Receptors Flashcards
(40 cards)
What are the four different cellular responses to extracellular signals?
- survive
- divide
- differentiate
- die
Define signal transduction.
Intracellular mechanism by which cells respond to changes in the extracellular environment.
Types of signal transduction.
- contact-dependent (ligand on signaling cell surface)
- paracrine signaling (most common)
- synaptic (type of paracrine. Cells not actually touching)
- endocrine (long distance, long term)
List the types of first messengers.
- amino acid derived
- peptide hormones
- cytokines
- growth factors
- fatty acid derived
- steroid hormones
- neurotransmitters
- amino acids
Describe how a neurotransmitter does not act directly on a muscle cell but still transduces its signal.
- acetylcholine activates nitric oxide receptor on endothelial cell, producing nitric oxide
- NO diffuses to muscle cell where it binds guanylyl cyclase to make cGMP, causing rapid relaxation of smooth muscle cell
Describe divergent signaling pathways.
binding of the same first messenger to same or different receptor types on different cells, causing different cellular responses.
ex: acetylcholine causes decreased contraction in heart muscle cell, contraction in skeletal muscle cell, and secretion in salivary gland cell
What does divergent signaling depend on?
the constituent subunits making up a receptor on the surface of a certain cell type.
ex: metabotropic glutamate receptor (mGluR) has many different subunits and different ones are presented on different cells.
Describe converging signaling pathways.
multiple ligand types, same receptor
ex: glucagon, LSH, ACTH, and FSH all stimulate adenylyl cyclase, causing generation of cAMp
If AC is maximally stimulated by one ligand-receptor class, what is the effect on other convergent ligands?
The other ligands will not be effective.
Describe the experimental evidence for convergent signaling.
- adrenaline receptors on RBCs. Cells treated with NEM to turn of AC inside cells, so the cells are no longer responsive to adrenaline
- these cells are fused with adrenal cortical cells with ACTH receptors with functional AC. Once fused, the cells are only responsive to ACTH, though adrenaline receptors are intact still.
- several hours after fusion, the cells are responsive to both ACTH and adrenaline, meaning that the AC’s inside the cell have been redistributed and are activated by more than one first messenger.
Why are cell surface receptors necessary for signal transfuction?
- cell membranes are impermeable to ions and polar molecules
- polar molecules, small or large, are 10,000x slower at diffusing through the membrane compared to water
- K+ takes 280 hours to diffuse through the bilayer, but much faster in the cytoplasm
How can you tell if a protein is associated with the lipid bilayer?
- primary structure - run of hydrophobic aa
- secondary structure - alpha helices
How can you tell which parts of a protein are associated with the lipid bilayer?
hydropathy plots - looks at aa sequence
-hydrophobicity versus aa residue number
(+) values that get higher as residue is more hydrophobic
(-) values that get more negative as the residue is more polar
In a hydropathy plot, where are the polar residues located with respect to the membrane?
You cannot tell which side of the membrane these residues reside on.
How can cell receptors be isolated/purified? Why do we want to isolate them?
We want to isolate novel cell receptors so we can determine their aa sequence and to know how they function.
- detergent solubilization (polar region stabilizes polar residue of the protein, hydrophobic region stabilizes hydrophobic residues)
- high salt (chaotropic solutions - disruption of hydrogen binding)
Which assay is used to determine the kinetics of ligand-receptor binding?
scatchard assay.. cells or membranes exposed to radioactive ligand. bound and free ligand are separated into fractions. unbound ligand is washed off, and amount bound to receptor is plotted as a function of the concentration of ligand added to the cell. The max amount of receptors bound is determined by the plateau of fluorescence.
What does the plot of a scatchard plot tell us about ligand-receptor binding kinetics?
the slope tells us -1/Kd, The larger this value, the tighter the association of the ligand to the receptor.
What receptor assay would be done to determine the second messenger responsible for transducing a signal?
biological response assay (uses isolated but intact organs)
Describe receptor state terminology.
channels: open vs closed
non-channel receptors: “r” form is inactive, “R” form is active
which forms of a receptor do agonist, antagonists, partial agonists, and inverse agonists bind to?
agonist: R (transduces a signal)
antagonist: both (does not transduce a signal)
partial agonist: both (can transduce a signal if it binds R. Cannot transduce if it binds r)
inverse agonist: r (cannot transduce)
How do agonists and antagonists affect the entropy of a receptor? Conformational changes?
agonists decrease entropy, antagonists increase entropy.
agonists cause conformational change in the receptor to transduce a signal. Antagonists and inverse agonists cannot induce a conformational change, therefore cannot transduce a signal.
Experiment to determine which side of the receptor a ligand binds to (extracellular vs intracellular)?
Cells are treated with extracellular protease trypsin to degrade any proteins on the cell surface. Then do mRNA assay of the downstream gene affected to measure ligand-receptor binding and compare to cells without trypsin treatment. If there is no binding, then can conclude that the receptor was on the cell surface.
Compare ligand concentrations needed to bind to the receptor vs to elicit a cellular response.
less ligand is needed to cause biological activity compared to that needed for measuring binding kinetics
How is a receptor defined?
- saturability (binding must reach a plateau)
- specificity
- reversibility (must be able to remove ligand)
- functional reconstitution (cell must be responsive to ligand)
