Cellular Receptors

1
Cellular Receptors

• Chapter 2

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

3
Molecular Recognition ? Specificity

• Cellular specificity
• Not all receptors in all cells, tissues
• Receptors selectively bind partic ligands
• Stereoselectivity
• No drug completely specific

4
(No Transcript)
5
Ligand/Receptor Interactions

• Reversible, bimolecular reaction
• D R ? DR ? DR ??? Response
• Where RReceptor w/ conformn change
• Each will have rate constant
• What does this remind you of??

6
Activating Drugs Agonists

• Drug/receptor binding
• ? conforml change in receptor
• ? actn downstream cell biochem pathway(s)
• ? tissue response
• May bind at separate site on receptor
• Allosteric modulators
• Increases response to natural agonist

7
(No Transcript)
8
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

9
Antagonists Bind Receptors

• BUT no activation occurs
• No conforml change in receptor, so no pathway
  response
• May keep agonists from binding
• Competitive
• Book ex curare blocks ACh from receptors of
  neuromuscular junction ? inhibn muscle
  depolarization ? paralysis
• Allosteric modulators may decrease natural
  agonist binding
• Best antagonists have efficacy0

10
(No Transcript)
11
Targets for Drug Action
12
Receptor Superfamilies

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

13
(No Transcript)
14
(No Transcript)
15
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 prots w/ multiple subunits

16
Book Ex Nicotinic Receptor

• Number of subunits differs w/ tissue
• Antagonists differ
• Allows selective blockade neuromuscular junction
• Multiple binding sites for Ach
• Excitatory
• ? incrd Na/K permeability ? incrd depoln ?
  incrd probability of action potential
• Direct transduction (no biochem intermediates)

17
(No Transcript)
18

• 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

19
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

20
(No Transcript)
21
G Proteins

• Intermediary mols
• Bind guanine nucleotides
• Extrinsic (periph) prots at inside bilayer
• Anchored to membr by fa chain
• Shuttle between receptor, target prots
• 3 subunits
• GTPase activity by a

22

• Resting state G prot trimer w/ GDP occupying
  site on a subunit
• Agonist binding receptor ? conforml change w/in
  cytoplasmic domain
• ? Receptor acquires high affinity for G prot ?
  binding G prot to receptor
• GTP replaces GDP
• bg duplex dissocs from a-GTP
• Diffuse along membr
• Assoc w/ enzymes, ion channels ? actn or deactn

23
(No Transcript)
24
(No Transcript)
25

• Termn activity w/ hydrol Pi from GTP w/ GTPase
  activity of a subunit
• Trimer reunites
• Single agonist binding can activate sev G-prot
  mols for sev prods/actn results
• ? Amplification

26

• Sev types G prots
• Interact w/ diff receptors
• Control diff effectors
• Gs stims enz adenylate cyclase, PLC, others
• Gi inhibits ad cyclase, PLC, others
• Agonist specificity

27
Cellular Responses

• Amplification of signal through second messengers
  that activate kinases
• cAMP
• Phosphatidylinositol
• ? Control regulatory enzs through covalent modn
• ? Large, varied cell responses
• GPCRs also control
• PLA ? eicosanoid release
• Ion channels ? depoln, transmitter release,
  contractility, etc.

28
(No Transcript)
29
Examples of GPCRs

• Receptors for
• ACh (muscarinic)
• Neuropeptides
• Ephinephrine
• Muscle (3 types), liver, fat, epithelium, neurons
  

30
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 ? actn ? dimerization
• Monomeric form inactive

31

• Dimerized receptors autophosphorylate tyr
  residues
• Phosphorylated tyr attracts, binds SH-2 domain
  protein
• Src Homology
• Conserved seq recognizes phosphotyrosine on
  receptor
• Various SH2-domain prots allow selectivity for
  spec receptors
• Some are enzymes
• Kinases
• Phospholipases

32

• Some SH-2 Domain prots are couplers for other
  cell prots w/ phosphorylated receptors
• Phosphorylation cascades
• Impt to cell division, diffn
• Ex Ras/Raf/MAP kinase pathway
• Impt to cancers

33
(No Transcript)
34

• SH-2 Domain prots as couplers contd
• Ex Jak-Stat Pathway
• Impt for cytokines, growth hormone, interferons
• Cytosolic kinase phosphorylates receptor dimer
• Various Jaks ? specificity
• SH-2 domain prots (Stats) attracted, phosphd,
  dimerize
• ? Nucleus ? gene expression

35
(No Transcript)
36
(No Transcript)
37
(No Transcript)
38
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

39
(No Transcript)
40

• Ligands lipophilic
• Traverse lipid bilayer
• Examples
• Steroid hormones
• Thyroid hormones
• Vitamin D
• Retinoic acid
• Impt to embryo devt

41

• Agonist binding to receptor ? conforml change
• ? Dimerization of receptors
• Dimers recognize specific base seqs on DNA near
  genes
• Hormone responsive elements
• 200 bp upstream from genes
• Binding DNA may activate or repress gene
  transcrn
• So ligand-activated transcrn factors

42
(No Transcript)
43
Other Targets of Drugs

• Ion Channels
• Ligands bind voltage (as well as ligand-gated)
  channels
• Block channel
• Affect gating
• Activation GPCRs ? phosphn channel prots
• Affect channel opening
• Ex opioids, b-adrenoreceptor agonists
• Modulation intracell Ca2, GTP, ATP
• Channels may bind these mols
• Ex sulfonylureas act at ATP-gated K channels
  on pancreatic B-cells

44
(No Transcript)
45

• 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

46

• Carrier molecules
• Impt for transport across cell membrs
• 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

47
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

48
(No Transcript)
49
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

50

• May be due to
• Change in receptors
• Phosphorylation inhibits ability to interact w/
  G proteins
• Slow conforml change
• Exhaustion of mediators
• Ex amphetamines relase amines from nerve
  terminals when endogenous amines depleted, drug
  doesnt work

51

• 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

52
(No Transcript)
53

• Physiological adaptation
• Receptor number not static
• Hormones may incr, decr receptor number
• Altered rate receptor synth
• Slow
• Upregn ? supersensitivity
• W/ antagonist
• W/ inhibn transmitter synth/release
• Downregn ? loss sensitivity
• W/ prolonged exposure to agonists

54
(No Transcript)