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Biochemistry II > Pentose Phosphate Pathway > Flashcards

Pentose Phosphate Pathway Flashcards

1
Q

What are the four major pathways that utilize glucose?

A
  1. The formation of structural polysaccharides
  2. The formation of glycogen
  3. Glycolysis
  4. Pentose phosphate pathway
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2
Q

Glycolysis breaks glucose down into two molecules of _____________ in the cytosol of most tissues.

A

Pyruvate

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3
Q

The pentose phosphate pathway breaks glucose down into ____________ in the cytosol, which can then be used for nucleotide synthesis.

A

Ribose

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4
Q

When do we want glycolysis to occur within cells?

A

When ATP levels are low

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5
Q

How do we know which pathway will occur?

A

Cellular conditions of ATP concentration and NADP+ concentration in the cytosol

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6
Q

What are the two primary roles of the pentose phosphate pathway?

A
  1. Ribose synthesis
    1. Nucleotide formation (ATP, NAD+, FAD)
  2. NADPH production
    1. Anabolic/biosynthetic pathways
    2. Reducing environment
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7
Q

In what tissues is the pentose phosphate pathway a significant pathway for glucose metabolism?

A

In actively dividing cells, including skin, tumor, bone marrow, liver, and adipose cells

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8
Q

Why is NADPH important in red blood cells?

A

It provides the reducing environment required to counter the oxidative stress placed on them by oxygen exposure

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9
Q

The pentose phosphate pathway can be broken down into two phases. What are they?

A
  1. The oxidative phase
  2. The non-oxidative phase
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10
Q

What is the role and/or purpose of the oxidative phase of the pentose phosphate pathway?

A

To produce ribose and NADPH

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11
Q

There are four enzymatic steps in the oxidative phase of the pentose phosphate pathway. What are they?

A
  1. Glucose-6-phosphate is converted to 6-phosphoglucono-delta-lactone via glucose-6-phosphate dehydrogenase
  2. 6-phosphoglucono-delta-lactone is converted to 6-phosphogluconate via glactonase
  3. 6-phosphogluconate is converted to ribulose-5-phosphate via 6-phosphogluconate dehydrogenase
  4. Ribulose-5-phosphate is converted to ribose-5-phosphate via phosphopentoisomerase
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12
Q

The pentose phosphate pathway begins with what molecule?

A

Glucose-6-phosphate

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13
Q

In the PPP, glucose-6-phosphate is converted to 6-phosphoglucono-delta-lactone via what enzyme?

A

Glucose-6-phosphate dehydrogenase

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14
Q

What type of reaction does glucose-6-phosphate dehydrogenase catalyze?

A

A redox reaction

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15
Q

What cofactor is required for glucose-6-phosphate dehydrogenase to oxidize glucose-6-phosphate to 6-phosphoglucono-delta-lactone?

A

NADP+

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16
Q

What enzyme in the PPP converted 6-phosphoglucono-delta-lactone to 6-phosphogluconate?

A

Lactonase

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17
Q

What reaction does lactonase catalyze in converted 6-phosphoglucono-delta-lactone to 6-phosphogluconate?

A

Ester hydrolysis

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18
Q

What are the products of an ester hydrolysis?

A

A carboxylic acid and an alcohol

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19
Q

What enzyme catalyzes the conversion of 6-phosphogluconate to ribulose-5-phosphate?

A

6-phosphogluconate dehydrogenase

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20
Q

What reaction does 6-phosphogluconate dehydrogenase catalyze?

A

An oxidative decarboxylation

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21
Q

What cofactor is required for 6-phosphogluconate dehydrogenase to facilitate the oxidative decarboxylation of 6-phosphogluconate to ribulose-5-phosphate?

A

NADP+

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22
Q

What enzyme catalyzes the formation of ribose-5-phosphate from ribulose-5-phosphate?

A

Phosphopentoisomerase

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23
Q

What reaction does phosphopentoisomerase catalyze?

A

An aldo-keto isomerization (a ketose to converted to an aldose)

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24
Q

What is the structure of glucose-6-phosphate?

A
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25
Q

What is the structure of 6-phosphoglucono-delta-phosphate?

A

C1 of cyclic glucose becomes a carbonyl (of an ester)

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26
Q

What is the electron movement (mechanism) for a cyclic ester hydrolysis?

A
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27
Q

Draw the mechanism for the cyclic ester hydrolysis of 6-phosphoglucono-delta-phosphate via lactonase.

A

Oxygen of carbonyl protonates forming an oxygen cation

Water attacks carbonyl carbon resulting in a temporary “diol”

A proton from water is transferred to the cyclic oxygen

A hydroxyl “kicks” back on the former carbonyl carbon, reforming a carbonyl, and is deprotonated (ultimately leaving behind a carboxylic acid on the former carbonyl and a hydroxyl group on the former cyclic oxygen)

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28
Q

6-phosphogluconate dehydrogenase catalyzes an oxidative decarboxylation. This is also known as a ___-_____ decarboxylation because you must first form a ___-____ acid before decarboxylation can occur.

A

Beta-keto

Beta-keto

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29
Q

Why is a coenzyme not required for the oxidative decarboxylation of 6-phosphogluconate?

A

Because a beta-keto acid intermediate is formed, which can be stabilized via resonance; this is not possible for decarboxylation events in which an alpha-keto acid is formed

30
Q

What is the mechanism for the oxidative decarboxylation of 6-phosphogluconate to ribulose-5-phosphate via 6-phosphogluconate dehydrogenase?

A
  1. A general base deprotonates the hydroxyl on the beta carbon, expelling a hydride ion
  2. The hydride ion protonates NADP+ forming NADPH
  3. The oxygen atom bound to the carbonyl group kicks back and expells carbon dioxide
  4. The oxygen atom on C2 of the resultant molecule kicks back and the alkene protonates, thereby forming ribulose-5-phosphate
31
Q

What is the structure of ribulose-5-phosphate?

32
Q

What is the structure of ribose-5-phosphate?

33
Q

What is the mechanism for the isomerization of ribulose-5-phosphate to ribose-5-phosphate via phosphopentoisomerase?

A

Add picture

34
Q

Is the oxidative phase of the pentose phosphate pathway reversible or irreversible?

A

Although technically reversible, most of the steps heavily favor the products, resulting in basically a irreversible process

35
Q

If the pentose phosphate pathway ends after the oxidative path, what is the net reaction?

A

G6P + 2 NADP+ + H2O –> ribose-5-phoshate + CO2 + 2 NADPH + 2 H+

36
Q

From which carbon of glucose does the carbon dioxide come in the oxidative decarboxylation of 6-phosphogluconate?

37
Q

Which phase of the pentose phosphate pathway involves the interconversion of 3, 4, 5, 6, and 7 carbon sugars?

A

The non-oxidative phase

38
Q

What is the role/purpose of the non-oxidative phase of the pentose phosphate pathway?

A

To recycle ribose so that we can make glucose (i.e., when we do not need NADPH)

39
Q

Is the non-oxidative phase of the pentose phosphate pathway reversible?

A

Yes; its reversibility is key

40
Q

What is the net reaction of the non-oxidative phase of the PPP?

A

2 xylulose-5-phosphate + ribose-5-phosphate 2 fructose-6-phosphate + glyceraldehyde-3-phosphate

3 C5 2 C6 + 1 C3

41
Q

What is the overview of the non-oxidative phase of the PPP?

A

C5 + C5 –> C3 + C7

C3 + C7 –> C6 + C4

C4 + C5 –> C6 + C3

3C5 –> 2F6P + 1C3

42
Q

What is the overall pathway of the non-oxidative phase of the pentose phosphate pathway?

A

3 C5 <–> 1 C3 + 2 C6

43
Q

What are the intermediates of the non-oxidative phase of the PPP?

A
  1. Glyceraldehyde-3-phosphate
  2. Sedoheptulose-7-phosphate
  3. Erythrose-4-phosphate
44
Q

Which three 5 carbon sugars are involved in the non-oxidative phase of the PPP?

A
  1. Ribulose-5-phosphate
  2. Ribose-5-phosphate
  3. Xylulose-5-phosphate
45
Q

What enzyme catalyzes the interconversion of ribulose-5-phosphate to ribose-5-phosphate?

A

Phosphopentoisomerase

46
Q

What enzyme catalyzes the interconversion of ribulose-5-phosphate to xylulose-5-phosphate?

47
Q

Xylulose-5-phosphate is a _____ epimer of ___________-___-____________.

A

C3 epimer

Ribulose-5-phosphate

48
Q

That are three enzymatic steps in the non-oxidative phase of the PPP. What are they?

A
  1. Xylulose-5-phosphate and ribose-5-phosphate are converted to glyceraldehyde-3-phosphate and sedoheptulose-7-phosphate
  2. Glyceraldehyde-3-phosphate and sedoheptulose-7-phosphate are converted to fructose-6-phosphate and erythrose-4-phosphate
  3. Erythrose-4-phosphate and xylulose-5-phosphate are converted to fructose-6-phosphate and glyceraldehyde-3-phosphate
49
Q

What is the structure of ribulose-5-phosphate?

A

Add picture

50
Q

What is the structure of xylulose-5-phosphate?

Hint - xylulose-5-phosphate is a C3 epimer of ribulose-5-phosphate

A

Add picture

51
Q

What is the structure of glyceraldehyde-3-phosphate?

A

Add picture

52
Q

What is the structure of sedoheptulose-7-phosphate?

53
Q

Is sedoheptulose-7-phosphate a ketose or an aldose?

54
Q

What is the structure of erythrose-4-phosphate?

55
Q

Is erythrose-4-phosphate a ketose or an aldose?

56
Q

What enzyme catalyzes the first step of the non-oxidative phase of the PPP?

(I.e., the conversion of xylulose-5-phosphate and ribose-5-phosphate to glyceraldehyde-3-phosphate and sedoheptulose-7-phosphate)?

A

Transketolase

57
Q

What enzyme catalyzes the second step of the non-oxidative phase of the PPP?

(Glyceraldehyde-3-phosphate + Sedoheptulose-7-phosphate <–> Fructose-6-phosphate + Erythrose-4-phosphate)

A

Transaldolase

58
Q

What enzyme catalyzes the last step of the non-oxidative phase of the PPP?

Erythrose-4-phosphate + xylulose-5-phosphate <–> fructose-6-phosphate + glyceraldehyde-3-phosphate

A

Transketolase

59
Q

The transketolase mechanism involves the transfer of __ carbon units utilizing ____?

A

Transfer of 2 carbon units

TPP

60
Q

What does TPP stand for?

A

Thiamine pyrophosphate

61
Q

The transketolase mechanism involves the transfer of a 2-carbon unit from a __________ donor to a ____________ acceptor.

A

Ketose donor

Aldose acceptor

62
Q

The transaldolase mechanism involves the ___________ side chain forming a Schiff base.

63
Q

What is a Schiff base?

64
Q

What acts as an electron sink for the transketolase mechanism? The transaldolase mechanism?

A

Thiamine pyrrophosphate acts as the electron sink in the transketolase mechanism

The Schiff base acts as the electron sink in the transaldolase mechanism

65
Q

What is the transketolase mechanism?

A
  1. TPP attacks the carbonyl carbon of the ketose donor
  2. The 5C ketose splits into a 2C and a 3C molecule
  3. The 2C donor - still attached to TPP - attacks an incoming aldose; the aldehyde group converts to a hydroxyl
  4. The carbonyl reforms on the 2C donor, expelling TPP and producing a 7C molecule
66
Q

What is the transalolase mechanism?

A
  1. Dehydration: lysine residue attacks carbonyl carbon of ketose sedoheptulose-7-phosphate, expelling water
  2. Schiff base formation
  3. C-C bond breakage: the hydroxyl group on C4 kicks back and breaks the C-C bond between C3 and C4
  4. Loss of erthryose-4-phosphate
  5. Nucleophilic attack by alkene of glyceraldehyde-3-phosphate (aldehyde becomes hydroxyl)
  6. Schiff base reformed and hydrolyzed, forming F6P
67
Q

What are the active components of both the transketolase and transaldolase enzymes? In other words, what are the “predict the products”?

68
Q

What is the important of glucose-6-dehydrogenase in the PPP?

A

It is the first enzyme in the oxidative phase of the PPP and produces NADPH as a byproduct, which is an allosteric regulator of glucose-6-dehydrogenase

69
Q

What is an allosteric regulator of glucose-6-phosphate dehydrogenase?

A

NADP+ and NADPH

70
Q

What activates glucose-6-phosphate dehydrogenase?

A

Increased levels of NADP+

Decreased levels of NADPH

71
Q

What inhibits glucose-6-phosphate dehydrogenase?

A

Increased levels of NADPH

Decreased levels of NADP+

Biochemistry II (19 decks)

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