Enzyme-Coupled Receptors, Molecular Mechanisms Of Flashcards
List the 6 classes of ECRs
Receptor tyrosine kinases
Tyrosine kinase-associated receptors
Receptor serine/threonine kinases
Histidine-kinase-associated receptors (bacteria)
Receptor guanylyl cyclases
Receptor-like tyrosine phosphates (very small group)
What do kinases lead to?
Phosphorylation events
What do phosphatases do?
Dephosphorylation
Enzyme which removes a phosphate group
What do receptor tyrosine kinases (class 1) do?
They phosphorylate tyrosine residues on a specific set of substrates
What are receptor tyrosine kinases (class 1) usually activated by?
Secreted growth factors and hormones
e. g. insulin (carbohydrate utilisation and protein synthesis)
e. g. epidermal growth factor (causes proliferation = cell division)
How many sub-classes of receptor tyrosine kinase (class 1) are there and how are they assigned?
There are 7 sub-classes of receptor tyrosine kinase They are assigned a sub-class based on their primary structure
How are signals transduced across the plasma membrane (RTKs)?
Structure of ECRs = a single α helix spanning the membrane so binding of a ligand does not cause a conformational change
Dimerisation or oligomerisation of the receptor sub-units occurs instead (= several receptors = several α helices = complex)
This causes a conformational change
Leads to signal transduction
Therefore oligomerisation of receptors causes reorientation of the internal α helices = initiates signalling = autophosphorylation events
Define: Oligomerisation
The formation of an oligomer from a monomer
Define: Oligomer
Molecule consisting of 2-100 repeating units
Define: Dimerisation
A compound formed of 2 identical simpler molecules (a kind of oligomer)
Can be due to growth factor
What does the reorientation of the α helices cause in receptor tyrosine kinases?
Causes the intracellular domain to have kinase activity = a kinase
How does autophosphorylation cause biological effects?
Growth factor binds and cause dimerisation of the 2 receptors = complex of 2 receptors and a growth factor
Reorientation of α helices occurs
Intracellular tyrosine kinase domains are activated - can now convert ATP to ADP and phosphorylate particular tyrosine residues (=autophosphorylation)
So receptor is now acting as a enzyme
Autophosphorylation causes an increase in its intrinsic activity (becomes even more active)
The phospho-tyrosine residues then act as sites for docking of signalling proteins (attracted by phosphorylation)
Formation of signalling complexes occurs
This will activate downstream events
How are signals generated by epidermal growth factor (EGF)?
PHOSPHORYLATION CASCADE:
Tyrosine kinase is the active form of the receptor
This will bind some intracellular proteins (e.g. GRB2, SOS) and forms a signalling complex
Activates another protein, RAS = a kinase (tethered to the membrane in its inactive GDP form)
GRB2 and SOS positively regulate RAS so exchange of GDP to GTP occurs
RAS will phosphorylate Raf1 –> Raf1 phosphorylates MEK –> MEK phosphorylates ERK
Therefore activation by 1 EGF activates multiple RAS’s and exponential activation from then on
—–
Conformational change of ERK = allows opening of a nuclear localisation 6 sequence
This causes translocation of the ERK into the nucleus
= Promotes gene transcription and cellular proliferation by phosphorylating c-Fos (transcription factor)
Describe the structure of an insulin receptor
Is a tyrosine kinase receptor
Exists as a tetramers (4 monomers) linked by disulphide bridges
Cross-membrane receptor
How does signalling through the insulin receptor occur?
Ligand (insulin) binds
IRS-1 & PI3K (a kinase) also bind - attracted by phosphotyrosine residues of the receptor
PIP2 (in the membrane) acted on by PI3K (kinase) = phosphorylation = PIP3
PIP3 acts on PKD1 (protein kinase)
PKD1 acts on Akt (kinase) –> Akt acts on GSK3 (kinase)
GSK3 phosphorylates glycogen synthase
This results in promotion of glycogen synthesis
What is the role of Akt?
2 roles:
1. Promotes glycogen synthesis by acting on GSK3
2. Phosphorylates glucose transporters inside of cell, causes them to translocate to the cell surface = more transporters on the cell surface bringing in the glucose to make glycogen
Therefore: brings in more glucose and makes more glycogen
Describe Tyrosine Kinase-Associated Receptors (Class 2)
Rely on the activity of cytosolic tyrosine kinases to activate intracellular signalling cascades (no tyrosine kinase activity itself)
Receptor requires dimerisation
The tyrosine kinases have an affinity for the C-terminal tail and bind to it
Example: Human Growth Hormone Receptor
What are cytosolic tyrosine kinases?
Located on the cytoplasmic side of the membrane
Fixed in position by binding to the cell-surface receptor and by covalent attachments to the lipid bilayer
What class are cytokine receptors?
Class 2: Tyrosine Kinase-Associated Receptors
What are cytokine receptors?
Rely on cytoplasmic kinases for signal transduction
Cytokines = signalling molecules
Have important roles in the regulation of the immune system
What are cytokines?
Large family of signalling molecules
e.g. chemokines, interleukins and tumour necrosis factor
What is the role of cytokines?
Cytokines regulate growth, maturation and the behaviour of subpopulations to immune cells
Therefore cytokines are important in determining how the body responds to infection and disease
What is tumour necrosis factor?
Part of cytokine family, family of peptides itself
Best known member = TNF-α
- Can promote apoptosis or cell death
- exerts its biological effect by activating TNF receptor 1
Explain the process of apoptosis by TNF-α
TNF receptor 1 = pre-assembled trimer, cross-membrane (trimerises to activate receptor = oligomerisation)
Scaffolding and signalling proteins bind to receptor = very large signalling complex formed
Complex will activate caspases (proteins) = proteolytic enzymes
Caspases cleave lots of other proteins which drives apoptosis cascade
Proteolysis of Caspase 8&10 = pro-form = active enzyme
This causes proteolysis of 3,6 and 7 = enzyme
Generation of apoptosis
What is the NFkB pathway?
Causes anti-apoptosis when activated
From TNF receptor 1 (same receptor which can cause apoptosis using caspases)
Depends on the concentration of TNF (lots of TNF = apoptosis)
What is the purpose of anti- TNF therapies?
TNFα is responsible for the destructive inflammatory processes of rheumatoid arthritis
Therapies block action of TNF (TNF promotes apoptosis when present in excessive concentrations)
Name the 2 drugs that are used in anti-TNF therapies
Monoclonal antibodies (Adalimumab) = anti-TNFα recombinant human IgG1 monolconal antibody Fusion proteins (Etanercept) = mimics receptor to prevent the binding of TNF to its cell-surface receptor = no excessive signalling
Name class 3 of enzyme-coupled receptors
Receptor serine/threonine kinases
These receptors will be phosphorylated on serine and threonine residues
Where are receptor serine/threonine kinases found?
Cytosol
What is TGFβ?
Transforming Growth Factor β (family of proteins)
Activates a serine/threonine kinase receptor
Active as a dimer - recruits a hetero-tetrameric receptor complex (by recruiting a fellow dimer)
What is the role of TGFβ?
TGFβ plays an important role in regulating the proliferation and differentiation of cells
How does TGFβ carry out its role?
TGFβ (ligand) binds to a type II homodimer and activates it
Activated type II then recruits a type I homodimer to form the activated TGFβ receptor complex
Smad family of proteins are then phosphorylated (activated) and translocate to the nucleus = phosphorylation cascade
This activates gene transcription
What are class 4 enzyme coupled receptors called?
Receptor Guanylyl Cylases
What are receptor guanylyl cyclases?
Single transmembrane proteins with an extracellular domain for agonists binding
How do receptor guanylyl cyclases have their effect?
Agonist binds
Induces receptor dimerisation and activates the intracellular cyclase domain to produce cyclic guanosine monophosphate (cGMP)
cGMP activates a cGMP-dependent protein kinase (PKG) - similar to cAMP & PKA for GPCRs
In summary, what do agonists of ECRs often act as
ECR agonists often act as:
Dimers
Trimers
Oligomers
What is required for the activation of signalling cascades?
Dimerisation or oligomerisation is required for the activation of signalling cascades
What plays a key role in the generation of the intracellular signal?
Trans-autophosphorylation of catalytic domains plays a key role in the generation of the intracellular signal
What do most signalling cascades lead to?
Most signalling cascades lead to the phosphorylation and activation of transcription factors
What is the role of transcription factors?
Transcription factors regulate gene transcription and are therefore responsible for producing the required biological effects (hours/days)
