Enzyme-Coupled Receptors, Molecular Mechanisms Of Flashcards

1
Q

List the 6 classes of ECRs

A

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)

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2
Q

What do kinases lead to?

A

Phosphorylation events

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3
Q

What do phosphatases do?

A

Dephosphorylation

Enzyme which removes a phosphate group

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4
Q

What do receptor tyrosine kinases (class 1) do?

A

They phosphorylate tyrosine residues on a specific set of substrates

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5
Q

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

A

Secreted growth factors and hormones

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

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6
Q

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

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

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

A

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

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8
Q

Define: Oligomerisation

A

The formation of an oligomer from a monomer

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9
Q

Define: Oligomer

A

Molecule consisting of 2-100 repeating units

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10
Q

Define: Dimerisation

A

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

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11
Q

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

A

Causes the intracellular domain to have kinase activity = a kinase

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12
Q

How does autophosphorylation cause biological effects?

A

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

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13
Q

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

A

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)

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14
Q

Describe the structure of an insulin receptor

A

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

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15
Q

How does signalling through the insulin receptor occur?

A

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

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16
Q

What is the role of Akt?

A

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

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17
Q

Describe Tyrosine Kinase-Associated Receptors (Class 2)

A

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

18
Q

What are cytosolic tyrosine kinases?

A

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

19
Q

What class are cytokine receptors?

A

Class 2: Tyrosine Kinase-Associated Receptors

20
Q

What are cytokine receptors?

A

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

21
Q

What are cytokines?

A

Large family of signalling molecules

e.g. chemokines, interleukins and tumour necrosis factor

22
Q

What is the role of cytokines?

A

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

23
Q

What is tumour necrosis factor?

A

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

24
Q

Explain the process of apoptosis by TNF-α

A

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

25
Q

What is the NFkB pathway?

A

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)

26
Q

What is the purpose of anti- TNF therapies?

A

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

27
Q

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

A
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
28
Q

Name class 3 of enzyme-coupled receptors

A

Receptor serine/threonine kinases

These receptors will be phosphorylated on serine and threonine residues

29
Q

Where are receptor serine/threonine kinases found?

A

Cytosol

30
Q

What is TGFβ?

A

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)

31
Q

What is the role of TGFβ?

A

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

32
Q

How does TGFβ carry out its role?

A

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

33
Q

What are class 4 enzyme coupled receptors called?

A

Receptor Guanylyl Cylases

34
Q

What are receptor guanylyl cyclases?

A

Single transmembrane proteins with an extracellular domain for agonists binding

35
Q

How do receptor guanylyl cyclases have their effect?

A

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

36
Q

In summary, what do agonists of ECRs often act as

A

ECR agonists often act as:
Dimers
Trimers
Oligomers

37
Q

What is required for the activation of signalling cascades?

A

Dimerisation or oligomerisation is required for the activation of signalling cascades

38
Q

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

A

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

39
Q

What do most signalling cascades lead to?

A

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

40
Q

What is the role of transcription factors?

A

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