3) Hormone Action Flashcards
What often occurs during the transmission of a signal? What does that result in?
- The signal is exponentially amplified
- Results in an on or off stage
Why must we possess specific receptors?
- Because there is a multitude of hormone signalling
- There cannot be interference among different receptors
What are target proteins?
Key regulatory proteins that determine the properties of a cell
Based on the signal receptor, what are the two main classes of pathways?
1) Pathways with cell-surface receptors
2) Pathways with intracellular receptors
What are the characteristics of cell-surface receptor ligands?
- Receive hydrophilic molecules (soluble in the extracellular fluid)
- Molecules bind to the outside of the cell (cell-surface receptor)
What are the characteristics of intracellular receptor ligands?
Receive lipophilic molecules, which partition themselves into the plasma membrane
What must hydrophobic molecules possess to travel in the blood? What allows them to interact with intracellular receptors?
- Must possess a carrier protein
- Dissociation from the carrier protein allows them to diffuse into the cell
What are the three structures that form a cell-surface receptor?
- Ectodomain
- Hydrophobic transmembrane domain
- Cytoplasmic domain (endodomain)
What is the ectodomain associated with?
- Amino terminus
- Area where the hormone binds
What is the hydrophobic transmembrane domain composed of?
- Domain that crosses the lipid bilayer
- Alpha helix
- Stretch of hydrophobic amino acids that are soluble in the lipid membrane
What is the function of the cytoplasmic domain of the cell-surface receptor?
- The endodomain activates the signal transduction mechanism
- The “lock and key” mechanism of the receptor and hormone results in the change in the transmembrane domain, activating a change in the cytoplasmic domain
How many amino acids are needed to cross the lipid bilayer membrane?
25 amino acids
Which end of the cell-surface receptor is associated with the amino end of the peptide? Which end of the cell-surface receptor is associated with the carboxyl end of the peptide?
- Amino (NH2): Ectodomain
- Carboxyl (COOH): Endodomain
Are the three domains of the cell-surface receptor functionally dependent or independent?
- Independent
- They are interchangeable
What are structural characteristics of the ectodomain of cell-surface receptors?
- Amino end of the peptide
- Rich in cysteine residues (S-S bonds for folding)
- Often glycosylated
What is the function of a free ectodomain?
May circulate as a hormone binding protein
What equilibrium exists in terms of circulating GH?
Equilibrium between GH bound to the ectodomain, and GH in the aqueous phase
How are ectodomains implicated in Grave’s disease?
- The ectodomain cleaved from the TSH receptor may induce antibodies
- Antibodies bind to the receptor and mimic TSH action
- Causes hyperthyroidism
What are conformational changes during a signalling cascade induced by?
- Phosphorylation of proteins
- Binding between proteins
Many signalling proteins are activated by phosphorylation at which amino acids?
- Serine
- Threonine
- Tyrosine
Which amino acid may be phosphorylated, and serves as a docking site for other proteins to interact?
Tyrosine (aromatic)
Which group is replaced by a phosphate group during phosphorylation?
A polar hydroxyl group is replaced by a phosphate group
What is the phosphate donor for protein phosphorylation?
ATP
Many signalling proteins are _____ that are activated (or inactivated) by phosphorylation.
kinases
What is the function of activated kinases?
- Phosphorylate other signalling proteins
- Induce signal amplification
What do activated kinases use as a phosphate donor?
- ATP
- They do NOT use their own phosphate group
How may phosphorylation be reversed?
Phosphatases
What are the three advantages of phosphorylation of proteins as a controlling mechanism for signal transduction?
- Rapid
- Reversible
- Easy to relay signals
Why is protein phosphorylation rapid?
Does not require new protein synthesis or protein degradation
Why is protein phosphorylation reversible?
Easily reversed by the action of protein phosphatases
Why does protein phosphorylation make it easy to relay signals?
Phosphorylation on tyrosine, threonine, or serine create binding sites for other proteins
___% of all cellular proteins are phosphorylated.
10
Which phosphorylated amino acids are more abundant? Which is less abundant?
- Phosphorylated serines are threonines are more abundant than phosphorylated tyrosines
- 100:1 ratio
The phosphorylation of which amino acid occurs at the beginning of a signal cascade?
Tyrosine
The intracellular domain of many receptors have or induce ________ _______ activity that is activated by a hormone binding to the receptor.
tyrosine kinase
What serves as a docking site for downstream signal proteins?
Phosphorylated tyrosines
What is the amino acid sequence that mediates docking to phosphorylated tyrosine? What is it diagnostic for?
- SH2 and SH3 domains
- Diagnostic for proteins involved in the signalling cascade
What are the three major types of cell-surface receptors?
1) Intrinsic tyrosine kinase activity
2) Recruited tyrosine kinase activity
3) G-protein coupled
What is the mechanism for intrinsic tyrosine kinase activity?
- The intracellular domain of the receptor itself is an inactive tyrosine kinase
- When a hormone binds to the ectodermal domain of the receptor, a conformation change occurs, allowing for the activation of the tyrosine kinase
What is the mechanism for recruited tyrosine kinase activity?
When a hormone binds to the ectodermal domain of the receptor, an activated tyrosine kinase is brought to the complex
What is the characteristic structure of GPCRs?
- 7 domain transmembrane region
- Associated with the inside of the cellular membrane
What is the characteristic structure of tyrosine kinase coupled cell-surface receptors?
Peptide chain crosses the cell membrane once
What are examples of intrinsic tyrosine kinase activity?
- Insulin
- IGF-1
How many domains compose the insulin receptor? What are the chains that compose the insulin receptor?
- Heterotetrameric
- 2 alpha and 2 beta chains
How is the heterotetrameric structure of the insulin receptor held together?
Disulfide bonds
How many precursor proteins are processed to form the insulin receptor?
A single precursor protein
How many insulin receptors are there per cell?
100 and 200 000 receptors per cell
Which tissues possess the largest number of insulin receptors?
- Adipocytes
- Hepatocytes
What receptor does the insulin receptor have homology with?
IGF-1
What is the sequence of events after insulin binding?
1) Autophosphorylation of intracellular domain of receptor
2) Docking and phosphorylation of IRS-1 and IRS-2
3) Activation of the two major signal pathways
Which two major signal pathways are activated by insulin binding to its receptor?
1) PI-3 kinase pathway
2) MAPK pathway
Where does autophosphorylation of the insulin receptor occur?
- Endodomain
- At tyrosine residues
What are the two primary functions of the PI-3K pathway?
1) Maintenance of active (dephosphorylated) glycogen synthase to increase glycogen synthesis
2) Movement of GLUT4 to the outer cell membrane to increase glucose uptake
What is the mechanism of the PI-3K pathway?
1) Phosphorylated IRS-1 activates PI-3K
2) PI-3K converts PIP2 to PIP3
3) PKB, bound to PIP3, is phosphorylated and activated
4) PKB phosphorylates GSK3, inactivating it
5) Inactive GSK3 cannot convert glycogen synthase to its inactive form, so GS remains active
6) PKB stimulates the movement of GLUT4 to the membrane
What is MAPK? What is its function?
- Mitogen activated kinase
- Central to many cellular responses
- Target proteins may be regulatory genes of cell division
What is the function of the MAPK pathway?
- Change of gene expression profile
- Increased division in some cells
What is the mechanism of the MAPK pathway?
1) Phosphorylated IRS-1 binds to Grb2, which binds to SOS, which binds to Ras
2) Ras causes GDP release and GTP binding
3) Activated Ras binds and activates Raf-1
4) Raf-1 phosphorylates MEK
5) MEK phosphorylates MAPK
6) MAPK moves into the nucleus and phosphorylates transcription factors
What are the metabolic effects of insulin?
- Increase glucose uptake
- Increase glycogen synthesis
- Increase glycolysis, Acetyl-CoA production
- Increase fatty acid synthesis
- Increase TG synthesis
What is the net effect of insulin?
Converts excess blood glucose into two forms of storage
What two forms of storage arise from the effects of insulin?
1) Glycogen in the liver and muscle
2) Triacylglycerols in adipose tissue
What do defects in the insulin receptor/signalling pathway lead to?
Insulin resistance syndromes
How many mutants have been identified in the insulin receptor alone?
Over 50