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What are G protein coupled receptors?

A family of receptors that act by altering the activity of effectors eg enzymes or ion channels.


How do G protein coupled receptors achieve their responses?

There is activation of one or more types of G proteins that allows a diverse range of cellular functions. Including muscle contraction and metabolic processes.


What is the structure of G proteins?

Heterotrimeric, they have 3 subunits - alpha, beta and gamma. The alpha site has a guanine nucleotide binding site to bind GTP. The beta and gamma subunits bind tightly to each other so function as one unit.


Under basal conditions, what form is the G protein ususally in?

The inner face of the plasma membrane in its heterotrimeric form. GDP is bound to the alpha subunit.


What happens when an activated receptor binds with the G protein?

It will have a high affinity for the G protein in its GDP state. A protein - protein intereaction then occurs which causes GTP to bind instead of GDP. (Causes the receptors affinity for the subunits to decrease so the subunits can interact with effectors)


What terminates effect interactions caused by G protein coupled receptors?

The intrinsic GTPase activity of the alpha subunit hydrolysing GTP to GDP. This causes the affinity of the alpha and beta gamma subunit to increase and the heterotrumer is reformed ready for reactivation.


How can G proteins be thought of as timers?

By the amount of time it takes for the GTP to hydrolyse.


What does G protein stimulate in the production of cAMP from adrenaline?

It has an intermediate role of stimulating adenylyl cyclase to produce cAMP. Also has additional effects independent of adenylyl cyclase stimulation.


What does G protein inhibit to reduce the levels of cAMP?

Inhibitory pathways that reduce cAMP by inhibiting adenylyl cyclase.
Also have additional effects independent of adenylyl cyclase


What receptor detects nor/adrenaline and what effect does this have? Include the G protein.

Beta Adrenoceptor, Gs G protein. It stimulates Adenylyl Cyclase and the respinse is Glycogenolysis or lipolysis


The M3 Muscarinic receptor is activated by Acetylcholine. What G protein is used and what is the effects?

Gq and the effects is that phospholipase C is stimulated which results in smooth muscle contraction.


The M2 Muscarinic receptor is activated by Acetylcholine. What G protein is used and what is the effects?

Gi which inhibits Adenylyl cyclase and stimulates K+ channels. This causes a slowing of the cardiac pacemaker.


What receptor does Light effect and what does this cause? Include the G protein.

The G protein is Gt, the light stimulates the Rhodopsin receptors and stimulates cyclic GMP and phosphodiesterase. This causes visual excitation.


How can an extracellular signal bring about a specific cellualr response using G proteins?

It can work via a specific GPCR that can activate a single or small sub population of G proteins and effectors in the cell due to there being huge diversity amongst G protein coupled receptors.


What does Cholera toxin and Pertussis toxin do to G proteins?

They are enzymes that ADP ribosylate specific G proteins.


How can Cholera toxin irreversibly activate Gs alpha?

It eliminates the GTPase activity of Gs alpha so it becomes irreversibly activated.


How can Pertussus toxin irreversibly inactivate Gi alpha?

It interferes with the GDP/GTP exchange on Gi alpha so it becomes irreversibly inactivated.


What causes Retinitis Pigmentosa?

A loss of function mutation in Rhodopsin.


What causes Nephrogenic Diabetes Insipidus?

Loss of funcgtion mutation in V2 vasopression receptor.


What causes Familial Male Precocious Puberty?

A gain of function mutation (Where the receptor is active without a ligand) to the LH receptor.


What are some examples of diseases caused by G protein coupled receptor mutations?

Retinitis Pigmentosa
Nephrogenic Diabetes Insipidus
Familial Male Precocious Puberty


What does Adenylyl Cyclase do?

Hydrolysis of cellular ATP to generate cyclic AMP. This the interacts with a specific protein kinase which then phosphorylates a variety of other proteins within the cell (to effect activity)


What does Phispholipase C do?

It is an enzyme that hydrolyses the membrane phosphilipid PIP2 to IP3. IP3 can then interact with specific intracellular receptors on the ER to allow Ca2+ to enter the cytoplasm.


What does Cyclic GMP Phosphodiesterase do?

In photoreceptive cells of the retina, regulates the breakdown of the second messenger cyclic GMP phosphodiesterase by Gt (transduction)


How can G protein coupled receptor pathways be deactivated?

1) Agonist receptor dissociation occurs.
2) While activated the receptor is phosphorylated by a variety of protein kinases that prevent it activating more G proteins. - receptor desensitisation.
3) May be that there are cellular factors that limit the lifetime and stimulate GTPase activity.
4) Cell contains high activity enzyme to metabolise messengers to return levels to basal.
5) Enzymatic cascades activated downstream of messenger/protein kinase activation to oppose their effects.


What are some examples of signalling pathways?

Regulation of chronotropy in the heart
Regulation of inotropy in the heart
Arteriolar Vasoconstriction
Modulation of Neurotransmitter release


What is the predominant receptor type of receptor in the heart for Ach detection?
Extra point - What does activation of these cause?

M2 muscarinic cholinoceptor.
Extra point - Increase plasma membrane permeability to K+ by having K+ channels open. This causes hyperpolarisation and slows the intrinisic firing rate so negative chronotropic effect.


Activation of what receptor causes regulation of Inotropy in the heart?
Extra point - How does it do this?

Activation of Beta adrenoceptors (Mainly Beta 1) increases the open probability of voltage gated calcium channels via Gs. Gs can interact with the VOCCs or with cAMP to PKA to phosphorylation (and activation) of VOCCs. All of this brings an influx of calcium and positive inotropic effect.


What is the biochemical basis of arteriolar vasoconstriction?

Sympathetic release of noradrenaline will act on alpha 1 adrenoceptors to stimulate phospholipase C and IP3 production via Gq. IP3 then causes Ca2+ to be released from the ER and initiates a contractile response.


How can G protein coupled receptors influence Neurotransmitter release?

Pre synaptic G protein coupled receptord can eg Pre synaptic mew-opiod receptors can be stimulated and the subunits liberated from the heterotrimer interact with VOCCs to reduce Ca2+ entry so less neurotransmitter can be released.