Second messengers - cyclic nucleotides and role of G-proteins Flashcards
(26 cards)
What is the general function of second messengers
allow for signal amplification
Activation of adenylyl cyclase creates many cAMP’s inside the cell – these activate many cAMP dependent protein kinases
What has an important role in generating second messengers
The cell membrane
What are key proteins that G-proteins can activate and what are their functions
Some G proteins activate adenylyl cyclase which increases intracellular cyclic AMP (cAMP) and activation of cellular kinase (heterotrimeric)
Some activate the enzyme phospholipase C – this increases intracellular inositol triphosphate (IP3) and diacyglycerol (DAG)
G protein associated Ras proteins (monomeric) transmit the signal from the single pass membrane protein (tyrosine kinase)
What does cAMP stand for
Cyclic adenosine monophosphate
How is cAMP formed in the cell
The cyclisation is an intramolecular attack on the 3’-OH of the ribose forming a phosphodiester bond
Energy release from the pyrophsophate (PPi) drives this reaction as it is thermodynamically favourable
What is the importance of ATP
It is cellular energy
The hydrolysis of ATP to ADP yields a favourable Gibbs-free energy of –7.3 cal/mol
What is adenylyl cyclase and how is it activated
Is an enzyme that coverts ATP to cAMP
Signals (like hormones) bind to GPCRs at the cell surface which activate the heterotrimeric G protein
The α-subunit of the activated G protein can interact with/activate adenylyl cyclase which converts ATP –> cAMP
Adenylyl cyclase is a membrane associated enzyme which is involved in the generation of cAMP
What domains does adenylyl cyclase have
Transmembrane and catalytic domains
What is cAMP’s function in cell signalling
Acts as an intracellular secondary messenger to a wide range of hormones, neurotransmitters and other signalling substances
Where is cAMP released from
It is released by the catalytic domain of adenylyl cyclase in the cytosol
How long will cAMP be produced for
It will be produced until adenylyl cyclase is switched off by lack of signal or by inhibitory G protein
What does cAMP do to amplify a signal
Cyclic AMP binds to and activates cyclic AMP dependent protein kinase (or PKA)
Usually cAMP levels increase due to adenylyl cyclase amplifying the signal
The produced cAMP activates PKA which then interacts with other
How is cAMP regulated
The generation and control of cAMP can occur at the signal/G protein level
Stimulatory signals – adrenaline activates the Gαs-subunit
Inhibitory signals – adenosine activates the Gαi-subunit
What is the function of phosphodiesterases (PDEs) in cAMP homeostasis
Cyclic AMP signalling control
The elevated cAMP must be removed once the signal has stopped
This family of enzymes converts cAMP to AMP
What happens if mutations in PDE genes occur
any mutations or dysfunctions in PDE genes is linked with the development of disease
Any mutation the PDE genes that affect enzymatic function means cAMP homeostasis could be lost resulting in disease like cancer
Due to importance of cAMP signalling
How does cholera toxin cause cholera
The toxin binds to the GM1 (ganglioside receptor M1) - this is expressed on the surface of the gut epithelial cells
Binding activates - Gα —> AC —> cAMP
What occurs after the binding of cholera toxin
Retrograde exocytosis of the cholera toxin occurs in these cells
A portion of the cholera is cleaved which then activates Gα then adenylyl cyclase then cAMP
Why does PKA do during cholera infections after cAMP production
PKA phosphorylates the CFTR which triggers the efflux of chloride ions into the gut lumen
How does the efflux of Cl- cause dehydration in cholera patients
Passive movement of water by osmosis to restore balance causes dehydration
What is the overall mechanism of action for cholera toxins
Therefore, the mechanism of action of cholera toxin is to activate adenylyl cyclase in gut epithelial cells and causes the production of cAMP, causing the loss of homeostasis
What are the 2 pathways cGMP is generated
The de novo pathway
The salvage pathway
What is GTP similar to
GTP is built from the purine base guanine and is very similar to ATP structurally
What is responsible for generating cGMP
Guanylyl cyclase
What are 2 forms of guanylyl cyclase
one is soluble and the other is membrane bound
