Enzyme-Coupled Receptors, Molecular Mechanisms Of

Question 1

List the 6 classes of ECRs

Answer 1

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)

Question 2

What do kinases lead to?

Answer 2

Phosphorylation events

Question 3

What do phosphatases do?

Answer 3

Dephosphorylation

Enzyme which removes a phosphate group

Question 4

What do receptor tyrosine kinases (class 1) do?

Answer 4

They phosphorylate tyrosine residues on a specific set of substrates

Question 5

What are receptor tyrosine kinases (class 1) usually activated by?

Answer 5

Secreted growth factors and hormones

e. g. insulin (carbohydrate utilisation and protein synthesis)
e. g. epidermal growth factor (causes proliferation = cell division)

Question 6

How many sub-classes of receptor tyrosine kinase (class 1) are there and how are they assigned?

Answer 6

There are 7 sub-classes of receptor tyrosine kinase
They are assigned a sub-class based on their primary structure

Question 7

How are signals transduced across the plasma membrane (RTKs)?

Answer 7

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

Question 8

Define: Oligomerisation

Answer 8

The formation of an oligomer from a monomer

Question 9

Define: Oligomer

Answer 9

Molecule consisting of 2-100 repeating units

Question 10

Define: Dimerisation

Answer 10

A compound formed of 2 identical simpler molecules (a kind of oligomer)
Can be due to growth factor

Question 11

What does the reorientation of the α helices cause in receptor tyrosine kinases?

Answer 11

Causes the intracellular domain to have kinase activity = a kinase

Question 12

How does autophosphorylation cause biological effects?

Answer 12

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

Question 13

How are signals generated by epidermal growth factor (EGF)?

Answer 13

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)

Question 14

Describe the structure of an insulin receptor

Answer 14

Is a tyrosine kinase receptor
Exists as a tetramers (4 monomers) linked by disulphide bridges
Cross-membrane receptor

Question 15

How does signalling through the insulin receptor occur?

Answer 15

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

Question 16

What is the role of Akt?

Answer 16

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

Question 17

Describe Tyrosine Kinase-Associated Receptors (Class 2)

Answer 17

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

Question 18

What are cytosolic tyrosine kinases?

Answer 18

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

Question 19

What class are cytokine receptors?

Answer 19

Class 2: Tyrosine Kinase-Associated Receptors

Question 20

What are cytokine receptors?

Answer 20

Rely on cytoplasmic kinases for signal transduction
Cytokines = signalling molecules
Have important roles in the regulation of the immune system

Question 21

What are cytokines?

Answer 21

Large family of signalling molecules

e.g. chemokines, interleukins and tumour necrosis factor

Question 22

What is the role of cytokines?

Answer 22

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

Question 23

What is tumour necrosis factor?

Answer 23

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

Question 24

Explain the process of apoptosis by TNF-α

Answer 24

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

Question 25

What is the NFkB pathway?

Answer 25

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)

Question 26

What is the purpose of anti- TNF therapies?

Answer 26

TNFα is responsible for the destructive inflammatory processes of rheumatoid arthritis
Therapies block action of TNF (TNF promotes apoptosis when present in excessive concentrations)

Question 27

Name the 2 drugs that are used in anti-TNF therapies

Answer 27

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

Question 28

Name class 3 of enzyme-coupled receptors

Answer 28

Receptor serine/threonine kinases

These receptors will be phosphorylated on serine and threonine residues

Question 29

Where are receptor serine/threonine kinases found?

Answer 29

Cytosol

Question 30

What is TGFβ?

Answer 30

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)

Question 31

What is the role of TGFβ?

Answer 31

TGFβ plays an important role in regulating the proliferation and differentiation of cells

Question 32

How does TGFβ carry out its role?

Answer 32

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

Question 33

What are class 4 enzyme coupled receptors called?

Answer 33

Receptor Guanylyl Cylases

Question 34

What are receptor guanylyl cyclases?

Answer 34

Single transmembrane proteins with an extracellular domain for agonists binding

Question 35

How do receptor guanylyl cyclases have their effect?

Answer 35

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

Question 36

In summary, what do agonists of ECRs often act as

Answer 36

ECR agonists often act as:
Dimers
Trimers
Oligomers

Question 37

What is required for the activation of signalling cascades?

Answer 37

Dimerisation or oligomerisation is required for the activation of signalling cascades

Question 38

What plays a key role in the generation of the intracellular signal?

Answer 38

Trans-autophosphorylation of catalytic domains plays a key role in the generation of the intracellular signal

Question 39

What do most signalling cascades lead to?

Answer 39

Most signalling cascades lead to the phosphorylation and activation of transcription factors

Question 40

What is the role of transcription factors?

Answer 40

Transcription factors regulate gene transcription and are therefore responsible for producing the required biological effects (hours/days)