Calcium & G-coupled Receptors Flashcards
In regards to the movement of ions, how an Na+ K+ ATPase functions?
3 Na+ ions out, 2 K+ ions in
Describe, in simple terms, the structure of PKA.
A combination of 4 subunits - 2 r subunits to which cAMP binds, and 2 c subunits which are the catalytic entity
List 3 ion transporters used to remove Ca2+ from the cell. Are these ion transporters passive or active transporters?
Plasma membrane Ca2+ ATPase (PMCA) - active
Smooth Endoplasmic Reticulum Ca2+ ATPase (SERCA) - active
Na+ Ca2+ Exchanger (NCX) - passive (antiporter)
What effect do agonists have on a receptor? In that case, what effect do antagonists have on a receptor?
Agonists activate a receptor - antagonists do not activate a receptor
Describe the structure of a G-protein coupled receptor?
A single polypeptide chain spanning the membrane 7 times - it has an extracellular N-terminus and an intracellular C-terminus
Describe the steps explaining how a substrate may activate a G protein.
The substrate binds the G protein-coupled receptor at either the N-terminus or by burying itself in the membrane of the receptor - this causes a conformational change in the receptor which releases the G protein - this causes the G protein to release GDP and bind GTP
What type of G-protein activates phospholipase C? What does phospholipase C catalyse?
Phospholipase C is actives by a Gq-type G-protein, and catalysts the cleavage of PIP2 to IP3 and DAG
What type of G-protein activates adenylyl cyclase? What type inhibits it?
Adenylyl cyclase is activated by Gs-type G-proteins, and is inhibited by Gi-type G-proteins
What molecules make up a G-protein?
A G-protein contains an alpha, beta, and gamma subunit (is heterotrimeric) - the beta and gamma subunits functionally act as one however
What cation is required for neurotransmitter release?
Ca2+ entry
What is the neuromuscular junction?
The synapse between a nerve and a skeletal muscle fibre (cell)
Are Ca2+ levels higher intracellularly or extracellularly?
Extracellularly
What is needed to increase the amount of Ca2+ entry at a nerve terminal?
Increase in frequency (as opposed to amplitude) of action potentials
What is the pore-forming subunit of a voltage-gated Ca2+ channel?
A1 (alpha 1) subunit
How many subunits does a voltage-gated Ca2+’channel have? How does this relate to a voltage-gated Na+ channel?
Both have 4 subunits, and share a very similar structure
What molecules can block L-type Ca2+ channels? Can you give 1 example?
DHP (dihydropyridines) e.g. nifedipine
What type,of Ca2+ channels are found in the heart?
R-type and T-type
How does increased intracellular Ca2+ affect Ca2+ channels?
Increased intracellular Ca2+ channels block the activity of Ca2+ channels
How do voltage-gated Ca2+ channels act in comparison to voltage-gated Na+ channels?
They act in a slower manner
What type of Ca2+ channels are found in skeletal muscle?
L-type
What breaks down acetylcholine in the synaptic cleft?
Acetylcholinesterase
What molecule brings Ca2+ vesicles to the pre-synaptic member ae to be released?
Synaptotagmin
What complex is associated with fusion of the Ca2+ vehicle to form a pore in the pre-synaptic membrane?
The SNARE complex
What molecules does a nicotinic acetylcholine receptor let pass through its pore?
Na+ and K+
How many molecules of acetylcholine are required to induce a conformational change and open a nicotinic acetylcholine receptor?
2 molecules of acetylcholine
If a nicotinic acetylcholine receptor lets through both Na+ and K+, which does Na+ flood into the cell and not K+?
The resting membrane potential of a cell is much closer to the equilibrium potential of K+ - as it is much further from the equilibrium potential of Na+, Na+ floods into the cell in an effort to reach equilibrium, which leads to depolarisation of the cell
State an example of a competitive nicotinic acetylcholine receptor blocker.
Tubocurarine
How can a competitive blocker of a nicotinic acetylcholine receptor (such as tubocurarine) be overcome?
Increasing the amount of acetylcholine (or the receptors endogenous substrate)
How do depolarising blockers act at nicotinic acetylcholine receptors?
They act to keep nicotinic acetylcholine receptors in a depolarised state, thereby deactivating adjacent Na+ channels meaning an action potential cannot propagate
Can you list an example of a depolarising nicotinic acetylcholine receptor blocker?
Succinylcholine
Explain the pathogenesis of myasthenia gravis.
Myasthenia gravis is an autoimmune disorder where antibodies attack nicotinic acetylcholine receptors on skeletal muscle - these inactivated nicotinic acetylcholine receptors lead to a reduced amplitude of end-plate action potentials, leading to muscle weakness and fatigue
How to nicotinic acetylcholine receptors and muscarininc acetylcholine receptors differ? How does this relate to their response?
Muscarininc receptors a G protein-coupled receptors, while nicotinic receptors are ligand-gated receptors - muscarininc acetylcholine receptors therefore act in a slower manner
How does Ca2+ binding to a protein instigate a change in the proteins function?
Through a conformational change
What are the approximate values of Ca2+ in the extracellular, cytoplasmic, and in the endoplasmic reticular compartments?
- extracellular = 1-2x10^-3 M
- cytoplasmic = 1x10^-7
- endoplasmic reticulum = 2-3x10^-4 M
List 3 transporters that are used in removing Ca2+ from the intracellular compartment of the cell.
- NCX - Na+ Ca2+ exchanger
- PMCA - plasma membrane Ca2+ ATPase
- SERCA - smooth endoplasmic reticulum Ca2+ ATPase
What receptors instigate Ca2+ induced Ca2+ release?
Ryanodine receptors - these are found in the membrane of the ER, and act to further increase Ca2+ concentrations in the cytosol, as a result of an increase in the cytosolic Ca2+
How may activation of G protein-coupled receptors in the cell membrane lead to an influx of intracellular Ca2+?
Gq proteins activates phospholipase C which goes on to cleave PIP2 into IP3 and DAG - IP3 then binds IP3 receptors on the membrane of the ER, which leads to efflux of Ca2+ from the ER into the cytoplasm
In normal muscle activity, when stimulation to contract stops, how are resting levels of Ca2+ regained?
The SER ATPase pump pumps Ca2+ back into the cell
Briefly outline how calcium buffers act to regulate cytosolic Ca2+ following increased levels in the cytoplasm.
Ca2+ buffers can bind up to 50 Ca2+ ions, reducing the spread of Ca2+ current from a channel, meaning it acts less on its effectors
List a Ca2+ buffer found in the cytosol, sarcoplasmic reticulum, and endoplasmic reticulum.
- cytosol - parvalbumin
- SR - calsequestrin
- SER - calreticulin
Describe the role of a Ca2+ sensor/trigger protein.
The Ca2+ sensor/trigger protein acts as an intermediate, binding Ca2+ and mediating its effects by then acting on its targets proteins itself
Give an example of a Ca2+ sensor/trigger protein, briefly describing how it acts, and why this is important.
Calmodulin - this binds 4 Ca2+ ions, which induces a conformational change in calmodulin - this is important, as many of the Ca2+ targets can’t actually bind Ca2+ themselves
Give a specific molecule that calmodulin acts on, and how it affects its activity.
Calmodulin can directly affect the activity of the PMCA (plasma membrane Ca2+ ATPase), increasing its sensitivity to Ca2+ when calmodulin is bound
How many subunits make up a G protein? What is interesting about their arrangement?
3 - an alpha, beta, and gamma subunit - the alpha subunit acts in dependent lay, while the beta and gamma subunits act together
What molecule binds a G protein to induce its active state?
GTP
How is a G protein turned off?
The alpha subunit of a G protein contains intrinsic GTPase activity, which hydrolyses GTP to GDP, and switches off the G protein
What is the second messenger of adenyl cylase? What is the secondary messenger of phospholipase C?
- adenyl cyclase - cAMP
- phospholipase C - IP3/DAG
Which molecule does cAMP binding regulate?
Protein kinase A
What happens in Gi activation?
The G protein would not release GDP and bind GTP, so would not be activated, and you would get no adenyl cyclase, cAMP, or protein kinase A activity
What molecule does adenyl cyclase act on to produce cAMP?
ATP
Why is signal amplification so widely used in the body?
Good economic use of a few ligands to produce a much greater response
What is ionotropy?
The force of muscle contraction
What type of receptor is a u-opioid receptor?
A G protein-linked receptor
