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How is the Occupancy-Response coupling process accomplished by?
By a numer of different molecular mechanisms (or transmembrane signaling pathways).
Each mechanism has evovled to transduce many different signals.
What type of Protein Families are invovled in these molecular signaling pathways?
- -receptors within cells and on cell membranes
- -signal transducer proteins
- -other molecules that generate, amplify, coordinate, and terminate postreceptor signaling by chemical second messengers
What are the Five Basic Mechanisms of transmembrane signaling?
- 1. Intracellular Receptors for Lipid-Soluble Agents
- 2. Ligand-Regulated Transmembrane Enzymes (Receptor Tyrosine Kinases)
- 3. Cytokine Receptors
- 4. Ligand-Gated Ion Channels
- 5. G-Proteins and Second Messengers
How does an Intracellular Receptor for Lipid-Soluble Agents work?
A lipid-soluble ligand (drug) crosses the plasma membrane and acts on an intracellular receptor (an enzyme or regulator gene transcription)
What are some examples of ligands that exert thier effects via Intracellular Receptors for Lipid-Soluble Agents?
- -nitric oxide
- -sex hormones
- -thyroid hormones
- -vitamin D
What type of receptors are Ligand-Regulated Transmembrane Enzymes (RTK)?
Receptors are transmembrane protiens consisting of extracellular (ligand-binding domain) and a cytoplasmic intracellular (enzyme domain).
What are some examples of intracellular enzyme domains in ligand-regulated transmembrane enzyme receptors?
protein tyrosine kinase, serine kinase, guanylyl cyclase
What connects the two domains in the receptor of a Ligand-Regulated Transmembrane Enzyme receptor?
a hydrophobic segment of the polypeptide that crosses the lipid bilayer of the plasma membrane.
How does a Ligand-Regulated Transmembrane Enzyme Receptor work?
Ligand binds to extracellular domain of receptor, activating (allosterically regulating) the enzymatic activity of the intracellular domain.
What are some examples of LIGANDS that exert their effect via the Ligand-REgulated Transmembrane Enzyme receptor?
insulin, epidermal growth factor (EGF), platelet-derived growth factor (PDGF), transforming growth factor (TGF), and other trophic hormones.
What is an important example of Ligand-Regulated Transmembrane Enzyme receptors?
Receptor Tyrosine Kinase
How does Receptor Tyrosine Kinases work?
1. Ligand binds to receptor's extracellular domain results in conformational change
2. conformational change causes receptor molecules to bind to one another (including tyrosine domains)
3. Tyrosine domains become enzymatically active, and phosphorylate one another (as well as downstream signaling proteins)
What happens once the receptor tyrosine kinases become active?
Activated RTKs phosphorylate tyrosine residues on different signaling protiens and allow a single activated receptor to modulate several biochemical processes
EX: each growth factor ligands initiates complex events with specific targets (altered membrane permeability to changes in gene expression)
What are effective therapeutic agents in treating cancer?
Specific inhibitors of growth factor-activated Receptor Tyrosine Kinases
(where over expression of growth factor receptors and excessive growth factor signaling occurs)
What is the mechanism for Cytokine Receptors?
- 1. Ligand binds to extracellular domain
- 2. transmembrane receptor protein is already bound to a protein tyrosine kinase (JAK)
- 3. Protien tyrosine kinase is activated by receptor-ligand binding
- 4.activated tyrosine kinase phophorylates tyrosine residues on receptor
- 5. Receptor binds to another set of proteins (STATs) and phosphorylates them
- 6. Once phosphorylated, STATs (Signal Transducers and Activators of Transcription) dimerize, dissociate from receptor, and travel to nucleus to regulate gene expression
What is the difference between RTKs and cytokine receptors?
Cytokine receptors and RTKs are similar in mechanisms.
However, cytokine receptors, the protein tyrosine kinase activity is NOT intrinsic to the receptor molecule.
A SEPERATE protein tyrosine kinase JAK (janus-kinase) family binds noncovalently to receptor
-kinase enzyme not part of receptor
What are some examples of ligands that bind and activate cytokine receptors?
growth hormone, interferons, other regulators of growth and differentiation
How does a Ligand-Gated Ion Channel work?
A ligand-gated transmembrane ion channel is induced to open or close by binding of a ligand
- 1. ligand binds
- 2. receptor changes conformation
- 3. channel opens/closes
What are some examples of natual ligands that bind to Ligand-Gated Ion Channels?
neurotransmitters acetylecholine, serotonin, B-aminobutyric acid (GABA), and excitatory amino acids (glycine, aspartic acid, and glutamic acid)
How does many important drugs regulate ion channels?
By mimicking or blocking the actions of endogenous ligands that regulate ion channels.
How does G-Proteins and Second Messengers work?
- 1. Ligand binds to the extracellular domain
- 2. Receptor stimulates GTP-binding signal transducer protein (G-Protein)
- 3. G-Protein activates Effector
- 4. Effector modulates production of intracellular second messengers
Where are G-Proteins located?
on the cytoplasmic face of the plasma membrane
What can be Effectors?
enzymes or ion channels
What Receptor protein molecules are coupled to G-Proteins?
Serpentine Receptors or 7-Transmembrane Receptors
All serpentine receptors transduce signals across membranes in same fashion
Many extracellular ligands act by increasing the intracellular concentrations of second messengers. What are some examples of second messengers via the G-protein coupled pathway?
cAMP, Calcium ions, phosphoinositides
What type of extracellular ligands produce cAMP as a second messenger?
catecholaimes via B-adrenoceptor, Histamine via H2 receptor, vasopression via V2 receptor, glucagon, FSH, LH, thyrotropin, parathyroid hormone
What is the effector molecule for cAMP and how does it work?
Adynylyl Cyclase, a transmembrane protein converts ATP into cAMP via Gs.
Gs stimulates anynylyl cyclase after being stimulated by ligands that act via specific receptor
How does the signal gets amiplified with G-proteins?
Binding and hydrolysis of GTP, amplyfies the transduced signal.
Amplification is attributed to the fact, active GTP-bound G protein remains active for a rel. long time (tens of seconds), because G-protein will have to undergo dephosphorylation to become inactive again.
Active G-protien (Gs) activates effector molecule, thus prolonging and transducing more signals
The duration of activation of adenylyl cyclase is dependant on what?
duration of activation of the G protein
How are G-Proteins activated?
- 1. G Protein is inactive when attached to GDP (Go)
- 2. GDP is phosphorylated to GTP
- 3. G Protein is active when attached to GTP (Gs)
- 4.GTP has to be dephosphorylated to GDP again for G Protein to become inactive
What are three well studied intracellular Second Messenger Signaling Pathways?
- cAMP (cyclic adenosine monophosphate)
- Calcium and Phosphoinositides
- cGMP (cyclic guanosine monophosphate)
What does cAMP mediate?
- -hormonal responses (breakdown of carbohydrates and triglycerides)
- -conservation of water by kidneys
- -calcium homeostatsis
- -increased rate and contraction force of heart muscle
- -production of adrenal and sex steriods
- -relaxation of smooth muscle
- -numerous other endocrine and neural processes
If cAMP is an example of a second messenger, what is the first messenger?
How does cAMP exert most of its effects?
By stimulating cAMP-dependent protein kinases
The specificity of cAMP's effects is attributed to the distinct protein substrates (usually enzymes) of kinases that are expressed in different cells.
When the hormonal stimulus ends, how are cAMP actions terminated?
- 1. Dephosphorylation of protein substrates (phosphatases)
- 2. cAMP degradation to 5'-AMP (phosphodiesterases, PDE)
Inhibition of cAMP degradation is one way some drugs exert thier effects, How?
If more cAMP accumulates and is not degraded, the response created by the kinases will be prolonged, due to the presence of the cAMP.
Second messenger signaling pathways may only oppose one another.
False. They may oppose and/or compliment one another in different cells.
ex: cAMP-mediated smooth muscle relaxation vs. Calcium-phosphoinositide-mediated smooth muscle contraction)
Are Protein Kinases important target for drug discovery efforts?
Yes. Protein Kinases are important targets for discovery efforts in a number of disease.
Specific inhibitors of protein kinases have potential as drugs, esp. in cancer chemotherapy
What is Imatinib an example of?
Imatinib, is an inhibitor of a tyrosine kinase which is activated by growth factor signaling pathways, used to treat chronic myelogenous leukemia.