04- Cell Signaling Flashcards
what are the three major classes of surface receptors for cell signaling?
- G-protein coupled receptor
- Enzyme-linked receptor
- Ion-channel linked receptor
what is a brief overview of the G protein-coupled receptor?
- 7 trans-membrane receptors
- act as receptors for many different ligands
- large amount of receptor diversity, but common mechanism of action
- transmit signals to intracellular targets via G proteins
- targets are plasma membrane bound enzymes or ion channels
what happens when there is no ligand attached to the GPCR?
it waits at the plasma membrane in an inactive state. In this inactive state, the alpha subunit is bound to GDP
What is the mechanism of activation of the GPCR?
- Ligand binds to the receptor on the extracellular domain of the GPCR and activates if
- This induces a conformational change that allows the cytosolic domain of the receptor to bind to the inactive G-protein
- This interaction activates the G protein and causes the alpha subunit to bind GTP instead of GDP (via the guanine nucleotide exchange factors- GEF)
- The activated G protein dissociates into activated alpha subunit and activated beta-gamma dimer. Each of these can activate target proteins
What happens when the ligand is removed from the receptor?
The alpha subunit will hydrolysis GTP back into GDP (via GTPase activating protein- GAP). This causes the G protein to become inactive and as a result it reassembles
What are the four main types of G protein?
Gs- stimulatory: activates adenylyl cyclase
Gi- inhibitory: inhibits adenylyl cyclase
Gq: activates phospholipase C-B
Go: activates K+ channels, inactivates Ca2+ channels
What is the mechanism of action of the Gs?
The activated alpha subunit can act by Gs (stimulatory G protein).
- The Gs activates adenylyl cyclase
- Adenylyl cyclase then converts ATP to cAMP by the release of 2 phosphate groups
- cAMP activates protein kinase A (PKA) which phosphorylates protein to produce a cellular response
What is the mechanism of action of the Gi?
The activated alpha subunit can act by Gi (inhibitory G protein).
1. The Gi inhibits the activation of adenylyl cyclase to produce cAMP
What are the two most important bacterial toxins that disrupt G proteins?
Cholera toxin and Pertussis toxin
How does cholera and pertussis disrupt the G protein (general).
They ADP-ribosylate arginine residues in G proteins and alter their activity. This reaction uses NAD+ as the donor for ADP-ribose and releases nicotinamide
What is cholera toxin caused by and what are the symptoms?
Cholera toxin is caused by the vibrio bacteria which is ingested through contaminated food and water.
The symptoms are diarrhea, fatigue and dehydration
How does Cholera toxin disrupt the G protein?
- Cholera toxins catalyze ADP ribosylation of the alpha subunit of the Gs of intestinal cells.
- It blocks the GTPase activity (alpha subunit is still active)
- This prevents GTP from being broken down which leads to the continuous activation of adenylyl cyclase of intestinal cells
- This leads to increased levels of cAMP in the cells
- This eats to continuous secretion of Cl-, HCO3 and water- which causes watery diarrhea and dehydration
What is pertussis toxin caused by and what are the symptoms?
Pertussis toxin is caused by the bordatella bacteria.
It causes whooping cough.
How does Pertussis toxin disrupt the G protein?
- Pertussis toxins catalyze ADP ribosylation of the alpha subunit of Gi of intestinal cells
- It prevents the displacement of GDP by GTP and blocks the inhibition of adenylyl cyclase by Gi
- This leads to increased levels of cAMP in the cells
- This leads to whooping cough symptoms
what is a brief overview of the enzyme-linked receptor, specifically tyrosine kinase-linked receptors (TKRs)?
- they are cell surface receptors that are directly linked to intracellular enzymes (kinases)
- they include receptors for most growth factors (NGF- nerve growth factor, EGF- epidermal growth factor, PDGF- platelet derived growth factor), insulin and Src
- the common structure is an extracellular N terminal with a ligand-binding domain and an intracellular cytosolic C-terminal domain with tyrosine kinase activity
- it can be a single polypeptide or dimer
What are some signal proteins that act via Tyrosine Kinase Receptors?
EGF- epidermal growth factor: stimulates cell survival, growth and proliferation
Insulin: stimulates carbohydrate utilization and protein synthesis
IGF1 and IGF2- insulin-like growth factors: stimulates cell growth and survival
NGF- nerve growth factor: stimulates survival and growth of some neurons
PDGF- platelet derived growth factor: stimulates survival, growth, proliferation and migration of cell types
VEGG- vascular endothelial growth factor: stimulates angiogenesis (formation of blood vessels)
What happens when there is no ligand attached to the TKR?
It exists as an inactive monomer. In this inactive state, the Ras protein is bound to GDP
What is the mechanism of activation of the TKR?
- The ligand which is normally a growth factor binds to the receptor and activates it
- This induces receptor dimerization
- The dimerization leads to the cross-phosphorylation of the receptor
- This increases kinase activity and produces binding sites for proteins with SH2 domains (e.g. GRB2)
- GRB2 with SOS bound to it, binds to the receptor complex which activates the SOS (SOS is a guanyl nucleotide release protein)
- Activated SOS causes the Ras protein to exchange GDP for GTP and GTP-bound Ras is now active
- Activated Ras then activates Raf-1 (MAP kinase kinase kinase)
- Activated Raf-1 then activates MEK (MAP kinase kinase)
- Activated MEK then activates MAPK (MAP kinase) which results in the amplification of the signal
- The final targets of the kinase cascade are transcription factors (fos and jun). Phosphorylation causes them to become active and bind to DNA causing changes in gene transcription.
What is the lipolysis pathway?
The breakdown of triglycerides stored in adipose tissue. They are hydrolyzed into glycerol and fatty acids.
When does lipolysis take place?
Low levels of glucose/insulin in the bloodstream
What are the enzymes that drive lipolysis?
- ATGL (adipose triglyceride lipase): catalyzes the hydrolysis of TAG to DAG
- HSL (hormone sensitive lipase): catalyzes the hydrolysis of DAG to MAG
- MGL (monoacyl glycerol lipase): catalyzes the hydrolysis of MAG to glycerol
- perilipin-1A: regulates lipolysis pathway
What are the components of the lipid droplet + adipocyte?
Inactive HSL enzyme is within the cytoplasm of the adipocyte.
Inactive CGI proteins are bound to inactive perilipin proteins on the surface of the lipid droplet.
Inactive ATGL enzyme and MGL enzyme are also on the surface of the lipid droplet
What is the mechanism of lipolysis?
When there are low levels of insulin/glucose in the bloodstream:
- Epinephrine comes and binds to the GPCR and activates it
- This induces a conformational change and allows the cytosolic domain of the receptor to bind to inactive G protein
- This interaction activates the G protein and causes the alpha subunit to bind GTP instead of GDP
- The activated G protein dissociated into activated alpha subunit and activated beta-gamma dimer.
- The activated alpha subunit acts on Gs protein
- The Gs activates adenylyl cyclase
- Adenylyl cyclase converts ATP to cAMP
- cAMP activates protein kinase A (PKA)
- Activated PKA acts on the HSL enzyme and phosphorylates it
- The phosphorylated HSL enzyme is moved to the surface of the lipid droplet
- Active PKA also phosphorylates perilipin-1A on the surface of the lipid droplet
- The phosphorylated perilipin protein detaches from the CGI proteins and associates with HSL enzyme
- The detached CGI proteins leave off and associate with the ATGL enzyme and activates it
- Active ATGL converts TAG to DAG
- Active HSL and perilipin protein converts DAG to MAG
- MGL enzyme converts MAG to glycerol
- DAG and MAG can also be converted into free fatty acids through hydrolysis
