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Tissues > T04 - Pentose Phosphate Pathway > Flashcards

T04 - Pentose Phosphate Pathway Flashcards

1
Q

What are the primary products of the PPP? (3)

A

NADPH

ribose

5 or 7-carbon carbonhydrates

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2
Q

Why is NADPH important? (2)

A

primary reducing power for biosynthetic pathways

protects against oxidative stress

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3
Q

Pompe disease results from

A

recessively-inherited deficiency in alpha-glucosidase

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4
Q

What is the function of the enzyme alpha-glucosidase?

A

digests glycogen to glucose

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5
Q

What are the effects of Pompe disease?

A

buildup of glycogen in lysosomes, leading to muscle weakness and organ damage

fatal in infants if not treated within 2 years

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6
Q

What are the linkages within glycogen?

A

glycosidic alpha-1,4 and alpha-1,6 linkages

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7
Q

The alpha-glycosidic bonds within glycogen cause it to adopt what shape?

A

forms helical-like configuration

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8
Q

What are the advantages of branches within glycogen? (2)

A

increased solubility

rapid release of glucose from glycogen

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9
Q

What enzyme is responsible for releasing glucose from glycogen?

A

glycogen phosphorylase

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10
Q

What enzyme is responsible for adding glucose to glycogen?

A

glycogen synthase

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11
Q

What is the primary function of liver glycogen?

A

maintain blood glucose levels

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12
Q

(T/F) Glycogen granules are absent from cells after a 24-hour fasting period.

A

True. Glycogen granules exist to maintain blood [glucose] between meals. If there are no meals, the glycogen granules are depleted.

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13
Q

What is the primary function of muscle glycogen?

A

provide source of ATP in muscle

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14
Q

What is the major limitation of muscle glycogen?

A

muscle glycogen is not available to other tissues because muscle cells don’t have glucose-6-phosphatase, which is required to make free glucose

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15
Q

What molecule is the primer for glycogen synthesis?

A

the protein glycogenin is the primer/initiator of glycogen synthesis

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16
Q

What molecule is the template (note: distinct from primer) for glycogen synthesis? Explain how this molecule forms.

A

glycosylated glycogenin is the template for glycogen synthase

“plain” glycogenin self-glycosylates by attaching glucose from UDP-glucose to its tyrosine residue

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17
Q

What molecule is the building block for glycogen synthesis?

A

UDP-glucose

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18
Q

Explain the difference between UDP-glucose and glucose.

A

UDP-glucose is an activated form of glucose because of high-energy diphosphate linkage → makes polymerization more energetically favorable

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19
Q

How is UDP-glucose synthesized? What drives this synthesis forward?

A

UTP + G-1-P → UDP-glucose + PPi, catalyzed by UDP-glucose pyrophosphorylase

irreversible hydrolysis of pyrophosphate drives reaction forward

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20
Q

Which enzymes are responsible for the alpha-1,4 and alpha-1,6 linkages?

A

glycogen synthase adds in alpha-1,4 fashion

branching enzyme breaks alpha-1,4 linkages and replaces it with alpha-1,6 linkage

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21
Q

In order for glycogen synthase to add UDP-glucose units, what structure must first exist in the glycogen molecule?

A

short pre-existing glucose polymer (4 units), which is synthesized by glycogenin

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22
Q

Describe how the branching enzyme of glycogen synthesis operates.

A

transfers block of 7 residues to interior site

breaks alpha-1,4 bond and replaces it with alpha-1,6 linkage

interior site is located at least 4 residues from another branch site

[tip: think “July 4th” for 7 residues/4 residues]

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23
Q

Write out the chemical equation for glycogen degradation/glycogenolysis.

A

(glycogen)n + Pi → (glycogen)n-1 + G-1-P

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24
Q

What enzyme breaks alpha-1,4 glycosidic linkages in glycogen?

A

glycogen phosphorylase

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25
Q

(T/F) G-1-P can freely diffuse across cell membranes.

A

False. It, like G-6-P, can’t cross cell membranes because of the negative charge of the phosphate.

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26
Q

How is G-1-P liberated from glycogen made available for metabolic processes?

A

phosphoglucomutase converts G-1-P to G-6-P, which is the molecule at the center of glucose metabolism

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27
Q

What enzyme breaks alpha-1,6 glycosidic linkages in glycogen?

A

debranching enzyme

28
Q

Describe how the debranching enzyme operates.

A

moves chain of 3 glucose residues to end of an adjacent glycogen branch

breaks alpha-1,6 linkage of remaining residue

remaining residue liberated as free glucose

29
Q

The debranching enzyme is an example of what kind of enzyme?

A

debranching enzyme = example of bifunctional enzyme

30
Q

What is the effect of epinephrine on glycogen? (2)

A

stimulates glycogenolysis in liver and muscle

[tip: think fight or flight]

31
Q

Draw out the cascade of events that occurs when glucagon activates protein kinase A.

A

glucagon → adenylate cyclase → cAMP → active PKA → active phosphorylase kinase → active phosphorylase a → glycogenolysis

32
Q

What is the effect of glucagon on glycogen?

A

glucagon stimulates glycogenolysis (to ensure there is adequate glucose for all tissues)

33
Q

Which form of glycogen synthase is active — phosphorylated or dephosphorylated?

A

dephosphorylated is active

[opposite of glycogen phosphorylase]

34
Q

Which form of glycogen phosphorylase is active — phosphorylated or dephosphorylated?

A

phosphorylated form is active

[opposite of glycogen synthase]

35
Q

The low glucagon and epinephrine levels in the period after eating a meal activate what protein?

A

protein phosphatase 1 (PP1)

36
Q

Protein phosphatase 1 (PP1) is activated/expressed in what period of time?

A

period of time after eating a meal, when glucagon and epinephrine levels are low

37
Q

What is the function of protein phosphatase 1 (PP1)?

A

activates glycogen synthase (by dephosphorylation)

inhibits glycogen phosphorylase (by dephosphorylation)

[yes, both mechanisms are by dephosphorylation]

38
Q

What does the term “favism” refer to?

A

refers to people who can’t tolerate Fava beans

39
Q

Favism results from what defect?

A

deficiency in G-6-P dehydrogenase (G6PDH)

40
Q

What is the significance of the enzyme G6P dehydrogenase (G6PDH)?

A

rate-limiting enzyme of PPP

41
Q

What is the rate-limiting enzyme of the PPP?

A

G6P dehydrogenase (G6PDH)

42
Q

What happens if a patient with favism eats fava beans?

A

most likely, acute hemolysis

43
Q

What are the symptoms of favism? (3)

A

jaundice

black urine

tired all the time

44
Q

What is a benefit of favism?

A

malaria resistance, because RBC membrane is too compromised to allow propagation of malaria parasite

45
Q

(T/F) Relatively speaking, the PPP consumes more glucose than glycolysis or glycogen synthesis.

A

False. The PPP consumes relatively smaller amounts of glucose compared to glycolysis or glycogen synthesis.

46
Q

Write out the overall reaction for the PPP.

A

G6P + 2 NADP+ → ribulose-5-phosphate + 2 NADPH + CO2

47
Q

Draw out the three steps of the PPP.

A
48
Q

How does NADPH maintain the reductive environment of the cytosol? (2)

A

glutathione reductase uses NADPH to reduce glutathione dimers to just glutathione

glutathione breaks disulfide bonds and maintains protein in reduced (-SH) state

49
Q

How is NADPH utilized in anti-bacterial mechanisms?

A

NADPH provides electrons for generation of ROS by phagocytic cells (NADPH oxidase) to kill invading bacteria

50
Q

How does NADPH protect cells from oxidative damage? Draw a diagram to support your answer.

A

glutathione peroxidase uses NADPH to convert H2O2 into water, which protects cells from oxidative damage

51
Q

The PPP is most active in tissues involved in…

Give three examples of such tissues.

A

fatty acid and steroid synthesis

for example:

liver

adrenal gland

adipose tissue

52
Q

Why is the PPP important in the eye?

A

cornea and lens lack mitochondria (therefore no TCA cycle) and have limited blood supply, so they use the PPP to metabolize glucose

53
Q

Why is the PPP important in RBCs? (2)

A

RBCs lack mitochondria (so no TCA cycle), making the PPP the only source of NADPH in RBCs

RBCs are susceptible to oxidative damage, which the PPP can help prevent

54
Q

What is the most common enzyme disorder in the world?

A

G6P dehydrogenase deficiency

55
Q

What is the most abundant small peptide in human cells?

A

glutathione

56
Q

What is the treatment for Pompe disease?

A

myozyme (recombinant acid alpha-glucosidase)

57
Q

Which two tissues have the highest concentration of glycogen in the body?

A

liver (5-10% of wet weight)

muscle (1-2% of wet weight)

58
Q

(T/F) Fat can be converted to glucose.

A

False

59
Q

In the polymerization reaction of glycogenesis, what serves as the leaving group?

A

UDP

60
Q

Write out the net chemical equation for glycogenesis/glycogen synthesis.

A

G6P + ATP + (glycogen)n + H2O → (glycogen)n+1 + ADP + 2 Pi

61
Q

How does glycogen phosphorylase break alpha-1,4 glycosidic linkages in glycogen?

A

uses inorganic phosphate to cleave the alpha-1,4 linkages

62
Q

What is the defect in Von Gierke’s disease?

A

glucose-6-phosphatase mutation

63
Q

What are the effects of von Gierke’s disease?

A

hypoglycemia

increased glycogen storage → hepatomegaly

lactic acidosis

64
Q

What is the most common treatment for von Gierke’s disease?

A

daily cornstach intake to prevent hypoglycemia

65
Q

Where in the cell does the PPP take place?

A

cytoplasm

66
Q

Does the PPP require ATP?

A

No.

Tissues (23 decks)

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