Pentose-Phosphate pathway

Question 1

What is the ratio of NADH to NAD+ in cells?

Why?

Answer 1

1/1000 (NADH/NAD+)

The reason is that NADH generated by glycolysis and CAC is efficiently used up by the mitochondria in the ETC.

Question 2

Why can’t NADH be used as a reducing power?

Answer 2

It is very low in concentration in the cell because it is efficiently used up by the mitochondria.

Question 3

What is used instead of NADH for its reducing power? Why can it be used?

Answer 3

NADPH.
Cannot be used in oxidative phosphorylation, mitochondria cannot used it.
NADPH is 100x more abundant than NADP+ and thus can carry out synthetic reactions.
The second reason is that NADPH is used to quench oxygen radicals.

Question 4

What are the two reasons for the use of NADPH?

Answer 4

1 - Reducing power for synthetic reactions

2 - Reducing power to neutralize oxygen radicals.

Question 5

When would we require a lot of NADPH?

Answer 5

During cell division, to generate ribose sugar for nucleic acids.
During periods of oxidative stress.

Question 6

What is the overall reaction of the pentose-phosphate pathway?

Answer 6

3 G6P + 6 NADP+ + 3 H2O –> 6 NADPH + 6H+ + 3 CO2 + 2 F6P + GAP

Question 7

How many enzymes are involved in the pentose-phosphate pathway?

Answer 7

7

Question 8

What are the different, major steps of the pentose-phosphate pathway?

Answer 8

1 - Oxidation reactions
2 - Isomerization and epimerization reactions
3 - C-C bond cleavage and formation reactions

Question 9

What are the different enzymes that fall under the oxidative reactions?

Answer 9

G6PDH
6-phosphogluconolactonase
6-phosphogluconate DH

Question 10

What are the enzymes that fall under the isomerization and epimerization reactions?

Answer 10

Ribulose-5-phosphate isomerase

Ribulose-5-phosphate epimerase

Question 11

What are the enzymes that fall under the C-C bond cleavage and formation reactions?

Answer 11

Transketolase

Transaldolase

Question 12

The pentose-phosphate pathway starts with G6P, why not glucose?

Answer 12

Glucose is immediately phosphorylated by HK.

Question 13

Per molecule of G6P, how much NADPH is produced?

Answer 13

2 NADPH

Question 14

Out of 3 glucose molecules, how many CO2 come out? How many F6P and GAP?

Answer 14

3 CO2
2 F6P
1 GAP

Question 15

What is the side product of the pentose-phosphate pathway?

Answer 15

R5P

Question 16

What is the main enzyme that controls the flux of this pathway?
How is it regulated?

Answer 16

G6PDH
Strongly inhibited by NADPH
As the concentration of NADPH goes up, the activity of this enzyme decreases.

Question 17

In the pentose-phosphate pathway, there is a very important side product. What is it and why is it not an end product?

Answer 17

R5P, used for generation of nucleic acids.

Not an end product since it gets recycled back to glucose or is used up.

Question 18

______ is used to generated reduced _______.

Answer 18

NADPH

Glutathione (GSH)

Question 19

_____ plays a critical role in quenching the oxyradicals in the cells.

Answer 19

GSH

Question 20

If ______ generation is inhibited due to a mutation in ___DH, cells become susceptible to oxidative damage.

Answer 20

NADPH

G6P

Question 21

People with a ______ defect are resistant to malaria.

Answer 21

G6PDH

Question 22

What occurs if oxygen is not given two electrons to become H2O?

Answer 22

Becomes superoxide, negatively charged, unpaired electron

Question 23

What enzyme deals with superoxides?

Answer 23

Superoxide dismutase

Question 24

What does superoxide dismutase do?

Answer 24

Converts superoxides to hydrogen peroxide

Question 25

How is hydrogen peroxide converted to water?

Answer 25

Glutathione peroxidase

Question 26

How does glutathione peroxidase neutralize hydrogen peroxide?

Answer 26

Reduced form of glutathione peroxidase gives a hydrogen to make 2 H20 and becomes oxidized.

Question 27

How goes oxidized glutathione peroxidase get reduced again?

Answer 27

Glutathione reductase, using NADPH, reduces oxidized glutathione peroxidase.

Question 28

What occurs if hydrogen peroxide accumulates?

Answer 28

Takes one proton/electron and gets converted to water and OH radical

Question 29

How are people with a defective version of G6PDH resistant to malaria?

Answer 29

Increased oxidative stress in the blood system causes invading parasite, which is very susceptible to oxygen radicals, to die.