Lecture 2: How Drugs Bind to Their Targets Flashcards
What are intracellular receptors?
receptors are inside the cell - this means the drugs need to be able to cross the plasma membrane
drugs that target these receptors must be lipid soluble (or have some other transport mechanism in order to cross the cell membrane)
classic example is the steroid hormones and their analogs (e.g. glucocorticoids, anabolic steroids, etc.)
mode of action is to bind to the LBD of a steroid hormone receptor, leading to displacement of HSP (heat shock protein) or other chaperone; this exposes a DNA recognition domain and leads to activation of transcription of target genes (receptor enters nucleus and binds DNA)
effects typically have slow onset and are long-lasting (not rapidly reversible)
What are G-protein coupled receptors?
a large fraction of therapeutic drugs target a family of transmembrane receptors called G-protein coupled receptors (GPCRs)
ligands bind to the extracellular side, and trigger a conformational change that activates a signaling cascade mediated by intracellular G-proteins
What does the activation of G-protein coupled receptors activate?
activation of the receptor promotes GDP-GTP exchange of the heterotrimeric G-protein
the G-alpha subunit has GTPase activity that acts like a molecular “timer” or “countdown” to terminate the signal; after GTP is hydrolyzed, the inactive G-protein complex reassembles and the system can reset
while active, the G-proteins can influence “effector” proteins that alter cellular activity
What are G-beta-gamma subunits?
can influence the activity of a variety of protein types including ion channels
How are GPCRs characterized?
are usually categorized based on the subtype of G-alpha that is associated
different G-alpha subtypes can influence different signaling cascades via effects on adenylate cyclase (AC)(Gs,Gi) or phospholipase C (PLC)(Gq)
What is the G-protein mediated signaling cascade via adenylate cyclase (AC)?
receptors coupled to Gs trigger increased activity of AC, leading to production of cyclic AMP (cAMP), which activates cAMP dependent protein kinases
some receptors are coupled to Gi, which suppress activity of AC
these responses generally happen very quickly and mediate moment to moment control of many physiological functions
What is the G-protein mediated signaling cascade via activation of phospholipase C (PLC)?
receptors coupled to Gq trigger increased activity of PLC, leading to production of inositol triphosphate (IP3) from breakdown of PIP2
IP3 triggers release of intracellular Ca2+ stores (ER), which can influence a variety of signaling pathways
other byproducts of PIP2 breakdown (DAG), lead to activation of protein kinase C and target substrates
these responses generally happen very quickly and mediate moment to moment control of many physiological functions
What are transmembrane receptors (TKRs)?
tyrosine kinase receptors are another example of how extracellular ligands can be translated into changes in cellular signaling
activation of TKRs is driven by dimerization of receptors in the presence of a ligand, causing the receptors to auto-phosphorylate and become activated
drugs that inhibit or stimulate the activation of these receptors will influence downstream signaling mechanisms
What are the characteristics of intracellular receptors (e.g., steroid hormone receptor)?
membrane permeable drugs? - YES, often steroid drugs (estrogens, glucocorticoids)
circulating form of drugs - bound to carrier globulin because of low solubility in plasma
speed of response - SLOW, requires DNA binding and activation of target genes
breakdown or termination of signal - SLOW, large fraction of hormone is bound, there is usually a large reserve; effects (gene expression) are also very slow to reverse (hours-days-weeks)
What are the characteristics of cell surface receptors (e.g., GPCR or TKR)?
membrane permeable drugs? - not necessary
circulating form of drugs - variable
speed of response - FAST, rapid transduction via protein conformational change and intracellular signaling cascades
breakdown or termination of signal - FAST, signal can rapidly terminate due to rapid GTPase cycle
What are ion channels?
the fastest mechanism of signaling in the body depends on electrical signals generated by ion channel proteins
ion channels allow ions to cross the plasma membrane very rapidly, leading to changes in membrane voltage (thus underlies transmission of signals in the nervous system, beating of the heart, etc.)
different ion channel types are controlled by distinct stimuli, such as ligand binding, or changes in membrane voltage
drugs that target ion channels after their response to these stimuli, or block the flow of ions
What are voltage-gated ion channels?
voltage-gated ion channels respond very rapidly to changes in membrane voltage
movements of charged amino acids in the transmembrane electric field change position in response to changes in voltage
this allows the protein to change its activity (open vs closed) in ~ milliseconds
What are ligand-gated ion channels?
ligand-gated channels open or close in response to binding of a small signaling molecule (various amino acids, neurotransmitters)
they play an enormously important role in synapses in the CNS
common drugs for psychiatric conditions target ligand gated channels (we will come back to this with an example)
What are the modes of signal transmission?
most of the drugs we will discuss in this course influence a cellular process, either by interacting with a transmembrane receptor (which translates an external signal into a cellular process), or by entering into a cell and interacting with an intracellular receptor
What are other broad classes of drug targets?
structural proteins (e.g. microtubules)
DNA replication machinery (foreign or native)
membrane transport proteins
enzymes
foreign proteins (e.g., bacterial cell walls, bacterial DNA/RNA machinery)
What is EC50?
effective concentration 50
refers to the concentration of drug that yields a 50% maximal effect
50% of your maximum response
potency
What is Emax?
this is the maximal biological effect observed with the drug
maximum biological response
efficacy
How do we describe the effects of a drug?
substances can influence receptor activity in a variety of different ways
agonism means that a substance/drug binds to a receptor and influences its activity; this is usually depicted as a “concentration-response” curve
you will often see drug concentrations plotted on a log scale
larger concentrations elicit a larger response
What are the two equations with Emax and EC50?
E = (Emax x [drug])/([drug]+EC50)
E = (Emax)/(1 + (EC50/[drug]))
these equations to describe concentration responses are equivalent, but you will often see the left equation written in textbooks because it avoids the problem of dividing by 0
the rearrangement on the right might help to illustrate that when [drug] = EC50, a half-maximal effect of the drug observed (Emax/2)
What is efficacy?
is a term that refers to the maximal drug effect
Emax, units relate to “effect”
What is potency?
is a term that refers to the concentration dependence (EC50)
a drug with strong potency has a low EC50 (only a small concentration is required to generate a large effect)
a drug with a weak potency has a high EC50 (large concentration is required to generate a large effect)
units of potency are a concentration or dose
What characterizes the differences between drugs potency and efficacy?
agonists can vary in efficacy and potency
receptors can have multiple agonists with distinct actions on the receptor
drugs with different potency exhibit different concentrations required for a particular effect
drugs with different efficacy exhibit a difference in the maximal effect that can be achieved
How are agonists characterized?
agonists are usually categorized based on their efficacy
What is a full agonist?
can generate the maximal observed effect
What is a parietal agonist?
can generate a fractional effect
What is an antagonist?
cannot generate a biological effect on their own
What is a inverse agonist?
cause suppression of basal activity
if a receptor does not have basal activity, then it would not have an inverse agonist
