How Drugs Act: Molecular and Cellular Aspects Flashcards
What is the job of the receptors?
▪ Protein molecule which function to recognize and respond to endogenous chemical signals
- recognize/bind specific endogenous ligands
- may also recognize/bind xenobiotics
What are receptors classified by?
ligand
What are the four superfamiles of receptors?
- Ligand-Gated Ion Channels
- G-Protein Coupled Receptors
- Kinase-Linked & Related Receptors
- Nuclear Receptors
What is the general name of the ligand-gated ion channels?
— “ionotropic” receptors
What is the general name of the G-protein coupled receptors?
— “metabotropic” receptors
— “7 trans-membrane spanning domain” receptors
— “heptahelical” receptors
— “serpentine” receptors
What is the general name of the kinase-linked and related receptors?
— large and heterogeneous group
— single trans-membrane spanning domain
What is the general name of the nuclear receptors?
“steroid superfamily”
What is the main example of a ligand-gated ion channel?
nicotinic acetylcholine receptor
What is the main example of a g-protein coupled receptor?
opioid receptors
What is the main example of a kinase-linked and related receptor?
insulin receptors
What is the main example of a nuclear receptor?
androgen receptors
Where are ligand-gated ion channels located?
plasma membrane
Ligand-Gated Ion Channels Composed of ____ of these subunits
4-5
Where are G-protein coupled receptors located?
plasma membrane
What is unique about kinase-linked and related receptors?
generally one transmembrane spanning domain
Why are there different receptor subtypes?
- different genes, different phenotypes
— different genes may encode for different subtypes - same gene, different phenotypes
— variation may arise from alternative mRNA splicing
— single nucleotide polymorphisms
What are the features of receptor subtypes?
- receptors in given family generally occur in several molecular varieties or “subtypes”
— similar architecture
— significant differences in amino acid sequence
— often different pharmacologic properties
— nicotinic acetylcholine receptor subtypes occur in different brain regions and these differ from subtype in muscle
What are examples of ligand-gated ion channels?
— nicotinic acetylcholine receptor (nAChR)
— gamma-aminobutyric acid type A receptor (GABAA)
— glutamate receptors
What are ligand-gate ion channels?
- share structural features with voltage-gated ion channels
- “ionotropic” receptors
What is important to know about nicotinic acetylcholine receptor (nAChR)?
— pentamer: four different polypeptide subunits
— transient opening of central aqueous channel
— Na+ flow from outside to inside cell
— cell depolarizes
— all occurs in milliseconds
What does the formation of an ion channel through the nicotinic acetylcholine receptor (nAChR) allow for?
movement of Na+ from outside cell to inside
What is the largest superfamily of receptors?
G-protein coupled receptors
What are examples of G-protein coupled receptors?
— muscarinic acetylcholine receptor (mAChR)
— opioid receptors (u, k, s)
— gamma-aminobutyric acid type B receptor (GABAB)
— serotonergic receptors
— adrenergic receptors ( and types)
— angiotensin II receptors (1, 2, 3, 4 types)
— endothelin receptors (A, B, C types)
— histamine receptors (1, 2, 3 types)
— photon receptors (retinal rod and cone)
How do G-protein coupled receptors work (specifically opioid receptors)?
- agonist binds to region inside receptor surrounded by
7 TM domains - conformational change in cytoplasmic side
- G-Protein affinity for nucleotide GDP is reduced
- GTP binds
- GTP-bound G-Protein dissociates from the receptor
- GTP-bound G-Protein engages downstream mediators (a.k.a. “effectors”)
What is the difference between an agonist versus a GTP-bound G-protein binding to a G-protein coupled receptor?
- agonist binds and dissociates rapidly
— a few milliseconds - activated GTP-bound G-proteins remain active much longer
— up to tens of seconds
— this produces significant signal amplification from one ligand-receptor interaction
How does amplification of G-protein coupled signaling occur?
- a single ligand (agonist) and interact with a single receptor outside but the G-protein once activated can be activated for 10s of seconds
- interacts with enzymes (effectors) downstream for a long time
How do you get heterogeneity in the response to a G-protein coupled receptor?
exact same receptor (same gene) in diferent tissues can get different responses because of…
- heterogeneity of G-proteins allow for substantial diversity in GPCR signaling in various tissues
Activated GTP-bound G-proteins
remain active much longer allowing for…
amplification of G-protein coupled receptors
What are the agonists of opioid receptors?
heroin
morphine
oxycodone
hydrocodone
What are the competitive antagonsists of opioid receptors?
naloxone
naltrexone
How does an opioid u type receptor work?
- morphine binds and causes a conformational change
*increased intracellular K+ hyperpolarizes cell making it refractory
*decreased intracellular Ca++ reduces neurotransmitter release and depolarization
Naloxone (an competitive antagonist of opioid) causes what kind of shift on a dose response curve for pain relief?
right shift
- you need twice the amount or morphine to get the same effect in the presence of naloxone
How does naloxone prevent death (what way does it shift the dose response curve for respiratory rate)?
- right shift
- increases the respiratory rate and keeps the patient breathing and alive
Why is fetanyl so dangerous?
Has a lower Kd than most opioids so it has a higher affinity for those receptors
- respiratory rate drops to zero
What is a protease activate receptor (PAR)?
- basically a protease cuts part of the receptor and that part activates the receptor
- activation of GPCR is normally a result of diffusible ligand in solution acting on a receptor
— but GPCR receptor activation can occur as a result of protease activation
What are examples of protease activated receptors (PAR)?
- thrombin: a protease involved in blood clotting activates PAR
- PAR-2 is activated by a protease released from mast cells following degranulation
Whta are the 2 main mechanisms that desenitization occurs?
- receptor phosphorylation
- receptor internalization
How does desenitization occur to B-adrenergic receptors (GPCR)?
- B-arrestin phosphorylates receptor reducing receptor affinity for G-proteins
- receptor can then be internalized
- all is rapidly reversible
one gene can give rise to multiple
subtypes of receptors through…
alternative mRNA splicing
one amino acid change can result in
different __________ of receptor
phenotypes
What is the function of kinase-linked and related receptors?
— involved mainly in events controlling cell growth and differentiation
— act indirectly by regulating gene
transcription
— signal transduction generally involves dimerization of two receptor molecules followed by autophosphorylation of tyrosine residues
All kinase-linked and related receptors have large extracellular ligand-binding domain connected via _________________ domain to an intracellular domain which has enzymatic activity
single membrane spanning
What are the three major families of kinase-linked and related receptors?
— Receptor Tyrosine Kinases (RTKs)
— Serine/Threonine Kinases
— Cytokine Receptors
What are examples of receptor tyrosine kinases (RTKs)?
- insulin receptors
- epidermal growth factor receptor
- nerve growth factor
- Toll-like receptors
How does the insulin receptor work?
- activates PI3 kinase pathway
» turns on or off gene expression
» activates glycogen synthesis - activates MitogenActivated Protein (MAP) Kinase pathway
» turns on or off gene expression
What are the features of serine/threonine kinase receptors?
— smaller group than RTKs
— structurally and functionally very similar to RTKs
— phosphorylate serine and threonine instead of tyrosine
What are examples of serine/threonin kinases?
transforming growth factor (TGF)
What are the features of cytokine receptors?
— interleukins, interferons, chemokines
— lack intrinsic enzymatic activity in intracellular domains
— associate and activate other kinases (JAK-STAT pathway)
What are ligand examples for nuclear receptors?
estrogens
progestins
androgens
glucocorticoids
mineralocorticoids
vitamin D
vitamin A (retinoid receptors)
fatty acids
What are the two main locations in the cell for nuclear receptors?
- cytoplasmic
- nuclear
What are the features of the “cytoplasmic” nuclear receptors?
- most are bound to Heat Shock Proteins when no ligand is present
- most form homodimers upon ligand binding
- some form heterodimers with Retinoid X Receptor (etc. thyroid hormone)
- translocate to nucleus to regulate gene expression
What are the features of the “nuclear” nuclear receptors?
- constitutively present in nucleus
- form heterodimers with Retinoid X Receptor (RXR)
How do androgen receptors (a type of nuclear receptor) work?
- androgen passively diffuses into the cell and interacts with the receptor in the cell
- displaces the heat shock protein
- transcription and translation
If an enzyme is a drug target what role does it usually perform?
— enzymes that are key rate-limiting steps in biochemical reactions are the best drug targets
— strategy is most often to reduce enzyme activity through drug inhibition
How can enzymes act as inhibitors?
non-competitive
— drug may covalently modify enzyme
competitive
— drug is often a structural analog of the naturally occurring substrate
* HMG-CoA Reductase Inhibitors
What is the main example of an enzyme acting as a drug target?
HMG-CoA Reductase Inhibitors
How does HMG-CoA Reductase Inhibitor (statins) work?
— competitively inhibit rate-limiting step in cholesterol biosynthesis in liver
* liver upregulates LDL receptors thereby reducing plasma LDL concentrations
* lovastatin is a good example
What is the circulating protein that uses monoclonal antibodies to recognize TNF-a, bind to it, and remove it?
Infliximab
Tumor Necrosis Factor-a (TNF-a) is elevated in what diseases?
— elevated in severe cases of aphthous ulcers
— elevated in rheumatoid arthritis, Crohn’s disease, psoriasis, ankylosing spondylitis
What type of carrier molecules (transporters) can be drug targets?
*Ion Transporters
— Na+/K+ ATPase
*Small Molecule Transporters such as…
— neurotransmitter uptake (norepinephrine, 5-HT, glutamate, etc.)
— organic ion transporters (organic acids and bases)
— p-glycoprotein (Multi-Drug Resistance)
What ion channels can be drug targets?
- voltage-gate ion channels
— Ca++ channels (L, T, N types)
— Na+ channels (fast and slow types)
— K+ channels (voltage- and ligand gated types)
What drug can inhibit Ca++ channels?
verapamil
