drug receptor interactions Flashcards
Selectivity
Binding to one receptor over another
Specificity
Only binding to one receptor
Therapeutic index
Dose that is lethal to 50%/dose that gives therapeutic effect in 50% of people
Want high TI
Potency
Refers to conc causing particular response rather than max response i.e EC50
EC50
Concentration that causes 50% of the maximal response
Effect of competitive antagonist
Occupies receptors so more agonist needed for given response so shift logconc/response curve right
Examples: Atropine (for ACh) and Mepyramine (for His)
Stereoisomers
Same formula and structure but with different 3D orientation
May have different affinity/efficacy so can act as antagonist
Racemate
Mix of both stereoisomers
Curve is shifted to the right by the inactive isomer (if no affinity)
If isomer has some affinity se lower slope straight line
Sidenafil citrate
Viagra
PDE5 inhibitor to inhibit cGMP-PDE to prolong cGMP lifetime for vasodilation
Steroid receptor sequence
Steroid binds receptor in cytoplasm/nucleus (if oes)
This causes receptor to dissociate from chaperone and to bind to specific parts of DNA to increase or decrease transcription
Structure of nuclear receptors
AF1 = N terminal
Zinc fingers = DNA binding domain
AF2 = Ligand binding domain
Cys-loop type ionotropic receptors
Pentameric
nAChR
GABA
5-HT3
Ionotropic glutamate type
Tetrameric
NMDA
Calcium release ionotropic receptor
RyR
IP3R
Tetrametric
nAChR structure
2alpha subunits, beta, delta, gamma
Linking of pore made by M2 helical segment
When ACh binds, alpha helices straighten and swing out to open the pore
Hill slope of 2
Membrane bound guanylyl cyclase
Receptor is a dimer
Binds extracellular ligand ANP from atrial muscle
Activated receptor converts GTP to cGMP which activates protein kinase G
Causes vasodilation, salt excretion and phosphorylation to decrease Na+ reabsorption to lower BP
Receptor ser/thr kinases
For heterodimers
Ligand binds type 2 receptor; this phosphorylates type 1 receptor to activate it
Recruits smad2/3 and phosphorylates
Smad can then oligomerise with co-smad
-> Smad-co-smad complex moves to nucleus and associates with DNA binding protein to target gene response element
Receptor tyrosine kinases
Form homodimers
Have intracellular catalytic domain and extracellular regulatory domain
Type 1: EGFR; binding causes dimerisation of single chain
Type 2: insulin receptor; 2 chains already linked as dimer so binding just causes conformational change
Type 3: PDGFR (split kinase domain)
Defects in TGFbeta pathway
TGFbeta mutations; colon cancer
Smad4 mutations; pancreatic cancer
RTK signalling cascade
Binding causes dimerisation and mutual phosphorylation to activate intrinsic tyrosine kinase
Recruits adaptor proteins with SH2/3 domains
Signalling cascade to make Ras-GTP (activated by all RTKs); hydrolysis sped up by GAP
Can activate kinase cascade
Receptors linked to soluble kinases
TMD dimerises upon agonist binding
Kinase domain associates with it (but encoded on separate gene)
Kinases phosphorylate themselves and receptors
E.g EPO (erythropoietin receptor); activates Jak-STAT signalling to release new RBCs
GPCR structure
7 TMDs 3 intracellular (2 and 3 used for G protein binding) Associated with heterotrimeric G protein; activation causes conformation change that leads to GDP loss and GTP binding
ADP ribosylating enzymes that alter alpha subunit activity
Cholera toxin: blocks GTPase activity on Gs so it stays active and increases cAMP
Pertussus toxin: prevents activation of Gi so adenylyl stays active and increases cAMP
Protein kinase A structure
2 regulatory domains that each bind 2 cAMP
Binding causes pseudo substrate part of regulatory domains to dissociate from catalytic subunits
These can phosphorylate CREB to alter gene expression
Gq/11 coupled receptors
Stimulates PLCbeta to cleave PIP2 to DAG and IP3
DAG stays in membrane and activates PKC
IP3 binds receptors on ER to cause Ca2+ channel opening
Phosphatidylinositol cycle
IP3 can be phosphorylated to IP4 (causes Ca2+ entry via PM)
OR dephosphorylated to IP2 to IP to inositol
Lithium involvement in phosphatidylinositol cycle
Inhibits Inositol-1-phosphatase that converted IP to inositol
This blocks recycling of inositol to make new PIP2
used for bipolar disorder
Mechanisms of GPCR desensitisation
1) Uncoupling receptor from G protein
2) Sequestration of receptors
3) Down-regulation of receptors e.g decreases receptor synthesis via effect of PKA on mRNA stability
Homologous desensitisation to uncouple GPCRs
Only on agonist occupied GPCRs
G protein coupled receptor kinases GRKs phosphorylate Ser/Thr residues in cytoplasmic tails
-> Then GRKs (ARK1/GRK2 and B-ARK2/GRK3) dissociate and arrestins can bind C terminal
This blocks ability of G protein to interact with loop 3 binding point
Arrestin can then promote receptor internalisation (sequestration)
Homologous desensitisation of Beta2 adrenoceptor
ARKs are B-ARK1 and B-ARK2
Phosphorylation of the C terminus increases affinity for Beta-arrestins that uncouple the receptor
Heterologous sensitisation
Can happen on non-agonist occupied GPCRs
GPCR stimulation causes rise in 2nd messengers e.g cAMP for beta2-adrenoceptor
These activate kinases (e.g PKA) that phosphorylate C terminal tail and loop 3
This causes uncoupling of alpha subunit from GPCR to stop further stimulation
(can also cause switch of G protein e.g from s to i)
NB: PKA could phosphorylate other similar receptors doesn’t need agonist bound
Agonist bias
Where an agonist specifically recruits one pathway and blocks another
Arrestin3 involvement in morphine analgesia
In arrestin-3-KO mice, morphine causes increasing analgesia and less response side effects (response depression, constipation)
- Therefore we want to block this arrestin
- TRV130 receptor activation is a potent bias agonist that preferentially goes down G protein route to analgesia rather than arresting route to desensitisation and side effects
Spare receptors
Receptors that don’t need to be occupied to get max response
There to sensitise the tissue so shift logconc/response curve left
These get occupied before we see a response of irreversible antagonism (e.g by BCM on mAChR)
