Transcript How Drugs Act

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 a
• “Resting state” G prot – trimer w/ GDP
occupying site on a 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
 bg duplex dissoc’s from a-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 a
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 a 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, b-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 b1-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