Signal Transduction Flashcards
Types of signals (6)
- Hormones and growth factors
- Neurotransmitters
- Smells
- Taste sensations
- Light
- Extracellular matrix
How is ECM a signal? What condition does it play an important role in?
It is the surface on which cells attach themselves. So, physical contacts via protein to protein that are made between cells and ECM are important in cancer biology.
Cancer cells do not attach themselves to ECM as well leading to metastasis.
Receptors- what do they do and where are they
Receive signal.
Majority are at PM, some inside.
Signal amplification involves (3)
G proteins
Second messengers
Activation of kinases and phosphatases which change activity of metabolic enzyme or growth promoting enzyme
Cellular response is caused by _____
Amplification of the signal
2 outcomes of signal transduction pathways
- Change the activity of a metabolic enzyme or group of enzymes using kinases/phosphatases
- Increase/decrease gene expression
Receptor specificity
What happens when a signal binds?
When a signal binds it is by NCIs –> can cause changes in NCI
All receptors are _______
Transmembrane
- outside to accept signal
- have a domain that is inside for signal to be relayed
- bind polypeptide hormones or other molecules which are not permeable to membrane
Amplification
Hormones do not have to be present in high concs to have a large effect.
When enzymes activate enzymes, _______
The number of affected molecules increases
What is the purpose of having a process with many steps?
Multiple points of regulation and ability to capture utilizable amounts of energy
Integration
Take signals coming from different directions –> pool together –> get a “net response”
Two hormones can:
- Work together and trigger similar types of responses
2. Oppose each other
Example of hormones that work together
Glucagon and epinephrine
Example of hormones that opposite other and their importance
Glucagon and insulin –> glucose homeostasis
Why is glucose homeostasis important?
Brain has high demand for glucose
Two types of signal transduction pathways to know
- G-Protein Coupled Receptors (GPCR)
2. Receptor Tyrosine Kinases
General characteristics of GPCR: (3)
What is the general mechanism
- has multiple alpha helices that snake in and out of membrane
- is an integral protein
- do NOT have enzymatic activity
- external signal binding to receptor activates an intracellular GTP-binding protein (G protein), which regulates an enzyme that generates an intracellular messenger
GPCR: What has enzymatic activity
The G Protein
NOT the receptor
2 Main differences between GPCR and receptor tyrosine kinases (RTKs)
- Receptor tyrosine kinases have enzymatic activity, GPCR do not. Therefore, the receptor can phosphorylate other proteins.
- Receptor tyrosine kinases do not have second messengers
General mechanism of receptor tyrosine kinases
Signal binding to extracellular domain stimulates enzyme activity in intracellular domain
GPCR interacts with ______
Heterotrimeric G protein
Heterotrimeric G protein
3 subunits- alpha, beta, gamma
What G protein subunits are anchored in PM by covalent bonds (lipid anchor)?
Importance?
Alpha and gamma.
Allows alpha to dissociate from beta and gamma when GTP binds and move laterally to bind to and activate adenylate cyclase
Alpha subunit of G-protein important characteristics (2)
- Where GTP binds for activation
2. Has GTPase activity. Therefore, it can hydrolyze GTP to terminate its own signal
When GTP binds to the alpha subunit, _______
A GTP/GDP exchange takes place.
GDP comes off, GTP comes on
Inactive vs. active GPCR
Inactive –> has GDP bound to Galpha
Active –> has GTP bound to Galpha
GPCR- Step 1
Epinephrine or glucagon binds to GPCR –> triggers a conformational change in G protein
GPCR- Step 2
GDP comes off alpha subunit and GTP comes on
Result: G-protein is now activated.
GPCR- Step 3
Galpha dissociates from beta and gamma and moves laterally in membrane to activate adenylate cyclase.
Physical contact triggers conformational change to activate catalytic function
GPCR- Step 4
AMPLIFICATION
Adenylate cyclase produces second messenger cAMP using ATP as a substrate.
Adenylate cyclase keeps producing _____ as long as _____
CAMP ; Galpha is bound
Properties of cAMP (3)
- polar
- charged
- diffuses away from membrane
GPCR: Step 5
- AMPLIFICATION *
CAMP activates multiple protein kinase A molecules
PKA structure and how it is activated
Made up of:
- 2 catalytic subunits
- 2 regulatory subunits
- 4 binding sites for cAMP (2 on each regulatory subunit)
The 2 regulatory subunits typically prevent PKA from always being active through NCI. But, when 4 cAMP bind to the regulatory subunit a conformational change disrupts the NCI between the regulatory and catalytic subunits and the catalytic subunit is now active.
GPCR- Step 6
Active PKA phosphorylates several enzymes to change their activity.
How? Use ATP as phosphate donor (ATP is split)
3 ways to terminate signal in GPCR
- GTPase activity of Galpha subunit
- Phosphodiesterase degrades cAMP
- Hormone dissociates from receptor
GTPase activity of Galpha subunit
Galpha hydrolyzes GTP to produce GDP
Galpha re associates with beta and gamma subunits
Phosphodiesterase
How?
Degrades cAMP and terminates signal
Cyclization is cleaved –> AMP made –> can be phosphorylated to ADP –> ATP
Caffeine affect on phosphodiesterase…result?
Inhibits phosphodiesterase therefore…
stops the breakdown of cAMP and HR increases
How does hormone dissociate from a receptor
Tissue that makes hormone is no long receiving signal so hormone conc is diluted out and eventually hormone will come off receptor
Cholera toxin mechanism
ADP-ribosylation:
Galpha subunit has Arg residue….
In the presence of ADP-ribose, cholera toxin transfer the ADP-ribose to the Arg. This is a large, covalent modification that targets Galpha in its active site
Result: Galpha cannot hydrolyze GTP so adenylate cyclase is constantly activated
What happens when adenylate cyclase is constantly activated due to ADP-ribosylation?
The Galpha subunit regulates the activity of a gated ion channel in small intestine –>
When G alpha is ADP-ribosylated, the channel remains open and Na+ rushes out and H2O follows…..gives rise to symptoms associated with cholera toxin
Glucagon (2)
- Small protein
- Released by Pancreas
Epinephrine (3)
- Modified amino acid
- beta adrenergic signaling associated with flight or fight response
- released from adrenal medulla
Glycogen (3)
- is what?
- stored in?
- degraded by?
- Large polysaccharide of glucose
- Stored in skeletal muscle and liver to maintain glucose homeostasis during overnight fast and exercise
- Degraded by phosphorylase a
Muscle has receptors for:
Epinephrine ONLY
- DOES NOT respond to glucagon.
Liver has receptors for:
Epinephrine AND glucagon
In the muscle, glycogen is broken down into glucose when?
What happens?
When epinephrine binds to receptors.
Glucose stays in the muscle and goes through glycolysis to provide energy for muscle contraction.
In the liver, glycogen is broken down into glucose when?
What happens?
Either epinephrine or glucagon bind to receptors.
Glucose goes into the bloodstream to maintain glucose homeostasis –> governs metabolism
Other second messengers in GPCR (4)
- In central nervous system, cGMP
- Ca+
- Inositol 1,4,5-triphosphate (IP3)
- Diacylglycerol (DAG)
Signaling through phosphotidylinositol
Phosphotidylinositol is present in low conc in the PM and can be cleaved to yield DAG and IP3 which are second messengers in signal transduction
Mechanism of signaling using phosphotidylinositol
- Phosphotidylinositol phosphorylated to become PIP2
- Phospholipase C breaks down by cleaving the P and inositol
- Left with DAG and IP3
DAG –> stays in membrane
IP3 –> diffuses away (charged)
GPCR-PI Cascade Mechanism
- IP3 diffuses away from membrane and binds to….
- IP3 binds to IP3 receptor on the smooth endoplasmic reticulum membrane
- IP3 receptor is a gated ion channel, so when IP3 binds the channel opens –> Ca2+ comes out and into cytoplasm
- Protein kinase C activated by DAG and Ca2+ binding
Protein kinase C activation keys
For maximal activity, PKC requires BOTH Ca2+ and DAG.
DAG can work by itself, but activity not as high.
PKC important in?
What hormones work by this cascade?
Growth control
Vasopressin and oxytocin
Epidermal Growth Factor (EGF) receptor structure
Extracellular EGF binding domain –> transmembrane helix –> intracellular kinase domain –> tyrosine rich C terminal tail
2 monomers of receptor
PKA and PKC typically phosphorylate ___ and ____ for metabolic control
Tyr and Ser residues
What happens when EGF bind to extracellular domain of receptor
The receptors dimerize. –> The two monomer subunits phosphorylate each other and activate receptor.
Mechanism of EGF
- EGF binds to receptor
- Dimerization of two monomer subunits –> phosphorylate each other –> activation of tyrosine kinase activity of the receptor (phosphates serve as binding sites for other proteins)
- Grb-2 binds to receptor
- Grb-2 interacts with Sos
- Sos triggers exchange of GDP for GTP on Ras to activate Ras
- Ras relays signal to interior of the cell
Ras
- Similar to Galpha
- Small protein, NOT a second messenger
- Has 2 amino acid residues that bind GTP
Important amino acid residues in Ras and consequences of mutations
Glycine 12 and Glutamine 61
Mutations in these residues are common in tumors. The mutations affect the ability of Ras to hydrolyze GTP therefore Ras always stays active –> entire growth signal continually keeps going
Ras relays signal to interior of the cell
Ras –> Raf–> MEK –> Erk1 or Erk2 –> goes on to affect many other protein for growth
In this cascade, kinases activate other kinases
Other example of RTK
Insulin Receptor
Insulin receptor structure
- A dimer
- alpha subunits (extracellular) and beta subunits (intracellular) connected by 2 S-S bonds
- Also a S-S bond the connects the alpha subunits
Binding of insulin to ____ subunit ______
To alpha subunit activates the beta subunit that contains the kinase activity
Mechanism of Insulin Receptor (RTK)
- Insulin binds to alpha subunit and….
- And activates the beta subunit. The beta subunit phosphorylate each other and the phosphates serves are binding sites for other proteins
- Insulin receptor substrate 1 (IRS-1) binds to receptor
- Receptor phosphorylates IRS-1 at tyrosine sites and those phosphates serve as binding sites
- PI-3-Kinase interacts with IRS-1
- PI-3-Kinase phosphorylates PIP2 –> PIP3 using ATP. PIP3 remains in the membrane and serves as a docking site for PDK-1
- PDK-1 binds to PIP3 and becomes activate
- PDK-1 phosphorylates Akt (protein kinase B) to activate it
Activated Akt (protein kinase B) leads to….
Many steps that lead to the recruitment of glucose transporters to the plasma membrane
Insulin sensitive tissues
Why?
Skeletal muscle and adipose tissue
Their transporters are low Km so when glucose concs rise in the blood they are already saturated –> insulin signal transporters in the cell to move to the membrane
Mechanism of recruiting glucose transporters to membrane of skeletal muscle and adipose tissue
- Akt triggers many proteins that cause the movement of vesicles containing glucose transporters (GLUT4)
GLUT4
Where?
Km?
- Specific to skeletal muscle and adipose tissue
- Low Km
What would happen if additional transporters were not brought to the membrane?
Because the transporters have low Km, they are working at saturation especially when glucose levels rise after a meal. Without the additional transporters, blood glucose levels would remain higher for longer
Insulin signal termination
Requires protein phosphatases and lipid phosphatases
