Receptors: Binding to Response Flashcards
Two properties of a receptor
Recognition and Transduction
Recognition
binds ligands reversibly with high affinity and specificity
Transduction
structural-dependent conversion of this binding into a cellular response, reflected in “activity” or “efficacy”
with in cell mechanism to propagate signal -> response
Receptor location
usually located in membrane communicates between outside and inside of cell; allows specific binding that turns into specific response
receptor binds
and responds to that binding -> cell response -> organ response -> binding response
Receptor vs enzyme
both bind with specificity but enzyme only convert substrate to product, this is not reversible except at binding site; receptors bind reversibility but aren’t converted, receptors leads to response
ligand
the thing binding to the receptor
3 criteria for receptor binding interaction
- Specificity
- Saturability
- Reversibility
Specificity
ligand should be structurally complimentary to receptor, producing a structurally specific and high affinity interaction
Saturabilty
finite number of receptors per cell should be present as revealed by saturable binding curve (only certain number of sites for ligand)
reversibility
after binding to receptor the ligand must dissociate in an unchanged form; concentration of drug decreases drug can dissociate and receptor can relax until next round
slower the rate the ___ the Kd
slower the rate the smaller the Kd and larger the on rate
If [L]= 0
not bound increase this to infinity; [RL] = tot # of receptors
Kd is when
1/2 of receptors are occupied
low Kd
high affinity
high Kd
low affinity
Saturation Isotherm Direct Plot
Concentration of bound ligand plotted as a function of concentration of free ligand producing rectangular hyperbola; Maximum binding approached asymptomatically as ligand concentration increases and is equal to concentration of receptors [R1]; concentration free ligand at which 50% binding sites occupied equal to Kd
Saturation Isotherm: Log Plot
concentration of bound ligand plotted as function of logarithm of free ligand concentration; this gives you a nice curve
Inhibition of Binding
competitive inhibition; ligands that bind to receptor at same site will inhibit each other’s binding competitively; if 1 is binding other can’t but this is reversible just increase concentration of one in the presence of other and get to saturation and compete out the other one
effect of competitive inhibition
decrease apparent affinity of measured ligand (L1) but have no effect on maximum binding of saturation ([Rt]); in presence of competitive inhibitor (L2) the apparent dissociation constant of the measured ligand (L1) increases (bc high Kd low affinity); magnitude or response directly proportional to amount of ligand bound; assumes Ed50= Kd
Kd’
apparent Kd this is what changes not actual Kd when there is presence of competitive inhibitor, this leads to a shift R; the Kd as in the property of the thing itself does not change
Binding to Response: Classical Theory
It was thought that response was directly related to agonist occupation of receptor but this is not the case for a lot of systems where things similar in structure to RA not enough to make response bc structural dependence to binding that can lead to conformational change in receptor that leads to response
Competitive inhibitor binding
can have competitive inhibitor that binds and doesn’t lead to conformational change but blocks to things that would
Classical Theory EC50
EC50 (concentration of agonist that produces 50% of maximum response) is = to KDapp (concentration of agonist at which 50% of receptors are occupied)
Classical theory true for
activated receptor (response and binding follow) but if you go downstream of this will cause depolarization and will curve at lower concentration
Binding to Response: Modified classical theory
Response can occur at much lower concentrations than binding and apparent affinity for response can be much higher than for binding b/c of amplification fo signal that produces “spare receptors” (ie there are more receptors than we need to produce max response bc if you knock out receptors you still get max response); cell amplifies stimulus so get response in cell that is shifted L so can get max response with less than 100% of receptors occupied
EC50 modified classical theory
EC50 < Kd app for binding
Partial agonists
an agonist that produces less than a maximal response when occupying all of the receptors
agonist vs antagonist
agonist activates receptors antagonists don’t activate receptors
full agonist
An agonist that produces a maximal response by occupying all or a fraction of the receptors
lower EC50 =
greater potency (lower [ ] where it has its effect = higher potency)
higher maximum response
higher efficacy
potency
binding interacting > binding affinity = more potent
efficacy
reflects conversion to activate state
efficacy of antagonist
is 0 because no conversion to active state, BUT this does have potency
Competitive antagonists
Ligands that bind to the receptor at same site as agonists but do not cause activation of receptor; effect of competitive antagonist is analogous to that of competitive inhibitor of binding; apparent affinity is decrease (EC50 increases; potency decrease), but no effect on maximum response (efficacy unchanged)
EC50
concentration of agonist that produces 50% of maximum response
antagonist in modified classical receptor theory
antagonist has efficacy of zero and thus produces no stimulus
to compete out antagonists
increase agonist concentration
competitive inhibitors are about what happens at
binding site so agonists and competitive antagonists are competitive inhibitors of each other
all competitive antagonists =
competitive inhibitors
not all competitive inhibitors are
competitive antagonists because agonists are competitive inhibitors of competitive antagonists but agonists lead to a response
inhibition at level of
binding
antagonist at level of
response
Noncompetitive antagonists
ligands that bind to receptor at a different site from agonists and prevent activation are noncompetitive antagonists (this = allosteric antagonist); this doesn’t keep agonist from binding but it does keep responses from happening; this doesn’t affect EC50 but does affect max response
Competitive antagonist vs noncompetitive antagonist EC50
competitive antagonist: EC50 increases, potency decreases, no response on maximum response (efficacy unchanged) noncompetitive antagonist (EC50 and potency not affected, but does affect max response)
Response to response
desensitization, down regulation, upregulation
desensitization
essentially every receptor system response to prolonged activation by “slowly” desensitizing or inactivating; receptor systems the cell will want to protect itself from getting worn out so as ligand binds it excites receptor system and system form cell turns itself off so even though ligand binding to neurotransmitter cell not responding
5 mechanisms by which cells “turn-off” their response to signals
Receptor sequestration, Receptor down-regulation, receptor inactivation, inactivation of signaling protein, production of inhibitory protein
Receptor sequesteration
remove receptor from cell membrane store it and put it in after drug gone for a while; prolonged response (these are what leads to drug tolerance)
receptor down regulation
remove receptor from cell and break it down; prolonged response (these are what leads to drug tolerance)
receptor inactivation
desensitization; receptor changes conformation to inactive state
production of inhibitory protein
in signal transduction system 2nd molecule made to feed back to receptor and turn it off; can turn receptor back on after ligand removed and can reset it
desensitization/ inactivation examples
- most ligand-gated ion channels close in prolonged presence of an agonist
- Voltage-gated ion channels close w prolonged depolarizatoin
- Many G- Protein coupled receptors are phosphorylated to inactive form by protein kinase A and bind arresting after phosphorylation by specific protein kinase
Down-regulation
G protein coupled beta-adrenergic receptors are endocytose after phosphorylation by specific protein kinase and binding of protein arrestin
B receptors over exposed to agonists
down regulate, if you take agonist away get response back; if keep stimulating will loose b receptors on cell membrane and it will take days to resynth b receptors and get back full response
chronic use of antagonists
if cell is under stimulated will upregulate its receptors (increase number of receptors expressed) so if you remove the antagonist expect a rebound effect b/c hypersensitive to increased number of receptors
partial agonists cardiac conditions
use instead of antagonists in treating cardiac conditions because avoids up regulation of receptors and hypersensitivity if remove antagonist
agonist
ligand that activates the receptor to cause a physiological response
spare receptors
refers to an observation of maximal response when only a fraction of total number of receptors are occupied
antagonist
ligands that does not activate the receptor and interferes with the response produced by an agonist (can be competitive or non competitive antagonist)
inhibitor
A ligand that prevents the binding of another ligand to the receptor
ED50 or EC50
Dose or Concentration of an agonist that produces half-maximal Effect or response
ID50
Dose of an antagonist that produces a half-maximal Inhibition
IC50
The Concentration of antagonist or inhibitor that produces half maximal Inhibition
