Furci - Membrane Receptors and Endocytosis Flashcards
What is the precursor of steroid hormones?
cholesterol
What are the three groups of steroid hormones?
- corticosteroids
- androgens
- estrogens
After steroid hormones are produced by gonads and adrenal cortex, they immediately diffuse out of _______ into _______.
- endocrine cells
- bloodstream
Are steroid hormones lipid-soluble and why is this important?
- lipid-soluble
- allows them to cross membranes
Where are steroid hormone receptors located?
inside the cell (cytosolic receptors)
Which hormones (steroid or peptide) are faster acting?
peptide
Which hormones (steroid or peptide) have a longer half-life?
steroid
Which hormones (steroid or peptide) are larger?
steroid
Do polypeptide hormones immediately enter the blood stream?
no
Where can polypeptide hormones be stored?
in endocrine cell vesicles
Are polypeptide hormones lipid-soluble or water-soluble?
water-soluble
Can polypeptide hormones pass readily through cell membranes?
no
What are polypeptide hormones sometimes called? Why did they get this nickname?
- “first messengers”
- their receptors are located externally on the cell so they are usually the first to send the message and then a “second messenger” carries this message within the cell
What type of molecules are usually “second messengers”? Give examples.
- low molecular weight signaling molecules
- cAMP or calcium
Describe the location and structure of a G-protein coupled receptor.
- integral membrane protein with an extracellular N-terminus
- 7 transmembrane alpha helices
- 3 extracellular and intracellular loops
- intracellular C-terminal tail membrane
- no intrinsic catalytic domain
Which part of the G-protein coupled receptor recruits the G-protein?
third intracellular loop
Where does the ligand bind to a G-protein coupled receptor?
in the pocket formed by the 7 alpha helices
What does the G-protein activate? What does that lead to?
- G-protein activates adenylyl cyclase which converts ATP to cAMP
- activates protein kinases in the cytosol (signal cascade)
What inactivates cAMP, thus turning off the cell response of a signal cascade?
phophodiesterase
Which subunit of the G-protein contains the GTP-binding site and intrinsic GTPase activity?
alpha subunit
How is the signal amplified in a G-protein coupled receptor?
each bound hormone can stimulate many alpha subunits of G-protein
What toxins target G-proteins?
- cholera toxin
- pertussis toxin
What does the cholera toxin do?
- ADP-ribosylates the G alpha subunit making it constitutively active
- increases in cAMP within intestinal epithelial cells leads phosphorylation of Cl- channels and efflux of water and electrolytes
What does the pertussis toxin do?
- ADP-ribosylates the G alpha subunit making it constitutively active
- enhances cAMP levels that inhibits neutrophil functions
What is the ratio of intracellular to extracellular calcium concentration? Why is this important?
- intracellular (100 nM) to extracellular (1 mM)
- allows for rapid changes in calcium concentration via hormone ligation
What protein binds calcium? Describe the structure of that protein.
- calmodulin
- has two globular domains (binds calcium) joined by a long alpha-helix
When calcium binds calmodulin, what happens?
conformational change that allows complex to bind to and modify target proteins (kinases) that initiate a signal cascade
What is the second messenger responsible for calcium mobilization?
phosphatidylinositol-4,5-bisphosphate (PIP2)
PIP2 is hydrolyzed by _______ to generate two second messengers: _______ and _______.
- PIP2-specific phospholipase C (PLC)
- inositol trisphosphate (IP3)
- diacylglycerol (DAG)
Which calcium second messenger is water-soluble and mobilizes calcium?
inositol trisphosphate (IP3)
Where is diacylglycerol (DAG) located and why? What is its function?
- anchored in plasma membrane due to hydrophobic fatty acid side chains
- activates key protein kinase C (PKC) family
True or false: Receptor-mediated endocytosis is selective.
true
Where do the vesicles of receptor-mediated endocytosis form? What do they contain?
- form at the surface of the membrane
- contain receptors and ligands
Describe the structure of the vesicles in receptor-mediated endocytosis.
- vesicles coated with three clathrin heavy chains and three light chains
- forms a polyhedral lattice
What is an example of receptor-mediated endocytosis?
iron transport via transferrin receptor
