Cardio - Biochemistry - Pentose Phosphate Pathway

Question 1

True/False.

Glucose molecules that just entered the liver are likely to both be phosphorylated to glucose 6-phosphate and enter both glycolysis and the pentose phosphate shunt.

Answer 1

True.

Question 2

Where does the pentose phosphate pathway take place?

What types of tissue needs are answered by increased pentose phosphate pathway activity?

Answer 2

In the cytosol;

NADPH needs (e.g. for fatty acid synthesis, cholesterol/steroid synthesis, glutathione reduction)

Question 3

What substance is very important as a reducing agent in fatty acid/steroid/cholesterol synthesis?

Via what pathway is it produced?

Answer 3

NADPH;

the pentose phosphate pathway

Question 4

What are the two phases of the pentose phosphate pathway?

Answer 4

Oxidative phase (irreversible)

Nonoxidative phase (reversible)

Question 5

Describe the general reaction that occurs in the oxidative phase of the pentose phosphate pathway.

Answer 5

Glucose-6-phosphate is oxidized to:

• 2 NADPH
• Ribulose-5-phosphate
• CO₂

(irreversible)

Question 6

Describe the general reaction that occurs in the nonoxidative phase of the pentose phosphate pathway.

Answer 6

Ribose-5-phosphate is converted to:

• Nucleic acids
• GAP (glyceraldehyde 3-phosphate)
• F6P

(reversible)

Question 7

What is the first enzyme of the oxidative phase of the pentose phosphate pathway?

This enzyme is especially important because it is the:

Answer 7

Glucose-6-phosphate dehydrogenase;

rate-determining enzyme

Question 8

What pathway uses ribose-5-phosphate as a substrate?

Answer 8

Nucleotide biosynthesis (DNA synthesis)

Question 9

NADPH is used as a reducing agent for which biosynthetic pathways?

Answer 9

Fatty acid synthesis

Steroid/cholesterol synthesis

Glutathione reduction

Question 10

What is the main difference between NADPH and NADH in their function/use?

Answer 10

NADPH - used for biosynthesis

NADH - used for energy production

Question 11

For each individual glucose molecule that enters the pentose phosphate pathway, how many NADPH are produced?

Answer 11

2 NADPH per G6P

Question 12

The pentose phosphate pathway committed step is performed by which enzyme?

Answer 12

Glucose-6-phosphate dehydrogenase

Question 13

Differentiate between the structure of ribulose-5-phosphate and ribose-5-phosphate sugars in the pentose phosphate pathway.

Answer 13

RibULOSE-5-phosphate = keto sugar

RibOSE-5-phosphate = aldose sugar

(Note: image is of trioses, not pentoses)

Question 14

The isomerization of ribulose-5-phosphate to ribose-5-phosphate occurs as part of what reaction in the oxidative phase of the pentose phosphate pathway?

Answer 14

The 4th (and final) reaction

(beginning of nonoxidative steps)

Question 15

Transketolase is the enzyme used in the 2^nd step of the nonoxidative phase.

What cofactor is required by this enzyme?

Answer 15

Thiamine pyrophosphate

(derivative of vitamin B1)

Question 16

What are the three enzymes of the nonoxidative phase of the pentose phosphate shunt?

Answer 16

Epimerase, transketolase, transaldolase

Question 17

Describe the ‘shell game’ that the nonoxidative phase of the pentose phosphate shunt uses to move carbons around and get from ribose 5-phosphate to any of the following:

glyceraldehyde 3-phosphate, fructose 6-phosphate, glucose 6-phosphate

Answer 17

Question 18

How does the nonoxidative phase of the pentose phosphate shunt get from a pentose (ribose 5-phosphate) back to trioses (glyceraldehyde 3-phosphate) and hexoses (fructose 6-phosphate, glucose 6-phosphate)?

Answer 18

2-carbon (taken from X5P) added to 5-carbon (R5P)

–> 7-carbon (S7P)

1-carbon taken from S7P (and added to 3-carbon (G3P) –> 4-carbon (E4P))

–> F6P

–> G6P

Question 19

What three glycolytic intermediates can be made from the pentose phosphate shunt?

From which phase?

Answer 19

Glyceraldehyde 3-phosphate, fructose 6-phosphate, glucose 6-phosphate;

nonoxidative

Question 20

The nonoxidative phase of the pentose phosphate pathway requires 3 enzymes for its four reactions

- epimerase, transketolase & transaldolase -

which one(s) require thiamine pyrophosphate as a cofactor?

Answer 20

Only transketolase

Question 21

Discuss the role of the epimerase used in the nonoxidative phase of the pentose phosphate pathway.

Answer 21

It converts ribulose-5-phosphate to its epimer xylulose-5-phosphate

Question 22

What two glycolytic intermediates are formed by the reactions of xylulose-5-phosphate (from the nonoxidative phase of the pentose phosphate pathway)?

Answer 22

Glyceraldehyde-3-phosphate;

fructose-6-phosphate

Question 23

As concentrations of NADPH increase in the cell, how will this regulate enzymatic function?

Answer 23

Strongly inhibits glucose-6-phosphate dehydrogenase

(initial enzyme in PPP)

Question 24

What is the role of NADPH in red blood cells?

Answer 24

Reduce glutathione

–>

Protect membrane from oxidative damage (free radicals)

Question 25

**True/False.** If a cell needs (1) only NADPH, or (2) only ribose 5-phosphate, or (3) both, it can tailor the pentose phosphate shunt to meet its needs.

Answer 25

**True**. *_Only NADPH_* - Ribulose 5-P turned back into G6P (goes back into oxidative phase) *_Both_* - Ribulose 5P turned into ribose 5-P *_Only ribose 5-phosphate_* - Oxidative portion shut down; nonoxidative runs in reverse (F6P and GAP --\> R5P)

Question 26

Describe how a cell can modify the pentose phosphate shunt for each of the following conditions of need: (1) only NADPH (2) only ribose 5-phosphate (3) both

Answer 26

_Only NADPH_ - Ribulose 5-P turned back into G6P (goes back into oxidative phase) _Only ribose 5-phosphate_ - Oxidative portion shut down; nonoxidative runs in reverse (F6P and GAP --\> R5P) _Both_ - Ribulose 5P turned into ribose 5-P

Question 27

How would a red blood cell regulate the two phases of the pentose phosphate pathway? Will it produce NADPH, ribose 5-phosphate, or both?

Answer 27

RBC has no nucleus (and so no DNA synthesis) --- oxidative phase will be uninhibited to produce _only_ NADPH *(the nonoxidative phase will take ribulose-5-phosphate and convert it to glucose-6-phosphate so it can reenter the oxidative phase)*

Question 28

How would a cell that requires a lot of nucleotide synthesis and no NADPH regulate the pentose phosphate pathway?

Answer 28

Only the nonoxidative portion matters, but in reverse --\> F6P and GAP are converted to R5P *(Oxidative phase will be shut down to prevent ANY NADPH production (which would inhibit glucose-6-phosphate dehydrogenase and prevent ribose-5-phosphate production).)*

Question 29

If the oxidative phase of the pentose phosphate pathway is effectively shut down, what would a cell use to create a steady stream of ribose-5-phosphate?

Answer 29

Glycolytic intermediates: glyceraldehyde-3-phosphate and fructose-6-phosphate

Question 30

How much energy can be made from NADPH in oxidative phosphorylation?

Answer 30

None; it is only a biosynthetic reducing agent. (It is _not_ like NADH.)

Question 31

What is the _major_ controlling factor for the pentose phosphate pathway? What other factor plays an important role?

Answer 31

NADPH concentrations; xylulose 5-phosphate levels

Question 32

Discuss the effect of rising xylulose-5-phosphate levels in the cell following a large meal. *(Note: Xylulose 5 -phosphate is produced in the pentose phosphate pathway.)*

Answer 32

Xylulose-5-phosphate activates phosphoprotein phosphatase (PP2A); fed state --\> dephosphorylates PFK-2/FBPase-2 enzyme --\> increases glycolysis / decreases gluconeogenesis

Question 33

What is the importance of creating xylulose-5-phosphate from ribulose-5-phosphate?

Answer 33

Xylulose-5-phosphate is a regulatory molecule for carbohydrate and lipid metabolism. \*X5P structure is also used to form glycolytic intermediates

Question 34

What is the role of phosphoprotein phosphatase (PP2A)?

Answer 34

Dephosphorylates PFK-2/FBPase-2 enzyme --\> Increases glycolysis / decreases gluconeogenesis

Question 35

When the PFK-2/FBPase-2 enzyme is dephosphorylated, what results?

Answer 35

PFK-2 is activated FBPase-2 is inhibited *(increases glycolysis / decreases gluconeogenesis)*

Question 36

When PFK-2 is activated by the regulatory activity of xylulose-5-phosphate, what are the downstream effects?

Answer 36

PFK-2 increases fructose-2,6-bisphosphate levels, activating glycolysis and inhibiting gluconeogenesis

Question 37

When xylulose-5-phosphate levels rise and induce their regulatory actions on phosphoprotein phosphatase and the PFK-2/FBPase-2 enzyme, what is the end result of this regulatory pathway?

Answer 37

Increased fructose-2,6-bisphosphate levels --\> increased levels of acetyl-CoA (through glycolysis)

Question 38

If high levels of glucose-6-phosphate leads to **increased NADPH**, and high levels of ribose-5-phosphate (also X5P) lead to **increased acetyl-CoA**, what metabolic process will ultimately follow?

Answer 38

NADPH and Acetyl-CoA both increase **fatty acid synthesis**

Question 39

How does the tripeptide (3 amino acids) glutathione help protect the cell membrane from reactive oxygen species (ROS)?

Answer 39

Glutathione is an antioxidant (the SH group is easily oxidizable); also, it and glutathione peroxidase decrease H₂O₂ levels

Question 40

What three amino acids make up a glutathione molecule?

Answer 40

Glutamine - cysteine (SH side chain here) - glycine

Question 41

Reduced glutathione (90%) is: Oxidized glutathione (10%) is:

Answer 41

Glutathione-SH Glutathione disulfide (glutathione-S---S-glutathione)

Question 42

How many molecules of glutathione are needed to reduce H₂O₂ to H₂O? What redox state do the glutathione molecule(s) need to be in?

Answer 42

**2**; reduced state (GSH, not GS-SG)

Question 43

What enzyme reduces glutathione (GSSG to 2 GSH)? What enzyme oxidizes glutathione (2 GSH to GSSG) and decreases intracellular hydrogen peroxide?

Answer 43

Glutathione reductase; glutathione peroxidase

Question 44

How does NADPH help with glutathione peroxidase-mediated protection against H₂O₂?

Answer 44

NADPH provides the reducing power for glutathione reductase

Question 45

What is the role of glutathione reductase?

Answer 45

Take an oxidized glutathione dimer (GSSG) and tranforms it into two molecules of reduced glutathione (GSH)

Question 46

What is the most common enzyme disorder?

Answer 46

Glucose-6-phosphate dehydrogenase (G6PD) deficiency

Question 47

Quinine (an antimalarial drug) is oxidative and causes an increase in reactive oxygen species (ROS), sometimes leading to a decrease in a patient's RBC count. How does quinine do this?

Answer 47

Hemolytic anemia (especially in individuals with G6PD deficiency) * Decreased NADPH ---\>* * Glutathione can't be reduced --\>* * Membrane damaged by ROS*

Question 48

An alcoholic man shows up to the ER in a hemolytic crisis. What is the most likely underlying cause?

Answer 48

Thiamine deficiency (affects production of NADPH by the pentose phosphate pathway)

Question 49

What are the steps of the respiratory (oxidative) burst (from O₂ to hypochlorite)? What cells perform this function to create reactive oxygen species?

Answer 49

NADPH + 2 O₂ -- *(NADPH oxidase)* --\> NADP⁺ + **O₂^-** 2 O₂^- -- *(superoxide dismutase)* --\> **H₂O₂** H₂O₂ -- *(myeloperoxidase)* --\> **HOCl** **monocytes, macrophages, neutrophils**

Question 50

What three enzymes are part of the respiratory (oxidative) burst? What NADPH-dependent enzyme helps limit its damaging effects?

Answer 50

NADPH oxidase, superoxide dismutase, myeloperoxidase; glutathione peroxidase

Question 51

What NADPH-dependent enzyme decreases the amount of H₂O₂ in the cell?

Answer 51

**Glutathione peroxidase** 2 GSH --\> GSSG H₂O₂ --\> 2 H₂O

Question 52

**True/False.** Oxidizing drugs can easily deplete reduced glutathione stores and increase the effects of oxidation damage on RBCs.

Answer 52

True.

Question 53

What population is at-risk for favism? What substances should they avoid? Why?

Answer 53

Individuals with glucose 6-phosphate dehydrogenase (G6PD) deficiency; oxidizing drugs, fava beans; loss of reduced glutathione --\> hemolytic anemia