Lecture 6 - Drug Targets Flashcards
What are receptors responsible for?
Transmitting a signal to a cell
What occurs in voltage-gated ion channels?
- Depolarization causes opening of ion channel
- Ions pass through
- Hyperpolarization causes closure of ion channel
What occurs when a drug binds to an ion channel?
Typically inhibits its function
Where can drug binding occur on an ion channel?
- Intracellular or extracellular part
- Can be w/in the pore of the channel itself or on the outside
Where and when drug binds to an ion channel depends on _____
The ion channel and the drug
What are some examples of drugs that bind to ion channels?
- Local anesthetics (sodium ion channel blockers)
- Calcium channel blockers
- Class 1 and 2 antiarrhythmics
How are ligand gated ion channels opened?
Binding of agonist to ion channel
Where does receptor binding normally occur in ligand gated ion channels?
Outside of membrane
What are some examples of ligand gated ion channels?
- Nicotine
- Acetylcholine
- GABAa
- AMPA receptors
Nicotinic acetylcholine receptor requires ____ ligands
2
What occurs in nuclear hormone receptors?
- Hormone enters nucleus and binds to nuclear hormone receptor
- Complex binds to response element of DNA
- Causes a change in mRNA expression => change in protein production and tissue response
What does a ligand bind on a nuclear hormone receptor and what does this cause?
- Ligand binds to ligand binding domain (AF-2)
- This causes a conformational change in receptor, which then causes a conformational change in the DNA binding domain (AF-1), which allows it to bind to DNA
What are some examples of ligands for nuclear hormone receptors?
Nuclear hormone like sex steroids or glucocorticoids
What is the receptor for estradiol?
Estrogen receptor
What is the receptor for testosterone?
Androgen receptor
What is the receptor for progesterone?
Progesterone receptor
What is the receptor for cortisol?
- Main receptor - glucocorticoid receptor
- Other receptor - mineralocorticoid receptor
What is the receptor for aldosterone?
- Main receptor - mineralocorticoid receptor
- Other receptor - glucocorticoid receptor (weak)
What is the binding of cortisol to mineralocorticoid receptor considered?
Off-receptor effect
What occurs in the estrogen receptor?
- Estradiol passively diffuses across cellular membrane and nuclear envelope
- Receptor dimerizes and binds to estrogen receptor alpha, which then binds to estrogen response element of DNA
- Causes a change in mRNA expression => protein and tissue response changes
What are transmembrane enzymes?
A group of receptors that have an extracellular ligand binding domain and an intracellular enzymatic domain w/in the same protein
What are 2 examples of transmembrane enzymes?
1) Epidermal growth factor
2) Insulin receptors
- Contain intrinsic tyrosine kinases in cytoplasmic domain
What are the ligands for the EGF receptor?
EGF or transforming growth factor alpha
Overgrowth of EGF receptors is found in ____
Epithelial cancers
What is the function of the EGF receptor?
Several signal transduction cascades leading to DNA synthesis and cell proliferation of epithelial cells
What occurs in the EGF receptor?
- Ligand binds causing EGFR dimerization and tyrosine kinase activity
- This leads to autophosphorylation of several EGFR tyrosine residues, causing activation of signal transduction cascades => DNA synthesis and cell proliferation
What does phosphorylated tyrosine act as in the EGFR?
A signal
What is an antagonist for an enzyme?
Inhibitor
What some important endogenous ligands for GPCR’s?
- Adrenaline / noradrenaline
- Acetylcholine
- Serotonin
- ADP
- Dopamine
- Histamine
- Glutamate
- Peptide hormones
What are the 3 components of GPCRs?
1) Receptor on outside of membrane binds ligand
2) G-protein on inside of membrane senses signal from ligand receptor interaction
3) 2nd messenger propagates signal throughout cell
What does an effector protein do?
Binds G-protein and produces or inhibits production of 2nd messenger
What is a GPCR composed of?
7 alpha helices that cross the membrane (also called transmembrane helix receptors)
Where is the ligand receptor of a GPCR located?
Extracellular side
Why is it called a G-protein?
Binds to GTP
Where is the signalling protein of a GPCR located?
Intracellular side
What is the function of a G-protein?
Transmit signal from receptor-ligand complex into the cell through biochemical cascade
Can one receptor activate only one G protein?
No, can activate multiple G-proteins
What happens to the GPCR after a ligand binds?
Affinity for GDP decreases and affinity for GTP increases
What happens after GTP binds to a GPCR?
- Dissociation of G alpha and G beta-gamma, which then interact w/ respective effector proteins
- G alpha hydrolyzes GTP to GDP and hydrolysis results in re-association of G alpha and G beta-gamma, eliminating signal
What does G alpha s do?
Activates adenylate cyclase => opens Ca2+ channels leading out of the cell
What is the 2nd messenger of G alpha s?
cAMP (produced)
What does G alpha i do?
Inhibits adenylate cyclase and opens K+ channels
What is the 2nd messenger of G alpha i?
cAMP (inhibited)
What does G alpha q do?
Activates phospholipase C beta
What is the 2nd messenger of G alpha q?
DAG and IP3 (produced)
What does Go do?
Activates receptors that inhibit Ca2+ channels leading out of cell
What does cAMP do?
Binds to and activates protein kinase A
What does protein kinase A do?
Phosphorylates many target proteins that amplify the signal again and produce many coordinated responses
What extinguishes the signal from cAMP and how?
- Phosphodiesterase
- By hydrolosing it into 5’AMP which does not bind to PKA
The active site of adenylate cyclase is blocked until ____
G alpha s binds
What needs to be bound to adenylate cyclase for it to produce cAMP?
G alpha s
What does G alpha s have intrinsic activity of and what does this mean?
- Has GTPase intrinsic activity
- Deactivates itself
What occurs when protein kinase A is stimulated through G alpha S?
A cascade of effects are initiated that liberate energy sources from storage and prevent storage of energy sources
What happens when membrane bound adenylate cyclase binds G alpha i?
Adenylate cyclase is inhibited and no cAMP is produced
Does G alpha i produce an inhibitory effect by itself?
No, with G beta-gamma subunits which open K+ channels, causing hyperpolarization of membrane
What does phospholipase C beta do once activated?
Hydrolyzes phosphatidylinositol diphosphate (PIP2) into the 2nd messengers DAG and IP3
Where is PIP2 found?
In membrane
Do DAG and IP3 leave the membrane?
- DAG is hydrophobic so stays in the membrane
- IP3 is hydrophilic, so it diffuses into cytosol
Do G alpha s and G alpha q have the same effect?
No, G alpha q counteracts the signal from G alpha s
What also happens after G alpha q binds to phospholipase C beta (besides PLC beta being activated)?
- Intracellular Ca2+ increases in smooth muscle causing contraction
- Ca2+ binds to calmudolin and stimulates PDE, which hydrolyzes cAMP to 5’AMP
What is the net effect of G alpha s?
To decrease intracellular Ca2+, which counteracts the effects of G alpha q
Can one GPCR activate only one G-protein?
No, can activate multiple
Does one G-protein have only one effector protein?
No, can have multiple
