Cellular Receptors

Chapter 2

Binding of Drugs in/to Cells

• Receptor = Drug “target”– Membrane protein– Enzyme– Nucleic acid

• Most drugs bind receptors by weak, noncovalent forces (what are these?)– May be reversed by pH change

Molecular Recognition Specificity

• Cellular specificity– Not all receptors in all cells, tissues

• Receptors selectively bind partic ligands– Stereoselectivity

• No drug completely specific

Ligand/Receptor Interactions

• Reversible, bimolecular reaction– D + R DR DR* Response– Where R*=Receptor w/ conform’n change– Each will have rate constant

• What does this remind you of??

Activating Drugs = Agonists

• Drug/receptor binding conform’l change in receptor act’n “downstream” cell biochem

pathway(s) tissue response

• May bind at separate site on receptor– “Allosteric modulators”– Increases response to natural agonist

Some definitions

• Affinity = tendency to bind receptor– Specificity– Association/dissociation constant

• Efficacy = tendency to activate receptor– Full agonists elicit max response– Partial agonists elicit submaximal

response

Antagonists Bind Receptors…

• BUT no activation occurs– No conform’l change in receptor, so no

pathway response• May keep agonists from binding

– Competitive– Book ex: curare blocks ACh from

receptors of neuromuscular junction inhib’n muscle depolarization paralysis

• Allosteric modulators may decrease natural agonist binding

• Best antagonists have efficacy=0

Targets for Drug Action

Receptor Superfamilies

• Ligand-gated ion channels• G protein-coupled receptors• Receptor tyrosine kinases• Nuclear hormone receptors

Ligand-Gated Ion Channels

• Brain, periph NS, excitable tissues (heart), neuromuscular junction– Nicotinic cholinergic receptors

(neuromusc)– GABA receptors (brain)– Glutamate receptors (brain)

change membr potential fast synaptic transmission

• Complex prot’s w/ multiple subunits

Book Ex: Nicotinic Receptor

• Number of subunits differs w/ tissue– Antagonists differ– Allows selective blockade

neuromuscular junction• Multiple binding sites for Ach

– Excitatory incr’d Na+/K+ permeability incr’d

depol’n incr’d probability of action potential

• Direct transduction (no biochem intermediates)

• Allosteric modulators may increase/decrease transmitter response in ligand-gated channels

• Ex: benzodiazepines– Antianxiety; sleep disorders– Bind GABA ligand-gated receptors

• GABA inhibitory

– Increases ability of GABA to open channels

G Protein Coupled Receptors

• Single subunit• 7 helices span bilayer• Agonists may bind extracell N-

terminal domain, or between helices– Few allosteric modulators known

• Cytoplasmic loop couples to G protein

G Proteins

• Intermediary mol’s• Bind guanine

nucleotides• Extrinsic (periph)

prot’s at inside bilayer– Anchored to

membr by fa chain– Shuttle between

receptor, target prot’s

• 3 subunits– GTPase activity by

• “Resting state” G prot – trimer w/ GDP occupying site on subunit

• Agonist binding receptor conform’l change w/in cytoplasmic domain

Receptor acquires high affinity for G prot binding G prot to receptor

• GTP replaces GDP duplex dissoc’s from -GTP

– Diffuse along membr– Assoc w/ enzymes, ion channels act’n

or deact’n

• Term’n activity w/ hydrol Pi from GTP w/ GTPase activity of subunit– Trimer reunites

• Single agonist binding can activate sev G-prot mol’s for sev prod’s/act’n results Amplification

• Sev types G prot’s– Interact w/ diff receptors– Control diff effectors

• Gs stim’s enz adenylate cyclase, PLC, others

• Gi inhibits ad cyclase, PLC, others

– Agonist specificity

Cellular Responses

• Amplification of signal through second messengers that activate kinases– cAMP– Phosphatidylinositol

Control regulatory enz’s through covalent mod’n

Large, varied cell responses• GPCRs also control

– PLA eicosanoid release– Ion channels depol’n, transmitter

release, contractility, etc.

Examples of GPCRs

• Receptors for – ACh (muscarinic)– Neuropeptides– Ephinephrine

• Muscle (3 types), liver, fat, epithelium, neurons

Receptor Tyrosine Kinases

• Single transmembr helix• Large extracell domain

– Agonist binding site• Large intracell domain

– Some incorporate tyr kinase activity– Cytokine receptors assoc w/ cytosolic

kinases• Agonist binding act’n

dimerization– Monomeric form inactive

• Dimerized receptors autophosphorylate tyr residues

• Phosphorylated tyr attracts, binds SH-2 domain protein– Src Homology– Conserved seq recognizes

phosphotyrosine on receptor– Various SH2-domain prot’s allow

selectivity for spec receptors– Some are enzymes

• Kinases• Phospholipases

• Some SH-2 Domain prot’s are couplers for other cell prot’s w/ phosphorylated receptors– Phosphorylation cascades– Impt to cell division, diff’n– Ex: Ras/Raf/MAP kinase pathway

• Impt to cancers

• SH-2 Domain prot’s as couplers – cont’d– Ex: Jak-Stat Pathway

• Impt for cytokines, growth hormone, interferons

• Cytosolic kinase phosphorylates receptor dimer

– Various Jak’s specificity

• SH-2 domain prot’s (Stat’s) attracted, phosph’d, dimerize

Nucleus gene expression

Nuclear Hormone Receptors

• Intracellular• Most in nucleus

– Some cytoplasmic• Three domains:

– Agonist binding domain at C-terminal

– Transcriptional control domain– DNA binding domain

• Highly conserved• “Zinc fingers”

• Ligands lipophilic– Traverse lipid bilayer– Examples:

• Steroid hormones• Thyroid hormones• Vitamin D• Retinoic acid

– Impt to embryo dev’t

• Agonist binding to receptor conform’l change

Dimerization of receptors• Dimers recognize specific base

seq’s on DNA near genes– Hormone responsive elements– ~200 bp upstream from genes

• Binding DNA may activate or repress gene transcr’n– So “ligand-activated transcr’n

factors”

Other Targets of Drugs• Ion Channels

– Ligands bind voltage (as well as ligand-gated) channels• Block channel• Affect gating

– Activation GPCRs phosph’n channel prot’s• Affect channel opening• Ex: opioids, -adrenoreceptor agonists

– Modulation intracell Ca+2, GTP, ATP• Channels may bind these mol’s• Ex: sulfonylureas act at ATP-gated K+

channels on pancreatic B-cells

• Enzymes– Drug may be substrate analog

• Competitive or irreversible inhibitor

– False substrate• Appears as substrate, so taken up• Not useful as product

– Ex: 5-FU blocks DNA synth

– Prodrugs• Metabolism active agent

• Carrier molecules– Impt for transport across cell membr’s– Have recognition sites for natural mol– Examples:

• Cocaine, antidepressants inhibit noradrenaline uptake

– Amphetamine acts as false substrate

• Loop diuretics affect Na+/K+/Cl- transporter in renal tubule

• Cardiac glycosides inhibit Na+/K+ pump

Single Agonist May Have Complex Effects

• Families of receptors for agonists– Ex: ACh receptors muscarinic, nicotinic

• Further subtypes

• Some receptors very specific• Some receptors bind similar ligands

– Book ex: dopamine structurally sim to norepi, can stim 1-adrenergic receptors

• Multiple receptor subtypes for one ligand can coexist in single cell

Regulation of Receptors

• Drugs, agonists decrease sensitivity of receptors to ligands– Fast: desensitization, tachyphylaxis– Gradual: tolerance, refractoriness,

drug resistance– Usually w/ continuous exposure

• Sensitivity can be increased• Sensitization, desensitization can

occur by ligand to same ligand or another

• May be due to– Change in receptors

• Phosphorylation – inhibits ability to interact w/ G proteins

• Slow conform’l change

– Exhaustion of mediators• Ex: amphetamines relase amines from

nerve terminals; when endogenous amines depleted, drug doesn’t work

• Loss of receptors– Binding agonist to receptor rapid

migration complex to coated pits• Membr invaginations surrounded by

clathrin

– Form intracell vesicles– Receptor dissociates

• Recycled to cell membr

– Agonist degraded in lysosomes• OR May be released outside cell

• Physiological adaptation– Receptor number not static– Hormones may incr, decr receptor

number• Altered rate receptor synth• Slow• Upreg’n supersensitivity

– W/ antagonist– W/ inhib’n transmitter synth/release

• Downreg’n loss sensitivity– W/ prolonged exposure to agonists