M&R S7 - Signal Transduction in Biological Membranes Flashcards Preview

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Flashcards in M&R S7 - Signal Transduction in Biological Membranes Deck (45):

What is signal transduction?

A process by which an extracellular signal (ligand binding to a receptor) can bring about an internal response in the cell


What are G-proteins?

A receptor superfamily that act by altering the activity of effectors (E.g. Ion channels, enzymes)

This is done through the activation of one or more types of Guanine nucleotide binding proteins (G-proteins)


What are some of the cellular functions controlled by G-protein receptors?

Muscle contraction
Light, smell and taste perception
Metabolic processes


Describe the structure of G-proteins

Heterotrimeric (3 distinct subunits)

Alpha, beta and gamma subunits bind tightly to one another and function as a single unit

Alph subunit has a guanine nucleotide binding site which binds to GTP and slowly hydrolyses it


In what form is G-protein found in basal conditions? (receptor inactivated)

Where is it found?

In heterotrimeric form

Bound to GDP

Found at the inner face of the plasma membrane


What happens to G-proteins when G-protein receptors are activated?

Activated receptors have a high affinity for GDP bound G-proteins and they will bind

A protein-protein interaction occurs leading to the release of GDP and the G-proteins subsequently binding to GTP

Once GTP bound, receptor affinity falls and GTP-alpha and beta-gamma are released separately to interact with effectors


By what mechinism is the interaction between effector and G-protein terminated?

Terminated by the intrinsic GTPase activity of the Alpha subunit

Once GTP is hydrolysed the affinity of the alpha subunit for the Beta-gamma subunit is increased and they will reform the heterotrimer to await reactivation


Why can G-proteins be considered as on/off switches?

What else can they be considered as?

GDP to GTP exchange and GTP hydrolysis can be considered the on/off switches

Can also be considered as timers

The length of time taken for GTP hydrolysis governs length of effector activation


G-protein activation can have one of two effects on the effector, what are these effects?



Give some examples of different G proteins



What G protein is activated when noradrenaline binds to Beta-adrenoceptors and what is the effect?

Gs (Alpha s-GTP)

Stimulates adenylyl cyclase


How might adenylyl cyclase activity be inhibited?

Noradrenaline binds to Alpha2 adrenoceptors


Ach binds to M2 cholinoceptors

The G-protein Gi is activated

Gi inhibits adenylyl cyclase


What does adenylyl cyclase do?

Creates cAMP


What two G-proteins bind to an effector other than adenylyl cyclase?



What is the action of Gq?

Stimulates Phospholipase C to cleave Posphatidylinositol-4,5-bisphosphonate (PIP2)

This results in Inositol-1,4,5-triphosphate (IP3) and Diacylglycerol (DAG) being produced


Where is rhodopsin found and what does it do?

In the eye, retinal photoreceptive cells (rods and cones)

It's a G-protein receptor that activates Gt

This in turn activates a phosphodiesterase enzyme that hydrolyses cGMP to 5'-GMP


For each G protein give:
- An example receptor
- It's effector and action on that effector
- An example physiological response to the effector in this case

- B-adrenoceptor
- Stimulates adenylyl cyclase
- Stimulates glycogenolysis, lipolysis

- M3 muscarinic
- Stimulates phospholipase C
- Smooth muscle contraction

- M2 muscarinic
- Inhibits adenylyl cyclase and stimulates K+ channels
- Slowing of cardiac pacemaker cells

- Rhodopsin
- Stimulates cGMP phosphodiesterase
- Visual excitation


Give a list of Adrenergic and muscarinic receptors and their G proteins along with that G protein's action on its effector


A1 - Gq - Stimulate phospholipase C

A2 - Gi - Inhibit adenylyl cyclase

B1+B2 - Gs - Stimulates adenylyl cyclase


M1+M3 - Gq - Stimulates Phospholipase C

M2 - Gi - Inhibits adenylyl cyclase


How many G-protein subunits are coded for in the human genome?

How many possible combinations of G proteins are there?

20 G-alpha
5 G-beta
12+ G-gamma

Over 1000 G-alpha-beta-gamma combinations


How many G-protein receptor types are there?

How specific is their action?

At least 800

Can interact with different G-alpha subtypes to activate/inhibit 10 or more effectors (enzymes/ion channels)


Why can an extracellular signal binding to a GPCR bring about specific cellular responses?

A specific GPCR will activate a single or small sub-population of G-proteins and effectors

Due to the large numbers of different receptor types and G-protein subtypes this will bring about a very specific response


Describe the action of Cholera toxin on the body from a biochemical standpoint

CTx will ADP-ribosylate the s-alpha subunit of Gs

This eliminates the GTPase activity of Gs-alpha and it becomes irreversible activated


Describe the action of Pertussis toxin on the body from a biochemical standpoint

PTx will ADP-ribosylate the i-alpha subunit of Gi

This interferes with the GDP/GTP exchange on Gi-alpha and it becomes irreversibly inactivated


What two conditions can be caused by loss of function mutations to GPCRs?

Name the GPCR that mutates in each case

Retinitis pigmentosa:
- Loss of function mutation to rhodopsin

Nephrogenic diabetes insipidus:
- Loss of function mutation to V2 vassopressin receptor


What condition can be caused by gain of function mutation to a GPCR?

What is the GPCR in question?

What is meant by 'gain of function' in this case?

Familial male precocious puberty:
- Gain of function mutation to the luteinising hormone receptor

The receptor is active without a ligand


How does adenylyl cyclase activity bring about increased/decreased cellular activity?

Hydrolyses ATP to generate cAMP which interacts with cAMP-dependent protein kinase (PKA)

PKA phosphorylates a variety of proteins within the cell to affect activity


How is adenylyl cyclase activity controlled?

Gi binding inhibits
Gs binding stimulates


Give some examples of the effects of adenylyl cyclase activation

Increased glycogenolysis and gluconeogenesis in the liver

Increased lipolysis in adipose tissue

Relaxation of some smooth muscle types

Positive ionotrophic and chronotrophic effects on the heart


Describe how Phospholipase C activation can affect cellular activity

Cleaves Posphatidylinositol-4,5-bisphosphonate (PIP2)

This results in Inositol-1,4,5-triphosphate (IP3) and Diacylglycerol (DAG) being produced

IP3 interacts with specific intracellular receptors (IP3 receptors) on the ER to allow Ca2+ to leave the ER lumen

DAG interacts with protein kinase Cs


Give a list of GPCRs that can activate Phospholipase C and their ligands

What G protein is responsible for Phospholipase C activation?

M1+3 muscarinic (Ach)

H1 receptors (Histamine)

5-HT2 receptors (5-HT aka. serotonin)


What physiological effects can Phospholipase C activation lead to?

Vascular, GI tract and airway smooth muscle contraction

Platelet aggregation

Mast cell degranulation (whatever that means)


What is the physiological effect of increased or decreased rhodopsin activity and what controls the level of rhodopsin activation?

Activated by a photon

In the dark, there is less cGMP breakdown and levels remain sufficient to open a secondary messenger controlled ion channel to allow Na+ and Ca2+ into the cell cytoplasm

On exposure to light there is more cGMP breakdown leading to decreased cGMP and the closure of the ion channel and membrane hyperpolarisation, thus altering output to the CNS


What are the secondary messengers that exert their effect via protein kinases?

For each, name the protein kinase they activate

- cAMP-dependent protein kinase (PKA)

- cGMP-dependent protein kinase (PKG)

- Protein kinase C (PKC)

- Ca2+/calmodulin-dependent protein kinase (CaM-kinase)


How do protein kinases activated by secondary messengers change cellular activity?

Cause phosphorylation of a distinct family of target proteins

These proteins may be enzymes, ion channels, transporters, structural proteins etc.

Their activities may be increased or decreased or unaltered by this covalent modification


Protein kinases activated by secondary messengers commonly phosphorylate what amino acid residues in the target proteins?

Serine or threonine


Give an example of regulation of Chronotropy in the heart by GPCRs

Ach release from parasympathetic nerves will bind to M2 muscarinic GPCRs on sinoatrial node cells

This results in the activation of Gi which increases the open probability of K+ channels

Increased membrane permeability to K+ causes hyperpolarisation resulting in a negative chronotropic effect


Give an example of ionotropic regulation in the heart by GPCRs

Sympathetic innervation (leading to release of noradrenaline) of the cardiac ventricles or circulation adrenaline will lead to activation of beta-adrenoceptors (mostly B1)

This results in activation of Gs which in turn activates adenylyl cyclase and leads to intracellular increase in cAMP levels

cAMP binds to PKA and causes phosphorylation of voltage operated Ca2+ channels (VOCCs)

which increased the open probability the VOCCs

The increase in intracellular Ca2+ brings about positive ionotropic effect (increase in force of contraction)


Explain how arteriolar vasoconstriction is brought about by GPCR activation

Sympathetic release of noradrenaline activates A1-adrenoceptors in the arterial smooth muscle

This activates Gq which stimulates Phospholipase C activity

Phospholipase C produces IP3 which releases Ca2+ from the ER and initiates contractile response


Give an example of how GPCR activation can lead to modulation of neurotransmitter release

Pre-synaptic u-opioid receptors can be stimulated by opioids (endogenous or analgesics)

This activates Gi and the beta-gamma subunit released from Gi interacts with Voltage operated Ca2+ channels (VOCCs) to reduce Ca2+ entry into the cell

This reduces neurotransmitter release


Why must GPCR - G-protein - Effector system allow for amplification?

A small signal (activation of a few GPCRs) requires amplification to generate a large intracellular response


By what mechanisms is the GPCR - G-protein - Effector system amplified?

Activated GPCR can activate more than one G-protein

Activated G-protein subunits (either alpha- GTP or beta-gamma subunits) can activate multiple effector molecules

Enzyme effectors can create/breakdown 100 - 1000s of secondary messengers

Ion channel effectors allows 100 - 1000s of ions to cross the plasma membrane

Secondary messengers often activate enzymes which can convert 100 - 1000s of molecules or initiate enzyme cascades


Over what time scale do activation/deactivation of signalling pathways in a cell occur?

Rapid, often a few seconds


What aspects of the GPCR signalling pathway facilitate deactivation?

Productive interaction of GPCR and G-protein weakens the GPCR-agonist binding, agonist dissociation more likely to occur

While activated the GPCR is vulnerable to protein kinases that phosphorylate the receptor and prevent it activating further G-proteins (receptor desensitisation observed in most GPCRs)

Active lifetime of a GTP-alpha G-protein subunit may be limited by cellular factors which stimulate the GTPase activity of the alpha subunit

Cells contain high levels of enzymes which metabolise secondary messengers

Enzymes/protein kinases activated by secondary messengers have their activities opposed or reversed


Give two examples of how a cell might metabolise secondary messengers

cAMP metabolised to 5'-AMP by phosphodiesterases

IP3 metabolised to inactive IP2 (inositol-1,4-bisphosphate) by 5-phosphatase activity


Give an example of how a cell might oppose the effects of secondary messengers

Target protein phosphorylation by protein kinases reversed by active cellular protein phosphatase activity