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► Med Misc 01 > M&R S7 - Signal Transduction in Biological Membranes > Flashcards

M&R S7 - Signal Transduction in Biological Membranes Flashcards

1
Q

What is signal transduction?

A

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

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2
Q

What are G-proteins?

A

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)

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3
Q

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

A

Muscle contraction
Light, smell and taste perception
Metabolic processes
Secretion

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4
Q

Describe the structure of G-proteins

A

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

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5
Q

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

Where is it found?

A

In heterotrimeric form

Bound to GDP

Found at the inner face of the plasma membrane

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6
Q

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

A

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

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7
Q

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

A

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

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8
Q

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

What else can they be considered as?

A

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

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9
Q

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

A

Activation

Inhibition

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10
Q

Give some examples of different G proteins

A

Gs
Gi
Gq
Gt

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11
Q

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

A

Gs (Alpha s-GTP)

Stimulates adenylyl cyclase

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12
Q

How might adenylyl cyclase activity be inhibited?

A

Noradrenaline binds to Alpha2 adrenoceptors

OR

Ach binds to M2 cholinoceptors

The G-protein Gi is activated

Gi inhibits adenylyl cyclase

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13
Q

What does adenylyl cyclase do?

A

Creates cAMP

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14
Q

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

A

Gq

Gt

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15
Q

What is the action of Gq?

A

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

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

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16
Q

Where is rhodopsin found and what does it do?

A

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

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17
Q

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
A

Gs:

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

Gq:

  • M3 muscarinic
  • Stimulates phospholipase C
  • Smooth muscle contraction

Gi:

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

Gt:

  • Rhodopsin
  • Stimulates cGMP phosphodiesterase
  • Visual excitation
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18
Q

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

A

Adrenergic:

A1 - Gq - Stimulate phospholipase C

A2 - Gi - Inhibit adenylyl cyclase

B1+B2 - Gs - Stimulates adenylyl cyclase

Muscarinic:

M1+M3 - Gq - Stimulates Phospholipase C

M2 - Gi - Inhibits adenylyl cyclase

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19
Q

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

How many possible combinations of G proteins are there?

A

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

Over 1000 G-alpha-beta-gamma combinations

20
Q

How many G-protein receptor types are there?

How specific is their action?

A

At least 800

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

21
Q

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

A

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

22
Q

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

A

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

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

23
Q

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

A

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

24
Q

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

Name the GPCR that mutates in each case

A

Retinitis pigmentosa:
- Loss of function mutation to rhodopsin

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

25
Q

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?

A

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

The receptor is active without a ligand

26
Q

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

A

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

27
Q

How is adenylyl cyclase activity controlled?

A

Gi binding inhibits

Gs binding stimulates

28
Q

Give some examples of the effects of adenylyl cyclase activation

A

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

29
Q

Describe how Phospholipase C activation can affect cellular activity

A

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

30
Q

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

What G protein is responsible for Phospholipase C activation?

A

M1+3 muscarinic (Ach)

H1 receptors (Histamine)

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

31
Q

What physiological effects can Phospholipase C activation lead to?

A

Vascular, GI tract and airway smooth muscle contraction

Platelet aggregation

Mast cell degranulation (whatever that means)

32
Q

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

A

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

33
Q

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

For each, name the protein kinase they activate

A

cAMP:
- cAMP-dependent protein kinase (PKA)

cGMP:
- cGMP-dependent protein kinase (PKG)

DAG:
- Protein kinase C (PKC)

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

34
Q

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

A

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

35
Q

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

A

Serine or threonine

36
Q

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

A

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

37
Q

Give an example of ionotropic regulation in the heart by GPCRs

A

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)

38
Q

Explain how arteriolar vasoconstriction is brought about by GPCR activation

A

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

39
Q

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

A

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

40
Q

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

A

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

41
Q

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

A

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

42
Q

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

A

Rapid, often a few seconds

43
Q

What aspects of the GPCR signalling pathway facilitate deactivation?

A

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

44
Q

Give two examples of how a cell might metabolise secondary messengers

A

cAMP metabolised to 5’-AMP by phosphodiesterases

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

45
Q

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

A

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

► Med Misc 01 (35 decks)

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