Transduction Systems 2 Flashcards
Are GPCR restricted to activating just one G protein?
No - one receptor may activate multiple G proteins with their own unique signalling properties
Can one G protein then only activate one effector?
No - one G protein may activate multiple effector enzymes of the same class or different classes with completely different signalling properties
How are G proteins classed?
According to which downstream effector they activate
What are the 3 main families of G-proteins?
- Gs
- Gi
- Gq
What downstream effector does the Gs family of G-proteins activate and what does this effector then do?
The effector is the enzyme adenylate cyclase which catalyses the formation of cAMP from ATP by removing two phosphates to increase cAMP levels
What downstream effector does the Gq family of G-proteins activate and what does this effector then do?
The effector is the enzyme phospholipase C which the phospholipid (PIP3) to produce the second messangers IP3 and DAG
What downstream effector does the Gi family of G-Proteins activate and what does this effector then do?
The effector is the enzyme adenylate cylase but in this case it is inhibited so it decreases cAMP level
Ultimately, what does Gs G-protein coupled signalling lead to?
protein kinase A activation which then goes to phosphorylate an enzyme called creb which can bind to DNA and regulate gene transcription
Describe the sequence of events involved in Gs coupled signalling?
- The agonist binds receptor
- The alpha subunit with GTP bound activates adenylate cyclase
- Adenylate cyclase catalyses the conversion of ATP to cAMP to increase cAMP levels
- cAMP then binds to PCA which is usually bound protein kinase A to keep it inactive - the binding of cAMP to PCA lets leads to PKA activation
- PKA can then enter the nucleus and phosphorylate an the Creb enzyme
- The Creb enzyme then joins as Creb binding protein on the DNA
- This complex then regulates gene transcription and therefore protein production
What is an example of a Gs Protein Coupled Receptors?
B-adrenoceptors
What is the agonist of B-adrenoceptors?
Adrenaline
With reference to the signalling method of Gs proteins, what effect does adrenaline have?
It leads to an increase in cAMP levels and therefore activation of proteins kinase A
There are two types of B-adrenoreceptors - what affect does adrenaline have on them?
B1-adrenoceptors are found on the heart and adrenaline therefore increases heart rate and force of contraction
B2-adrenoceptors are found on the lunhs and adrenaline causes relaxation of the bronchial smooth muscle and therefore more air can get into the lungs
What is salbutamol?
An B2 selective agonist
What is salbutamol effect? What would it be used to treat?
It relaxes bronchial smooth muscle, increases air into lung and improves respiration so is therefore used in asthma
What is atenolol?
A B1 selective antagonist
What is atenolols effect? What would it be used to treat?
Blocks the effect of adrenaline on the heart so overall It reduces heart rate and force of contraction - used to treat hypertension
What are the sequence of events involved in the signalling with a Gq protein?
- Agonist binds receptor
- GTP bound alpha subunit activates the enzyme phoshplipase C
- Phospholipase C then catalyses the breakdown of the phospholipid (PIP3) found in the membrane
- PIP3 is broken down into DAG which stays in the membrane and IP3 which leaves the membrane
- IP3 then binds to receptors on the endoplasmic reticulum and causes them to release calcium ions into the cytoplasm
- The increase in calcium ions and also the increase in DAG causes activation of protein kinase C
- The overall result is smooth muscle contraction
What is an example of a Gq Coupled Protein Receptors?
a-adrenoceptors
What is the agonist for a-adrenoceptors
Adrenaline
Where are A-adrenoceptors found and what then is the effect of adrenaline on this tissue?
Found in smooth muscle and adrenline causes muscle contraction (due to increase in calcium ions)
What is an agonist for the a-adrenoceptors in the smooth muscle?
Prazosin
What kind of antagonist is prazosin and what effect does it have on the smooth muscle?
It is a reversible competitive antagonist and therefore prevents contraction of the smooth muscle by adrenaline which causes vasodilation and therefore a reduction in blood pressure
What could prazosin be used to treat?
Hypertension
