How drugs act? Flashcards
What are the 5 major protein targets for drug action?
- Receptors
- Enzymes
- Specific Circulating Plasma Proteins
- Carrier Molecules (Transporters)
- Ion Channels
Protein molecule which function to recognize and respond to endogenous chemical signals • recognize/bind specific endogenous ligands • may also recognize/bind xenobiotics Classified based on ligands Grouped into 4 major superfamilies
Receptors
What are the 4 superfamilies of receptors?
- Ligand-Gated Ion Channels
- G-Protein Coupled Receptors
- Kinase-Linked & Related Receptors
- Nuclear Receptors
How many subunits are found in ligand-gated ion channels?
4-5 subunits
Which subfamily of receptors form dimers?
Kinase-Linked
variation may arise from ______:
- single gene can give rise to more than one receptor isoform
- splicing can result in inclusion or deletion of one or more mRNA coding regions giving rise to short or long forms of protein
- big role in G-protein coupled receptors
alternative mRNA splicing
• share structural features with voltage-
gated ion channels
• “ionotropic” receptors
Ligand-Gated Ion Channels
— nicotinic acetylcholine receptor (nAChR)
— gamma-aminobutyric acid type A receptor
(GABAA)
* inhibitory neurotransmitter
— glutamate receptors [N-methyl-D-aspartate
(NMDA), a-amino-3-hydroxy-5-methylisoxazole-4-propionic
acid (AMPA), and kainate types]
* excitatory neurotransmitter
are examples of what subfamily of receptor?
Ligand-gated ion channels
— best characterized of all cell-surface receptors
— pentamer: four different polypeptide
subunits
* 2 a, 1 b, 1 g, and 1 d, MW from 43K-50K each
* each subunit crosses plasma membrane 4 times
— acetylcholine binds sites on a subunits
— conformational change occurs
— transient opening of central aqueous channel
— Na+ flow from outside to inside cell
* down electro-chemical gradient
— cell depolarizes
— all occurs in milliseconds
• nAChR
- largest superfamily of receptors
- “metabotropic” receptors
- 7 transmembrane spanning domains
G-Protein Coupled Receptors
• The following are examples of _____ receptors:
— muscarinic acetylcholine receptor (mAChR)
— opioid receptors (m, k, d)
— gamma-aminobutyric acid type B receptor (GABAB)
— serotonergic receptors (5-hydroxytryptamine or 5-HT, 1-7 types)
— adrenergic receptors (a and b 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)
G-Protein Coupled Receptors
______ of Opioid Receptors
— heroin, morphine, oxycodone,
hydrocodone
Agonists
______ of Opioid
Receptors
— naloxone, naltrexone
• Competitive Antagonists
— 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
* protease cleaves off part of N-terminal domain of receptor
* “tethered agonist”: remaining attached domain is free to interact with ligand-binding domain
— examples:
* thrombin: a protease involved in blood clotting activates PAR
* PAR-2 is activated by a protease released from mast cells following degranulation
• Protease Activated Receptors (PAR)
— applicable to all GPCRs — occurs via 2 main mechanisms * receptor phosphorylation * receptor internalization — example: * b-adrenergic receptors * b-arrestin phosphorylates receptor reducing receptor affinity for G-proteins * receptor can then be internalized * all is rapidly reversible
• Desensitization
What are the 3 ways a GPCR can be desensitized?
Phosphorylation
Internalization in lysosome
Endocytosis
— 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 have large extracellular ligand-binding domain connected via single membrane spanning domain to an intracellular domain which has enzymatic activity
Kinase-Linked & Related Receptors
— all have large extracellular ligand-binding domain connected via single membrane spanning domain to an intracellular domain which has enzymatic activity
Kinase-Linked & Related Receptors
What are the three major families of Kinase-Linked & Related Receptors?
— Receptor Tyrosine Kinases (RTKs)
— Serine/Threonine Kinases
— Cytokine Receptors
— insulin receptor * activates PI3 kinase pathway » turns on or off gene expression » activates glycogen synthesis * activates Mitogen- Activated Protein (MAP) Kinase pathway » turns on or off gene expression
• Receptor Tyrosine
Kinases (RTKs)
— smaller group than RTKs — structurally and functionally very similar to RTKs — phosphorylate serine and threonine instead of tyrosine — e.g. transforming growth factor (TGF)
• Serine/Threonine Kinases
— interleukins, interferons, chemokines,
etc
— lack intrinsic enzymatic activity in
intracellular domains!!
— associate and activate other kinases
* binds and activates Janus Kinase (Jak)
* Jak binds and activates Signal Transducers
and Activators of Transcription
» a.k.a. the “Jak-STAT” pathway
* downstream turns on or off gene expression
• Cytokine Receptors
— ligand-activated transcription factors — ligand examples: * estrogens, progestins, androgens, glucocorticoids, mineralocorticoids, vitamin D, vitamin A (retinoid receptors), fatty acids, etc. — two main locations in the cell * cytoplasmic * nuclear — ligand-binding and DNA-binding domains
Nuclear receptor
Of all receptor superfamilies, which one must have ligand get into cell to activate receptor?
Nuclear receptor
— cytoplasmic
* most are bound to Heat Shock Proteins when no ligand is present
* most form homodimers upon ligand binding (e.g. steroid receptors)
* some form heterodimers with Retinoid X Receptor (e.g. thyroid hormone)
* translocate to nucleus to regulate gene expression
— nuclear (e.g. fatty acid receptors)
* constitutively present in nucleus
* form heterodimers with Retinoid X Receptor (RXR)
— interact with hormone response elements on genes to regulate gene expression
• Example: Androgen Receptors
• Nuclear Receptors (“Steroid Superfamily”)
— ______ that are key rate-limiting steps in biochemical reactions are the best drug targets
— strategy is most often to reduce this activity through drug inhibition
enzymes
— competitively inhibit rate-limiting step
in cholesterol biosynthesis in liver
* liver upregulates LDL receptors thereby
reducing plasma LDL concentrations
• HMG-CoA Reductase Inhibitors
“Statins”
— monoclonal antibodies that recognizes TNF-a
— bind TNF-a removing it from circulation
• Infliximab and adalimumab
- important in moving substances across lipid bilayer membranes
- often good drug targets as they regulate key cellular events
Ion and Small Molecule Transporters
- neurotransmitter uptake (norepinephrine, 5-HT, glutamate, etc.)
- organic ion transporters (organic acids and bases)
- p-glycoprotein (Multi-Drug Resistance)
Small molecule transporters
— protective role in moving potential toxicants out of gastrointestinal epithelial cells back into lumen to prevent absorption
— overexpressed in certain tumor cells leading to drug resistance
— can be blocked by drugs
* could increase absorption of some drugs
* could potentially increase activity of anti-cancer drugs
• p-glycoprotein (Multi-Drug Resistance)
________ important everywhere
— establishes electrochemical gradient by moving Na+ out and K+ in against concentration gradient
— requires energy (ATP) to function
— key in all muscle contraction, nervous conduction, ion gradient establishment, etc
— often provides the driving force for other ion transporters
— can be inhibited by drugs (e.g. digoxin, a cardiac glycoside used for heart failure)
• Na+/K+ ATPase
• structure — very similar in structure and function to ligand-gated ion channel receptors • Ca++ channels (L, T, N types) • Na+ channels (fast and slow types) • K+ channels (voltage- and ligand- gated types) — produce at least 9 different K+ currents in heart, vascular smooth muscle, and other tissues such as pancreas
Voltage-Gated Ion Channels
— channels open or close depending upon the electrical gradient (voltage) across the plasma membrane * resting membrane potential ~ -90 mV * depolarized membrane potential ~ 0 mV — channels change opened/closed or activated/resting states as electrical potential changes from -90 mV to +10 mV (inside relative to outside) — channels often susceptible to binding by various compounds, including xenobiotics
• voltage-dependent gated ion channels
— Ca++ Channel Blocker
* binds to L-type Ca++ channels in heart and vascular smooth
muscle
* blocks movement of Ca++ from outside to inside
* reduces cardiac contraction (negative inotropic effect)
* slows cardiac conduction (negative chronotropic effect)
* reduces vascular smooth muscle contraction
* reduces blood pressure
Verapamil
______ inhibits
L type Ca channel
reduces heart rate and contractility
Verapamil
