Lecture 3.1: G-Protein Coupled Receptors (GPCRs) Flashcards
What is Cell Signalling?
The ability to perceive and respond appropriately to ‘information’ in the environment
Principles of Biological Signalling (4 Steps)
1) Reception
2) Transduction
3) Response
4) Amplification
What are GPCRs?
Membrane bound receptors that utilise G proteins in the transduction process
How many GCPRs does a human have?
800-1000 GPCRs
What signals do GCPRs transduce? (4)
- Sensory Information (phototransducers, taste)
- Hormones
- Neurotransmitters
- Growth Factors
What processes do GCPRs play mission critical roles in? (7)
- Neurotransmission
- Blood Pressure
- Regulation
- Vascular Function
- Glucose Metabolism
- Immune Response
- Developmental Processes
Why are GPCRs important in terms of drugs?
- Have been identified as drug targets
- 1/3 of prescribed drugs act at GPCRs
What drugs act at GCPRs? (5)
- Angiotensin receptor blockers
- Bronchodilators
- Anti-histamines for allergy
- H2 receptor antagonists
- GPCRs in oncology as they regulate a number of
signalling pathways in cancer cells
General Structure of GCPRs
- Extracellular N terminus
- Intracellular C terminus
- 7 transmembrane α-helices
- 3 intracellular and 3 extracellular loops
Classification of GCPRs: 2 Types
- Class A-F
- GRAFS System
Class A of GCPRs
Rhodopsin like (includes neurotransmitters and hormones)
Class B of GCPRs
Secretin Receptor Family
Class C of GCPRs
Metabotropic Glutamate Receptors
Class D of GCPRs
Fungal Mating Pheromone Receptors
Class E of GCPRs
cAMP Receptors
Class F of GCPRs
Frizzled and Smoothened Receptors
GRAFS System
Glutamate
Rhodopsin
Adhesion
Frizzled
Secretin
What 3 subunits are G-proteins are made of?
α (alpha)
β (beta)
γ (gamma)
How do the α & γ subunits of G-proteins interact with the cell membrane?
They are covalently bound via a range of lipid moieties to the membrane – v.complex!
At rest GDP (guanosine diphosphate) binds
to what subunit?
α-subunit
When a ligand binds to the receptor GTP
binds to the α-subunit, what does this cause?
Dissociation of the trimer into the α-subunit and a β-γ dimer occurs
How many genes code for α?
20
How many genes code for β?
6
How many genes code for γ?
12
How many permutations of G-proteins are possible?
c.1400
G-Protein Subtype Gs: Effector? Second Messenger?
Effector: Adenylyl cyclase (+)
Second Messenger: cAMP ↑
G-Protein Subtype Gi: Effector? Second Messenger? (3 of each)
Effector:
* Adenylyl cyclase (-)
* K+ channel(+)
* VOCC Ca2+ channel(-)
Second Messenger:
* cAMP↓
* K+↓
* Ca2+ ↓
G-Protein Subtype Gq: Effector? Second Messenger?
Effector: Phospholipase C (+)
Second Messenger: DAG & IP3 → Ca2+↑
G-Protein Subtype Gt: Effector? Second Messenger?
Effector: cGMP phosphodiesterase (+)
Second Messenger: cGMP ↓
G-Protein Subtype Golf: Effector? Second Messenger?
Effector: Adenylyl cyclase (+)
Second Messenger: cAMP ↑
What is another name for the G-Protein Subtype Gt? Why is it important?
- Also called transducin
- Linked to rhodopsin i.e. it transducers photons to
action potentials
What is another name for the G-Protein Subtype Golf? Why is it important?
- Also gustducin
- Involved in the transduction of taste (bitter, sweet
and umami)
What are protein kinases?
Enzymes that when activated phosphorylate substrates
What does PKA (Protein Kinase A) target? (3)
- Phosphorylates phosphorylase kinase which in turn
phosphorylates glycogen phosphorylase leading to
glycogen breakdown - Phosphorylation of Cl- channels in small intestine (Cl-
enters lumen, Na+ enters lumen = H2O entering n
lumen) - Phosphorylation of transcription factors (e.g. CREB),
that when phosphorylated are able to initiative
transcription)
How does adrenaline mobilise glycogen breakdown in muscle cells? (6 Steps)
- Chemical Signal: Adrenaline
- Receptor Activation: β-adrenoreceptor
- Signal Transduction: Dissociation of G-Protien,
Activation of adenylyl cyclase - Message Generation: ↑ cyclic AMP
- Enzyme Activation: Activation of PKA, Phosphorylation
of glycogen phosphorylase - Biological Response: Glycogen breakdown
Why are Phosphodiesterases important?
- They promote the conversion of cAMP → AMP
(analogous for cGMP) - Their role in ‘switching off’ signalling pathways offers
the potential for targeting them with drugs
Theophylline can inhibit type IV Phosphodiesterases, what effect does this have?
Airway relaxation
Sildenafil (Viagra) inhibits type ….. Phosphodiesterases, & therefore promotes the vasodilatory effects of
cGMP?
V
Activation of a muscarinic receptor by acetylcholine
Slide 22
What is signal amplification?
A few molecules of an agonist may evoke significant
intracellular responses
Monomeric G-Proteins
Often called Small GTPases (to distinguish them from GPCRs) and consist of a single unit (~Gα subunit)
What do Monomeric G-Proteins bind at rest?
At rest bind GDP, free in cytoplasm
How do monomeric G-proteins work? When are they active?
Only active when GTP bound (therefore inactivated by GAPs and activated by GEFs)
Ras the monomeric G-protein, why is it important?
- Important role (via PKs) in cytoskeletal reorganisation,
cell polarity, cell cycle progression, gene expression - Mutations in Ras genes associated with some 30% of
tumours - Three types of Ras in humans considered to be
oncogenes - Quite often mutations occur in the GTPase region – G-
protein always on (development of GAPs as tumour
suppressors?)
The process of G-protein insertion and G-Protein degradation is known as…?
G-protein trafficking
What are GPCRs regulated by?
G-protein receptor kinases (GRKs)
How do GRKs regulate GCPRs?
- Phosphorylation of activated (ligand bound) GPCRs by
GRKs allows the binding of β-arrestin proteins - Β-arrestins act as scaffolds for endocytosis leads to
transient receptor internalisation
