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biochemistry: exam 3 > Biochemical Signaling: Pathways > Flashcards

Biochemical Signaling: Pathways Flashcards

1
Q

What does 7TM stand for?

A

Seven-transmembrane-helix receptors

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2
Q

What are the three most common types of membrane receptors that mediate information transfer into the cell?

A

1) Seven-transmembrane-helix receptors
2) Dimeric membrane receptors that recruit protein kinases
3) Dimeric protein receptors that are protein kinases

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3
Q

What is the largest class membrane receptors?

A

7TM

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4
Q

About what percentage of drugs in use alter 7TM receptors?

A

50%

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5
Q

What is the structure of 7TM receptors?

A

Seven helices spanning the membrane bilayer.

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6
Q

How do 7TM receptors work?

A

Binding of ligand on the part of the receptor outside of the cell induces a conformational change detectable within the cell.

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7
Q

What type of receptor is a beta-adrenergic receptor? What does it bind?

A

Seven-transmembrane-helix receptor (7TM); it binds epinephrine (also called adrenaline)

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8
Q

What receptor binds epinephrine? What kind of receptor is it?

A

beta-adrenergic receptor; 7TM

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9
Q

What does ligand binding to 7TM receptors lead to?

A

Conformational changes in the cytoplasmic domain of the receptor, leading to activation of a heterotrimeric GTP-binding protein (G protein).

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10
Q

What does an unactivated G-protein consist of?

A

1) An alpha subunit bound to GDP (most important)
2) A beta subunit
3) A gamma subunit

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11
Q

Why is an unactivated G-protein a heterotrimer?

A

It consists of three different subunits

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12
Q

What happens when a G-protein is activated?

A

1) GDP is exchanged for GTP
2) The alpha subunit dissociates from the beta and gamma subunits, forming G-alpha and G-beta, gamma units.
3) This dissociation transmits the signal that the ligand has bound to the receptor.

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13
Q

What does the “G” in G-protein stand for?

A

guanyl nucleotide

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14
Q

True or false: A single hormone-receptor complex can stimulate nucleotide exchange in only one G-protein heterotrimer.

A

False; It can stimulate nucleotide exchange in many G-protein heterotrimers, giving an amplified response.

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15
Q

What does the activated G protein do?

A

Stimulates activity of adenylate cyclase, a second messenger that increases the concentration of cAMP

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16
Q

What enzyme increases cyclic AMP by forming it from ATP?

A

adenylate cyclase

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17
Q

What does GPCR stand for?

A

G-protein coupled receptors

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18
Q

What does the effect of the increased concentration of cyclic AMP in the cell depend on?

A

The type of cell involved.

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19
Q

What are most effects of increasing the concentration of cAMP mediated by?

A

The activation of a protein kinase (an enzyme).

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20
Q

What enzymes phosphorylate a substrate at the expense of a molecule of ATP?

A

kinases

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21
Q

What does PKA stand for?

A

Protein kinase A

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22
Q

What does PKA consist of?

A

2 regulatory (R) chains and 2 catalytic (C) chains

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23
Q

What activates the R2C2 complex of protein kinase A?

A

cyclic AMP

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24
Q

Which chains of PKA does cAMP bind to?

A

The 2 regulatory chains

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25
Q

What happens when cAMP binds to the regulatory chains of PKA?

A

The 2 catalytic chains are released,

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26
Q

What does activated PKA do?

A

Phosphorylates specific serine and threonine residues in many target proteins, altering their activity.

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27
Q

What turns off the cAMP cascade?

A

cAMP phosphodiesterase

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28
Q

How does cAMP phosphodiesterase turn off the cAMP cascade?

A

It converts cAMP into AMP, which does not activate PKA.

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29
Q

What happens after the cAMP cascade is turned off by cAMP phosphodiesterase?

A

The C and R subunits of PKA rejoin to form the inactive enzyme.

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30
Q

How is the signal initiated by 7TM receptors switched off?

A

G-alpha subunits have intrinsic GTPase activity that hydrolyzes the bound GTP to GDP and Pi (inorganic orthophosphate) and thereby deactivates itself.

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31
Q

What happens as hormone concentration decreases?

A

The hormone dissociates from the receptor and the receptor returns to its initial unactivated form.

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32
Q

True or false: Ligand-receptor interaction is reversible.

A

True

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33
Q

True or false: The bound GTP on G alpha acts as a built in clock that spontaneously resets the G alpha subunit after a short period of time.

A

True

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34
Q

What does the likelihood of a receptor remaining in an inactive state depend on?

A

The concentration of hormone in its environment.

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35
Q

What are cholera and whooping cough due to?

A

Altered G-protein activity

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36
Q

What is the bacterial toxin that causes cholera?

A

Choleragen

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37
Q

What is the result of choleragen infection?

A

Choleragen modifies the G-alpha-s (s=stimulatory) protein so that it is trapped in the active GTP-bound form. This causes loss of NaCl and water into the intestine (causes cholera).

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38
Q

What is the result of pertussis infection?

A

Pertussis modifies the G-alphai (I=inactive) protein, trapping it in the inactive form, rendering it inactive (causes whooping cough)

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39
Q

What does the hormone vasopressin regulate?

A

water retention

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40
Q

What does PIP2 stand for? What is it?

A

Phosphatidylinositol 4, 5-bisphosphate; a phospholipid present in cell membranes.

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41
Q

What does the binding of vasopressin to its 7TM receptor lead to?

A

Activation of phospholipase C, which cleaves PIP2 into two second messengers, IP3 and DAG

42
Q

What is the Ga protein that activates phospholipase C called?

A

Gaq

43
Q

What does activated phospholipase C enzyme do?

A

It cleaves PIP2, forming 2 second messengers: 1) Diacylglycerol (DAG) and 2) Inositol 1, 4, 5 triphosphate (IP3)

44
Q

What is a soluble molecule that can diffuse from the membrane that is released when an activated phospholipase C enzyme cleaves PIP2?

A

Inositol 1, 4, 5 triphosphate (IP3)

45
Q

What is a second messenger that stays in the membrane that is released when phospholipase C enzyme cleaves PIP2?

A

Diacylglycerol (DAG)

46
Q

True or false: IP3 causes a cascade of phosphorylation to elicit a response from the cell.

A

False

47
Q

What does IP3 cause?

A

Rapid release of Ca2+ from the ER and SR (sarcoplasmic reticulum–in muscle) into the cell cytoplasm.

48
Q

What membrane protein does IP3 associate to allow the flow of Ca2+ from the ER into the cytoplasm?

A

IP3-gated channel or IP3 receptor

49
Q

What is the key difference between the actions of the activated G Protein when epinephrine and vasopressin are received by 7TM receptors?

A

With epinephrine, the Gas protein activates adenolate cyclase; with vasopressin, the Gaq protein activates phospholipase C.

50
Q

What types of processes does the release of Ca2+ from the ER and SR trigger?

A

1) Smooth muscle contraction

2) Glycogen breakdown

51
Q

True or false: The half-life of IP3 is very short.

A

True

52
Q

What do DAG and Ca2+ activate?

A

Protein kinase C (PKC)

53
Q

What does protein kinase C (PKC) do?

A

Phosphorylates serine and threonine residues in target protons

54
Q

How is IP3 turned off?

A

It has a short half-life and is rapidly converted to derivatives that have no effect on IP3-gated channels.

55
Q

How is it theorized that lithium ion works to treat bipolar disorder?

A

May inhibit the recycling of IP3.

56
Q

Which kinase requires both DAG and Ca2+ to be activated?

A

Protein kinase C (PKC)

57
Q

What level of structure in 7TM membranes is changed by ligand binding?

A

Tertiary structure

58
Q

What level of structure in dimerizing receptors is changed by ligand binding?

A

Quaternary structure

59
Q

How does a growth hormone receptor exist in the absence of a bound hormone?

A

As a monomer

60
Q

What is the activated form of a growth hormone receptor?

A

A dimer

61
Q

How is the dimer formed when the growth hormone binds?

A

The growth hormone binds to two monomeric receptors, creating the dimer.

62
Q

What is the structure of a GH (growth hormone) receptor?

A

Single, membrane spanning helix

63
Q

What causes 2 JAK2 proteins to phosphorylate each other when GH binds to its receptor?

A

GH binds two monomeric receptors, drawing them together into a dimer; this pulls each receptor’s JAK2 proteins together inside the cell and allows them to phosphorylate each other.

64
Q

What does JAK2 stand for?

A

Janus kinase (Tyrosine protein kinase)

65
Q

When does JAK2 become activated?

A

When it is cross-phosphorylated by its partner JAK2

66
Q

What is the structure of insulin?

A

It is a polypeptide hormone that consists of two monomeric chains linked by two disulfide bonds.

67
Q

What class of membrane proteins does an insulin receptor belong to?

A

The tyrosine kinase class

68
Q

What does each insulin receptor consist of?

A

1) An extracellular alpha subunit
2) An intracellular beta subunit that spans the membrane with a single transmembrane protein
3) The two subunits are linked by two disulfide bonds

69
Q

What causes the two subunits in an insulin receptor to dimerize?

A

Insulin binding

70
Q

What causes the insulin-receptor to become an activated tyrosine kinase?

A

Cross-phosphorylation of beta subunits when the dimer is formed due to insulin binding

71
Q

How are docking sites for insulin receptor substrates (IRS) formed on the insulin receptor?

A

By phosphorylation of additional sites on the receptor when insulin binds to create the dimer

72
Q

What happens to the IRS proteins that dock on the insulin-receptor tyrosine kinsase?

A

They are phosphorylated

73
Q

What do phosphorylated IRS proteins do?

A

Act as adaptor proteins (i.e., they are recognized by other proteins, most importantly phosphoinositide 3-kinase)

74
Q

What is the most important protein that recognizes phosphorylated IRS proteins?

A

Phosphoinositide 3-kinase

75
Q

What does phosphoinositide 3-kinase do when it recognizes phosphorylated IRS proteins?

A

Phosphorylates membrane-bound PIP2 to form membrane-bound PIP3

76
Q

What does membrane-bound PIP3 do?

A

Activates PDK1 (PIP3 Dependent Protein Kinase)

77
Q

What does activated PDK1 do?

A

Phosphorylates and activates PKB (also called Akt), causing it to detach from the membrane.

78
Q

What does Akt (PKB) do once it is detached from the membrane?

A

Moves through the cell phosphorylating enzymes to 1) stimulate glycogen synthesis and 2) move GLUT 4 transporters to the cell surface to allow glucose entry into the cell.

79
Q

What is GLUT 4?

A

A glucose transporter

80
Q

How is insulin signaling terminated?

A

By protein phosphatases that remove phosphates and deactivate enzymes

81
Q

What three types of phosphatases are responsible for terminating an insulin signal?

A

1) Protein tyrosine phosphatase
2) Lipid phosphatases
3) Protein serine phosphatases

82
Q

What activates the enzymes that terminate an insulin signal?

A

Insulin binding to the receptor (both activates and starts a cascade for deactivation)

83
Q

What enzymes hydrolyze PIP3 back to PIP2 to help terminate the insulin signal?

A

Lipid phostatases

84
Q

What enzymes dephosphorylate activated protein kinases such as Akt in the insulin signal pathway?

A

Protein serine phosphatases

85
Q

What enzymes remove phosphoryl groups from tyrosine in the insulin receptor to help terminate the signal?

A

Protein tyrosine phosphatases

86
Q

What are transmembrane receptors that have tyrosine kinase domains within their intracellular domains?

A

Receptor tyrosine kinases (RTK’s)

87
Q

What stimulates the growth of epidermal and epithelial cells?

A

Epidermal growth factor

88
Q

What does binding of EGF to the extracellular domain of its receptor cause?

A

1) Dimerization

2) Cross-phosphorylation and activation

89
Q

Which adapter protein links the cross-phosphorylation of an EGF receptor to chains of phosphorylation that stimulate cell growth?

A

Grb-2

90
Q

What causes Grb-2 to bind to receptor proteins of tyrosine kinase on the intracellular domain of EGF receptors?

A

Cross-phosphorylation of the EGF receptor

91
Q

What does Grb-2 do when it binds to the EGF receptor?

A

Recruits Sos protein

92
Q

What does Sos do when it is activated by Grb-2?

A

Binds to Ras and activates it.

93
Q

What family of proteins is Ras a member of?

A

small G Proteins (or small GTPases)

94
Q

How are small G proteins (or GTPases) like heterotrimeric G proteins?

A

They cycle between a GDP inactive and a GTP active form and cause the final phosphorylation events.

95
Q

How do G proteins (or GTPases) differ from heterotrimeric G proteins?

A

They are smaller and monomeric

96
Q

What do activated GTPase proteins stimulate?

A

Cell growth and differentiation

97
Q

By what mechanism does Sos activate Ras?

A

Sos opens up the nucleotide binding pocket of Ras, allowing GDP to escape and GTP to enter.

98
Q

What does GEF stand for? Which protein in the EGF signal pathway falls in this category?

A

Guanine-nucleotide exchange factor; Sos

99
Q

True or false: Ras, like the Ga protein in the 7TM pathway, contains intrinsic GTPase activity that terminates the signal and returns the system to the inactive state.

A

True

100
Q

What terminates the signal in the EGF pathway?

A

GTPase activity that is begun when Ras is activated

biochemistry: exam 3 (23 decks)

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