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ESA 2- Membranes and Receptors > Signal Transduction in Biological Membranes > Flashcards

Signal Transduction in Biological Membranes Flashcards

1
Q

What are G-Protein Coupled Receptors?

A

A family of receptors

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2
Q

How do G-protein coupled receptors act?

A

By altering the activity of effectors (e.g. enzymes / ion channels)

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3
Q

How do G-protein achieve alteration of effectors?

A

Via the activation of one or more types of guanine nucleotide binding proteins (G-proteins)

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4
Q

What are G-proteins responsible for?

A

A diverse range of cellular functions, including muscle contraction, stimulus-secretion coupling, catabolic and anabolic metabolic processes and light, smell, and taste perception

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5
Q

What is meant by G-proteins being heterotrimeric?

A

They are made up of three distinct subunits, alpha, beta and gamma.

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6
Q

Which sub-units in G-proteins bind tightly together?

A

ß and gamma

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7
Q

How do the ß and gamma sub units in G proteins function?

A

As a single unit

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8
Q

What does the α sub-unit of G proteins have?

A

A guanine nucelotide-binding site

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9
Q

What does the guanine nucleotide-binding site do?

A

Binds GTP and slowly hydrolyses it to GDP (GTPase activity)

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10
Q

How is G-protein present under basal conditions?

A

At the inner face of the plasma membrane, predominantly in its heterotrimeric form, with GDP bound to the α-subunit

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11
Q

What has a high affinity for G-protein under basal conditions?

A

Activated receptor (agonist bound)

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12
Q

What occurs when the activated receptor binds with the G-protein?

A

A protein-protein interaction occurs, leading to GDP being released by the α-subunit and binding GTP in it’s place

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13
Q

What does the activated G-protein receptors act as?

A

A guanine nucleotide exchange factor (GEF)

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14
Q

What happens once GTP has bound to the α-subunit of the G-protein?

A

The affinity of the receptor for both α-GTP and the ßγ subunits is decreased

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15
Q

What is the result of the decreased affinity of α-GTP and ßγ subunits?

A

Both are subsequently released, and are able to interact ith effectors

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16
Q

How is the effector interaction terminated?

A

By the intrinsic GTPase activity of the α-subunit hydrolysing GTP → GDP

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17
Q

What happens once effector interaction has been terminated?

A

The affinity of the α-subunit for the ßγ subunit increases, and the ßγ-subunit increases, and the αßγ heterodimer is reformed and awaits reactivation by an agonist-activated receptor to reinitiate cycle

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18
Q

What can G protein can be thought of as?

A

On/off switches and timers

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19
Q

What is the ‘on switch’ of G proteins?

A

Receptor-facilitated GDP/GTP exchange

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20
Q

What is the ‘timer/off switch’ of the G proteins governed by?

A

The length of time taken for GTP hydrolysis

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21
Q

What did our understanding of G-protein mediated systems first come from?

A

Attempts to understand how hormones such as adrenaline brought about the formation of the second messnger cyclic AMP

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22
Q

Where does the G-protein have an intermediate role?

A

Where the stimulatory Gs stimulates adenylyl cyclase to produce cAMP

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23
Q

How are other pathways similar to the Gs pathway?

A

There are other, similar transduction pathways that employ a similar 3-component transduction pathway

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24
Q

What exists antagonistically to Gs pathways?

A

Inhibitory (Gi) pathways

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25
Q

What do Gi pathways do?

A

Reduced cAMP levels by inhibiting adenylyl cyclase

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26
Q

How are Gi proteins like Gs proteins?

A

They have additional effects independent of adenylyl cyclase inhibition, including effects on ion channels and signalling pathways involved in cell growth and differentiation

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27
Q

Have G-protein families that exert their actions on effectors other than adenylyl cylase been discovered?

A

Yes

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28
Q

What do Gq proteins do?

A

Preferentially interact with the membrane bound enzyme phospholipid phosphatidylinositol 4,5-bisphosphate (PIP 2 ) to generate the second messengers 1,4,5-triphosphate (InsP 3 ) and diacylglycerol (DAG)

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29
Q

Where is rhodopsin present?

A

In mammalian retinal rod cells

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30
Q

What is the function of rhodopsin?

A

Light-sensing

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31
Q

What does rhodopsin do?

A

Activates a G protein (called transducing, or Gt), which in turn activates a phosphodiesterase enzyme that hydrolyses cylic GMP to 5’-GMP

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32
Q

What is the receptor for adrenaline/noradrenaline?

A

ß-adrenoreceptor

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33
Q

What is the G-protein for adrenaline/noradrenaline?

A

Gs

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34
Q

What is the effector for adrenaline/noradrenaline?

A

Stimulates adenylyl cyclase

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35
Q

What is the physiological response to adrenaline/noradrenaline?

A

Glycogenolysis, lipolysis

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36
Q

What is the receptor for acetylcholine?

A

M3-Muscarinic

M2-Muscarinic

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37
Q

What is the G-Protein for acetylcholine when a M3 receptor is used?

A

Gq

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38
Q

What is the effector for acetylcholine when a M3 receptor is used?

A

Stimulates phospholipase C

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39
Q

What is the physiological response to acetylcholine when a M3 receptor is used?

A

Smooth muscle contraction

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40
Q

What is the G-protein for acetylcholine when a M2 receptor is used?

A

Gi

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41
Q

What is the effector for acetylcholine when a M2 receptor is used?

A

Inhibits adenylyl cyclase

Stimulates K channel

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42
Q

What is the physiological response to acetylcholine when a M2 receptor is used?

A

Slowing of cardiac pacemaker

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43
Q

What is the receptor for light?

A

Rhodopsin

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44
Q

What is the G-protein for rhodopsin?

A

Gt

45
Q

What is the effector for rhodopsin?

A

Stimulates cyclic GMP phosphodiesterase

46
Q

What is the physiological response to light?

A

Visual excitation

47
Q

What are the adrenergic receptors?

A

α1
α2
ß1
ß2

48
Q

What G-protein α-subunit does the α1 receptor utilise?

A

Q

49
Q

What G-protein α-subunit does the α2 receptor utilise?

A

I

50
Q

What G-protein α-subunit does the ß1 receptor utilise?

A

S

51
Q

What G-protein α-subunit does the ß2 subunit utilise?

A

S

52
Q

What does stimulation of the α1 receptor do?

A

Stimulates phospholipase C

53
Q

What does stimulation of the α2 receptor do?

A

Inhibits adenylyl cyclase

54
Q

What does stimulation of the ß1 receptor do?

A

Activates adenylyl cyclase

55
Q

What does stimulation of the ß2 receptor do?

A

Activates adenylyl cyclase

56
Q

What are the cholinergic receptors?

A

M1
M2
M3

57
Q

What G-protein α-subunit does the M1 receptor utilise?

A

Q

58
Q

What G-protein α-subunit does the M2 receptor utilise?

A

I

59
Q

What G-protein α-subunit does the M3 receptor utilise?

A

Q

60
Q

What does stimulation of the M1 receptor do?

A

Activates phospholipase C

61
Q

What does stimulation of the M2 receptor do?

A

Inhibits adenylyl cyclase

62
Q

What does stimulation of the M3 receptor do?

A

Activates phospholipase C

63
Q

How many Gα proteins does the human genome encode for?

A

20

64
Q

How many Gß proteins does the human genome encode for?

A

5

65
Q

How many Gγ proteins does the human genome encode for?

A

12+

66
Q

How many combinations of Gα-γ are possible?

A

Over 1000

67
Q

How many receptor types that can interact with Gα subtypes are there?

A

At least 800

68
Q

How many enzyme/ion channel effectors can be activated/inhibited by interactions with Gα subunits and receptors?

A

10 or more

69
Q

How do G-proteins bring about a specific cellular response?

A

An extracellular signal works via a specific GPCR to activate a single, or small sub-population of G-proteins and effectors in the cell

70
Q

What can be used in the experimental manipulation of the G-protein cycle?

A

Cholera Toxin (CTx) and Pertussis Toxin (PTx)

71
Q

What do CTx and PTx do?

A

They are enzymes that ADP-ribosylate specific G-proteins

72
Q

What does CTx do specifically?

A

Eliminates the GTPase activity of G s α

73
Q

What is the result of the elimination ofthe GTPase activity of G s α?

A

G s α is irreversibly activated

74
Q

What does PTx do specifically?

A

Interferes with the GDP/GTP exchange on G i α

75
Q

What is the result in the interference withthe GDP/GTP exchange on G i α?

A

G i α becomes irreversibly inactivated

76
Q

What can genetic changes in GPRCs result in?

A

Loss of function or gain of function mutations

77
Q

Give 3 examples of conditions caused by mutations in GPCRs

A

Retinitis Pigmentosa
Nephrogenic Diabetes Insipidus
Familial Male Precocious Puberty

78
Q

What causesRetinitis Pigmentosa?

A

A loss-of-function mutation to rhodopsin

79
Q

What causesNephrogenic Diabetes Insipidus?

A

A loss-of-function mutation to V2 vasopressin receptor

80
Q

What causesFamilial Male Precocious Puberty?

A

A gain-of-function mutation (receptor active without ligand) to the LH receptor

81
Q

What is adenylyl cyclase?

A

An integral plasma membrane enzyme that can either be activated (via Gs) or inhibited (Gi) by activation of different receptors

82
Q

What does adenylyl cyclase do?

A

Hydrolyses cellular ATP to generate cyclic AMP

83
Q

What does cyclic AMP do?

A

Interacts with a specific protein kinase, which in turn phosphorylates a variety of other proteins within the cell to affect activity

84
Q

What specific protein kinase does cyclic AMP interat with?

A

Cyclic AMP-dependent protein kinase (PKA)

85
Q

What can receptors that activate adenylyl cyclase, and therefore increased cellular cyclic AMP levels, cause?

A

Increased glycogenolysis and gluconeogenesis in the liver
Increased lipolysis in adipose tissue
Relaxation of a variety of types of smooth muscle
Position inotropic and chronotropic effects in the heart

86
Q

What is phospholipase C?

A

An enzyme that hydrolyses the membrane phospholipid (PIP 2 ) to IP 3

87
Q

What is phospholipase C activated by?

A

Gq

88
Q

How does IP 3 exert its effects?

A

By interacting with specific intracellular receptors on the endoplasmic reticulum to allow Ca 2+ to leave the lumen of the ER and enter the cytoplasm

89
Q

What is cyclic GMP phosphodiesterase?

A

A specialised mechanism found in the photoreceptive cells of the retina

90
Q

What doescyclic GMP phosphodiesterase do?

A

Regulates the breakdown of the second messenger cyclic GMP phosphodiesterase by Gt

91
Q

What happens once a receptor has productively interacted with a G-protein?

A

The binding of the agonist is weakened, and agonist-receptor dissociation is likely to occur

92
Q

What is the receptor susceptible to whilst activated?

A

A variety of protein kinases that phosphorylate the receptor and prevent it from activating further G-proteins

93
Q

What is the problem with the activated receptor being susceptible to phosphorylation?

A

It compromises an important part of the receptor desensitisation phenomenon observed for most of the GPCR

94
Q

What may the active lifetime of α-GTP be limited by?

A

Cellular factors that stimulate the intrinsic GTPase activity of the Gα subunit

95
Q

What does the enzymatic activities in the cell favour?

A

The basal state

96
Q

What is the result of enzymatic activities in the cell being such that the basal state is favoured?

A

Cells contain high activity enzymes that metabolise second messengers, rapidly returning their levels to the basal

97
Q

How is the effect of second messenger/protein kinase activation opposed?

A

Enzymatic cascades are activated downstream that oppose the effect

98
Q

What can the rate at which the sinoatrial node fires an action potential be affected by?

A

Ach release by the parasympathetic nerves

99
Q

What is the predominant receptor in regulation of chronotropy in the heart?

A

M2 muscarinic cholinoceptors

100
Q

What does activation ofM2 muscarinic cholinoceptors cause?

A

An increase in the open probability of K+ channels via Gi

101
Q

What does an increase in plasma membrane permeability to K+ cause?

A

Hyperpolarisation, slowing the intrinsic firing rate, resulting in a negative chronotropic effect

102
Q

What is inotropy?

A

The force of heart contraction

103
Q

What can influence inotropy?

A

Sympathetic innervation of the cardiac ventricles (and/or circulating adrenaline)

104
Q

How is a positive inotropic effect in the heart bought about?

A

Activation of the ß-adrenoreceptors (predominantly ß1) increases the open probability of voltage operated calcium channels via Gs.
Gs both interact directly with the VOCCs, and indirectly via cyclic AMP → PKS → phosphorylation and activation of VOCCs
The influx of Ca brings about a positive inotropic effect

105
Q

How is arteriolar vasoconstriction bought about?

A

Sympathetic release of noradrenaline acts on α1-adrenoreceptors to stimulate phospholipase C and IP3 production via Gq
IP3 releases ER Ca, and initiates a contractile response

106
Q

What can pre-synaptic GPCR influence?

A

Neurotransmitter release

107
Q

Give an example of where pre-synaptic GPCR influence neurotransmitter release?

A

Pre-synaptic µ-opiod receptors can be stimulated, either by endogenous opiods or by analgesics such as morphine to couple Gα1 proteins

108
Q

How is neurotransmitter release reduced by GPRCs?

A

The Gßγ subunits are liberated from the heterodimer and interact with VOCCs to reduce Ca entry, thus reducing neurotransmitter release

ESA 2- Membranes and Receptors (13 decks)

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