptors also act as ligands– “SH2 proteins” recognize Tyr-P and bind

– Binding activates SH2 proteins

– Creation of signaling complex

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Example

kinase P04

SH2 protein

PLC-

substrate

product

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G protein-coupled Receptors

• Largest class of receptors• Wide range of ligands• Wide range of binding “methods”

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G protein-coupled Receptors

• Differ from other classes

• Seven transmembrane domains

• All act through combination of G protein and effector protein

RG

E

A B

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G Proteins

• Made of three subunits:

• has three functions:– Detects ligand-bound

receptor (gets active)

– Activates effector

– Turns itself off

GDP

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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

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Effectors

• Enzymes– Synthesize product when activated– Modify proteins when active

• Ion channels– Ions flow down gradient– Change in electrical state

• Activated effectors produce effects

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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

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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