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Biochem 2 (CHEM 441B) > Ch. 14 > Flashcards

Ch. 14 Flashcards

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1
Q

What is the pentose phosphate pathway (PPP)?

A

Metabolic pathway in which glucose-6-P is oxidized then decarboxylated, generating ribulose-5-P and NADPH

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2
Q

Where does the PPP occur?

A

In the cytoplasm

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3
Q

What are the important functions of the PPP?

A
  1. Reduce 2 molecules of NADP+ to NADPH for each molecule of glucose-6-P that is oxidatively decarboxylated to ribulose-5-P
  2. Produce ribose-5-P from glucose-6-P
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4
Q

What does NADPH function as?

A

A strong reductant (electron donor)

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5
Q

What is the [NAD+]/[NADH] in liver cells?

A

~1000

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6
Q

What is the [NADP+]/[NADPH] in liver cells?

A

0.01

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7
Q

What are the 2 phases of the PPP?

A
  1. Oxidative
  2. Non-oxidative
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8
Q

Flux through both phases of the PPP is regulated to meet what 3 metabolic states?

A
  1. If increased NADPH is required
  2. If nucleotide pools need to be replenished
  3. If ATP levels in the cell are low
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9
Q

What happens in the PPP if increased NADPH is required?

A

Fructose-6-P and glyceraldehyde-3-P are used to resynthesize glucose-6-P, thereby maintaining flux through the oxidative phase

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10
Q

What happens if nucleotide pools need to be replenished?

A

The bulk of ribulose-5-P is converted to ribose-5-P, stimulating nucleotide biosynthesis

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11
Q

What does the PPP accomplish for the cell? (i.e. what are its functions)

A
  • Generate NADPH (needed in lots of pathways and to detox ROS)
  • Produce ribose-5-P for nucleotide synthesis
  • Regenerate glucose-6-P (to maintain NADPH production)
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12
Q

What is the over all net reaction of the PPP?

A

6 Glucose-6-P + 12 NADP+ + 6 H2O –>
4 Fructose-6-P + 2 Glyceraldehyde-3-P + 12 NADPH + 12 H+ + 6 CO2

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13
Q

What are the key enzymes in the PPP?

A
  • G6PD
  • Transketolase and transaldolase
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14
Q

What is G6PD?

A

The enzyme that catalyzes the initial step in the PPP, converts glucose-6-P to 6-phosphogluconolactone
- Commitment step
- Feedback inhibited by NADPH

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15
Q

What is transketolase?

A

The enzyme that catalyzes the transfer of 2-carbon units among sugars in the PPP

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16
Q

What is transaldolase?

A

The enzyme the catalyzes the transfer of 3-carbon units among sugars in the PPP

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17
Q

What do transketolase and transaldolase do together?

A

Catalyze the reversible carbon shuffle reactions of the non-oxidative phase

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18
Q

What are examples of the PPP in everyday biochemistry?

A

G6PD deficiency
- Most common enzyme deficiency in the world
- Affects >400 million people
- 90% decrease in enzyme activity results in inability of RBCs to make enough NADPH to protect the cells from ROS
- ROS generated by antimalarial drugs and compounds in fava beans

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19
Q

Which reaction(s) in the oxidative phase of the PPP is/are reversible?

A
  • Lactonase
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20
Q

Which reaction(s) in the oxidative phase of the PPP is/are NOT reversible?

A
  • G6PD
  • 6PGD (6-Phosphogluconolactonate dehydrogenase)
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21
Q

Since glucose-6-P is a substrate for glycolysis and the PPP, what controls the overall metabolic flux through these pathways?

A

The [NADP+]/[NADPH] in the cytosol
- Acts as a rheostat to regulate G6PD activity

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22
Q

What is glutathione (GSH)?

A

Reduced form is an electron donor in redox reactions

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23
Q

What are the electrons from GSH used for?

A
  • To keep cysteine residues in hemoglobin in a reduced state
  • To reduce ROS and hydroxyl free radicals that damage proteins and lipids via oxidation-induced cleavage reactions
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24
Q

What is primaquine?

A

Antimalarial drug that causes oxidative stress in RBCs, which creates a hostile environment for the malaria parasite
- Leads to acute hemolytic anemia in people with a G6PD deficiency

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25
Q

What is gluconeogenesis?

A

Metabolic pathway used for the production of glucose from noncarbohydrate sources

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26
Q

How many enzymes do glycolysis and gluconeogenesis share?

A

7 enzymes

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27
Q

Which enzymes are glycolytic-specific?

A
  • Hexokinase
  • Phosphofructokinase-1
  • Pyruvate kinase
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28
Q

What are the carbon sources for gluconeogenesis?

A
  • Lactate
  • Amino acids
  • Glycerol
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29
Q

How do plants get carbon for gluconeogenesis?

A

Use the Calvin cycle to convert CO2 to glyceraldehyde-3-P

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30
Q

What does gluconeogenesis accomplish for the cell?

A
  • Liver and kidneys use it to generate glucose from noncarbohydrate sources for export to other tissues that use glucose for energy
  • Plants use it to convert CO2 to glyceraldehyde-3-P
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31
Q

What is the overall net reaction of gluconeogenesis?

A

2 Pyruvate + 2 NADH + 4 ATP + 2 GTP + 6 H2O –> 2 NAD+ + 2 H+ + 4 ADP + 2 GDP + 6 Pi

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32
Q

What are the key enzymes in gluconeogenesis?

A
  • Pyruvate carboxylase
  • Phosphoenolpyruvate carboxykinase
  • FBPase-1
  • Glucose-6-phosphotase
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33
Q

What is pyruvate carboxylase?

A

ATP-dependent enzyme that carboxylates pyruvate to generate oxaloacetate
- MITOCHONDRIAL ENZYME

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34
Q

What does pyruvate carboxylase maintain flux through?

A

Maintains flux through the citric acid cycle in the presence of acetyl-CoA

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35
Q

What is phosphoenolpyruvate carboxykinase?

A

Catalyzes a decarboxylation reaction in gluconeogenesis that converts oxaloacetate to phosphoenolpyruvate and CO2

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36
Q

What is FBPase-1?

A

Converts fructose-1,6-BP to fructose-6-P and opposes the PFK-1 in glycolysis

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37
Q

What is glucose-6-phosphotase?

A

Converts glucose-6-P to glucose and opposes the hexokinase/glucokinase reaction in glycolysis

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38
Q

What are examples of gluconeogenesis in everyday biochemistry?

A

Cori cycle
- Short intense exercise –> lactic acid build up in muscle because of anaerobic glycolysis
- Warm down period to increase circulation and remove lactate from muscle
- Lactate goes to liver where it’s converted to glucose and sent back to muscle to replenish glycogen

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39
Q

What are the 3 bypass reactions in gluconeogenesis?

A
  1. Pyruvate carboxylase and phosphoenolpyruvate carboxykinase (pyruvate kinase)
  2. FBPase-1 (PFK-1)
  3. Glucose-6-phosphotase (hexokinase)
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40
Q

What happens in the first bypass reaction in gluconeogenesis?

A
  • Pyruvate carboxylase uses phosphoryl transfer energy in ATP to drive carboxylation reaction that converts pyruvate to oxaloacetate
  • Oxaloacetate is decarboxylated by phosphoenolpyruvate carboxykinase and converted to phosphoenolpyruvate
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41
Q

What is biotin?

A

Cofactor for pyruvate carboxylase
- Functions as carrier of carboxyl groups in enzymatic reactions

42
Q

What happens in the second bypass reaction in gluconeogenesis?

A

Fructose-1,6-BP is converted to fructose-6-P

43
Q

What does a low energy charge lead to in terms of gluconeogenesis/glycolysis?

A

Low energy charge –> high AMP levels –> activation of PFK-1 (increase flux through glycolysis) and inhibition of FBPase-1 (decrease flux through gluconeogenesis)

44
Q

What happens in the third bypass reaction in gluconeogenesis?

A

Free glucose is generated that can be exported to blood

45
Q

Where is glucose-6-phosphotase located?

A

ER lumen

46
Q

Where is hexokinase located?

A

Cytosol

47
Q

What does fructose-2,6-BP do?

A

Reciprocally regulates glycolysis and gluconeogenesis
- Increase flux through glycolysis
- Decrease flux through gluconeogenesis

48
Q

What inhibits pyruvate kinase?

A

ATP and alanine (decrease flux through glycolysis)

49
Q

What stimulates pyruvate carboxylase?

A

Acetyl-CoA (increase flux through gluconeogenesis)

50
Q

What is PFK-2/FBPase-2?

A

Dual-function enzyme that catalyzes the interconversion of fructose-6-P and fructose-1,6-BP

51
Q

What are the 2 functional domains of PFK-2/FBPase-2?

A
  1. Kinase activity that phosphorylates fructose-6-P into fructose-1,6-BP
  2. Phosphotase activity that dephosphorylates fructose-1,6-BP into fructose-6-P
52
Q

What is the Cori cycle?

A

Mechanism where lactate produced by anaerobic glycolysis in muscle is converted in liver cells to glucose via gluconeogenesis

53
Q

Where do glycogen degradation and synthesis occur?

A

In the cytosol

54
Q

What is the substrate for glycogen degradation and synthesis?

A

Nonreducing (free) ends

55
Q

What kind of bonds does glycogen have?

A

α(1–>4) and α(1–>6)

56
Q

What are the 4 key enzymes in glycogen degradation and synthesis?

A
  1. Glycogen phosphorylase
  2. Glycogen synthase
  3. Glycogen branching enzyme
  4. Glycogen debranching enzyme
57
Q

What does glycogen phosphorylase do?

A

Catalyzes a phosphorolysis reaction using inorganic phosphate to remove glucose from nonreducing ends of glycogen

58
Q

What reaction does glycogen phosphorylase catalyze?

A

Glucose-1-P –> glucose-6-P

59
Q

What is glucose-6-P used for?

A

Glycolysis in muscles or gets dephosphorylated in liver

60
Q

What is glycogen phosphorylase activity activated by?

A

Epinephrine and glucagon signaling

61
Q

What does glycogen synthase do?

A

Adds glucose residues to nonreducing ends of glycogen using UDP-glucose as the substrate

62
Q

What is glycogen synthase activated by?

A

Insulin signaling

63
Q

What do glycogen branching and debranching enzymes do?

A

Mediate formation and cleavage of α(1–>6) glycosidic bonds

64
Q

What tissues have a lot of glycogen?

A

Liver and skeletal muscle

65
Q

What are glycogen particles?

A

Formed by 20-40 glycogen core complexes

66
Q

What % by weight is liver glycogen?

A

~10% by weight

67
Q

What is liver glycogen for?

A

Glucose source for export when there are limited dietary sources (between meals)

68
Q

What % by weight is muscle glycogen?

A

~1-2% by weight

69
Q

What is muscle glycogen for?

A

Used to generate glucose-6-P (chemical energy source in aerobic and anaerobic
glycolysis)

70
Q

What purpose does glycogen degradation and synthesis serve in animals?

A
  • Liver glycogen: short term energy source
  • Muscle glycogen: glucose source during exercise
71
Q

What is the net reaction of glycogen degradation at nonreducing ends?

A

Glycogenn units of glucose + Pi —> Glycogen n-1 units of glucose + Glucose-1-P

72
Q

What is the net reaction of glycogen synthesis?

A

Glycogenn units of glucose + Glucose-1-P + ATP + H2O —> Glycogen n+1 units of glucose + ADP + 2Pi

73
Q

What is an example of glycogen degradation and synthesis and everyday biochemistry?

A

Endurance athletes carbohydrate loading
- Short period of intense exercise followed by ingesting 10g/kg of carbs –> 2x muscle glycogen in 24 hours

74
Q

What enzyme initiates glycogen degradation?

A

Glycogen phosphorylase

75
Q

What is pyridoxal phosphate (PLP)?

A

Coenzyme for glycogen phosphorylase
- Acid-base catalyst that donates proton to Pi

76
Q

How is PLP bound to glycogen phosphorylase?

A

Through a lysine residue

77
Q

What does it mean when glycogen phosphorylase is processive enzyme?

A

Stays attached to glycogen substrate a cleaves α(1–>4) bonds and releases glucose-1-P until it gets too close to α(1–>6) branch point

78
Q

Glycogen phosphorylase removes glucose until it is how many glucose away from the branch point?

A

4 glucose

79
Q

What are the 2 steps of the debranching enzyme remodeling the substrate?

A
  1. Debranching enzyme transfers 3 glucose units to nearest nonreducing end
  2. Bifunctional debranching enzyme cleaves all α(1–>6) bond to release free glucose
80
Q

How much glucose-1-P vs. glucose does complete glycogen degradation release?

A

~90% glucose-1-P and ~10% glucose

81
Q

What are the 2 conformations of glycogen phosphorylase?

A
  • R state (active)
  • T state (inactive)
82
Q

What reaction does phosphoglucomutase catalyze in glycogen degradation?

A

Glucose-1-P –> Glucose-6-P

83
Q

How does unphosphorylated glycogen phosphorylase shift from T state to R state?

A

By binding AMP (allosteric effector)

84
Q

What shifts unphposphorylated glycogen phosphorylase back to the R state?

A

Glucose-6-P (allosteric inhibitor; competes with AMP)

85
Q

What enzyme initiates glycogen synthesis?

A

Phosphoglucomutase

86
Q

What reaction does phosphoglucomutase catalyze in glycogen synthesis?

A

Glucose-6-P –> Glucose-1-P

87
Q

What does glycogen synthase use as a substrate to catalyze the glycosyltransferase reaction?

A

UDP-glucose

88
Q

What is glycogen synthase activity controlled by?

A
  • Inhibited by phosphorylation (shift to T state)
  • Activated by dephosphorylation (shift to R state)
89
Q

What are the 2 catalytic activities of glycogenin?

A
  1. Glycosyltransferase activity: uses UDP-glucose as donor to generate O-linked glycosidic bond between glucose to tyrosine residue
  2. Glycogen synthase activity: extends glycogen chain to 8 glucose residues
90
Q

What happens when glucose is added to a liver cell extract?

A
  • Rapid decrease in glycogen phosphorylase activity
  • Steady increase in glycogen synthase activity
91
Q

What does low blood glucose lead to?

A

Glucagon signaling

92
Q

What does high blood glucose lead to?

A

Insulin signaling

93
Q

What does protein phosphotase 1 do?

A

Dephosphorylates specific protein targets

94
Q

What stimulates protein phosphotase 1 activity?

A

Insuling signaling

95
Q

What causes von Gierke disease?

A

Deficiency in liver glucose-6-phosphotase

96
Q

What causes Hers disease?

A

Defect in liver glycogen phosphorylase

97
Q

What causes Pompe disease?

A

Deficiency in lysosomal α-1,4-glucosidase

98
Q

What causes McArdle disease?

A

Defect in muscle glycogen phosphorylase

99
Q

What causes Cori disease?

A

Defect in liver muscle glycogen debranching enzyme that impacts its ability to fully degrade glycogen molecules

100
Q

What causes Andersen disease?

A

Defect in liver glycogen branching enzyme that results in format of large linear glycogen molecules

Biochem 2 (CHEM 441B) (23 decks)

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