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BioChem Exam 4 > Lecture 28: Glucose Catabolism (Pentose Phosphate Pathway) > Flashcards

Lecture 28: Glucose Catabolism (Pentose Phosphate Pathway) Flashcards

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1
Q

Pentose Phosphate Pathway Overview

What is the Pentose Phosphate Pathway, generalized?

A

alternate pathway for oxidation of glucose to generate NADPH and metabolize 5-carbon sugars

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2
Q

Pentose Phosphate Pathway Overview

What are the main products of Pentose Phosphate Pathway?

A

NADPH and Ribose-5-phosphate

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3
Q

Pentose Phosphate Pathway Overview

Why is NADPH important?

A

It is an electron Donor
* Used in reductive biosynthesis of fatty acids and steroids
* repair of oxidative damage

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4
Q

Pentose Phosphate Pathway Overview

Why is different betwen NADH and NADPH?

A

NADPH has an additional phosphate

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5
Q

Pentose Phosphate Pathway Overview

Why is Ribose-5-Phosphate important?

A

It is a biosynthetic precursor of nucleotides
* used in DNA and RNA synthesis
* synthesis of some coenzymes

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6
Q

Pentose Phosphate Pathway Overview

Where does the Pentose Phosphate Pathway occur?

A

the cytosol

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7
Q

Pentose Phosphate Pathway Overview

What are the two phases of Pentose Phosphate Pathway?

A
  1. Oxidative phase
  2. Nonoxidative phase
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8
Q

Pentose Phosphate Pathway Overview

What are some important aspects about the oxidative phase?

A

produces NADPH
irreversible

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9
Q

Pentose Phosphate Pathway Overview

What are some important aspects about the nonoxidative phase?

A

produced ribose-5-phosphate
interconcerts phosphorylated sugars
reversible reactions to feed glycolysis

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10
Q

Oxidative Phase

What is the overall equation for this phase?

A

Glucose-6-phosphate + 2 NADP + H2O
–>
ribulose-5-phosphate + 2 NADOH + 2 H + CO2

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11
Q

Oxidative Phase

What does this phase produce?

A

NADPH and ribulose-5-phosphate

ribULOSE not ribOSE

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12
Q

Oxidative Phase

What enam catalyzes the first reaction and what is the result?

A

Enam: glucose-6-P dehydrogenase (rate limiting step)
Product: sigma-Phosphoglucono-b-lactone
Releases NADPH

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13
Q

Oxidative Phase

What enam catalyzed the second reaction and what is the result?

A

enam: lactonase
product: 6-phosphogluconate

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14
Q

Oxidative Phase

What enam catalyzed the third reaction and what is the result?

A

enam: 6-phosphogluconate
product: ribulose-5-phosphate
Releases NADPH

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15
Q

Oxidative Phase

Why is NADP used as the hydrogen acceptor instead of NAD?

A

The Km for glucose-6-phosphate dehydrogenase is 1000x lower using NADP than it is using NAD

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16
Q

Non-Oxidative Phase

What does this phase do?

A

isomerizes Ribulose-5-phosphate into ribose-5-phosphate

17
Q

Non-Oxidative Phase

What is it that catalyzes this isomerization?

A

Phosphopentose isomerase

18
Q

Non-Oxidative Phase

What are the two enams in this reaction?

A

Transketolase
Transaldolase

19
Q

Non-Oxidative Phase

What will transKetolase transfer?

A

2-C units

20
Q

Non-Oxidative Phase

What will transAldolase transfer?

A

3-C units

21
Q

Non-Oxidative Phase

What are the 3 reactions, their enam, and the net reaction?

A
  1. C5 + C5 <-> C3 + C7 (Transketolase)
  2. C3 + C7 <-> C6 + C4 (Transaldolase)
  3. C4 + C5 <-> C6 + C3 (Transketolase)
  4. 3 C5 <-> 2 C6 + C3 (net reaction)
22
Q

Non-Oxidative Phase

What can Glucose-6-phosphate be regenerated from?

A

3-C glyceraldehyse-3-phosphate or the 6-C fuctose-6-phosphate via enzymes of Gluconeogenesis

23
Q

Non-Oxidative Phase

How many six-carbon sugars can be created from 6 five-carbon sugars?

A

5
(6 ribose can be converted into 5 glucose)

24
Q

Non-Oxidative Phase

Why would glucose be regenerated during this phase?

A

When tissues, like liver and adipose, require more NADPH than ribose-5-phosphate

25
Q

Net Result

What is the net result from adding the oxidative phase and nonoxidative phase?

For tissues requiring more NADPH than R-5-P

A

Glucose-6-phosphate + 12 NADP + 7 H2O
–>
6 CO2 + 12 NADPH + 12 H + Pi

26
Q

Regulation of Pentose Phosphate Pathway

What controls the entry of glucose-6-phosphate into this pathway?

A

glucose-6-phosphate dehydrogenase

27
Q

Regulation of Pentose Phosphate Pathway

Which peice is a strong inhibitor of glucose-6-phosphate dehydrogenase?

A

NADPH

it competes with the NADP for the active binding site

28
Q

Regulation of Pentose Phosphate Pathway

How is the synthesis of glucose-6-phosphate dehydrogenase induced?

A

increased insulin/glucagon ratio after a high carbohydrate meal

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