Pentose Phosphate Pathway

Question 1

In the PPP, glucose-6-phosphate in the cytosol is is oxidized to

Answer 1

• Ribose 5-phosphate –> nucleotide synthesis
• CO₂
• NADPH –> used for other pathways like
  
  • FA and steroid synthesis
  • Monooxygenase detoxification
  • Glutathione defense system against reactive oxygen species

Question 2

If the cell has a lot of NADH, it gets used for ___

If the cell has lot of NADPH, it gets used for ____, requiring an active PPP.

Answer 2

NADH –> E production via ETC

NADPH –> biosynthetic reactions, requiring an active PPP

Question 3

What are the purposes of the oxidative & nonoxidative phases of the PPP?

Answer 3

Oxidative phase: generate 2 NADPH and the 5-carbon phosphopentose sugar; counter ROS damage

Nonoxidative phase recycles the pentose phosphate back into useful glycolytic intermediates, which can be further metabolized

Question 4

What are the 4 enzymes of the PPP oxidative phase, in order?

Answer 4

1. Glucose-6-phosphate dehydrogenase (oxidation)
2. Lactonase (hydrolysis)
3. 6-phosphogluconate dehydrogenase (oxidative decarboxylatoin)
4. Phosphopentose isomerase (isomerization)

​–> D-Ribose5-phosphate

Question 5

Which is the pathway commitment step of PPP?

Describe its regulation.

Answer 5

Glucose 6-phosphate + NAD –^{<em>G6P</em> <em>dehydrogenase</em>}–> 6-phosphoglucono-lactone + NADPH

Allosteric regulation - high [glucose-6-phosphate] from glycolysis promotes this rxn; high [NADPH] inhibits it

Question 6

When does the ring structure of lactone get hydrolyzed into its straight chain linear form?

Answer 6

6-Phosphoglucono-lactone –^{<strong><em>Lactonase</em></strong>}–> 6-Phosphogluconate

Question 7

In the third PPP reaction,

Answer 7

• 6-phosphogluconate dehydrogenase oxidizes 6-phosphogluconate
• Another NADPH is produced
• CO₂ formed, generating Ribulose 5-phosphate

Question 8

Fourth PPP reaction

Answer 8

Ribulose 5-phosphate –^{phosphopentose isomerase}–> Ribose 5-phosphate

This is a ketose-to-aldose conversion

Question 9

___ is a precursor for nucleic acid synthesis

Answer 9

Ribose 5-phosphate

(an aldose)

Question 10

Cells that actively synthesize steroids or FA (need NADPH, but not nucleotide synthesis), need to get rid of all the Ribose-5-phosphate from the oxidative phase.

What happens to it?

Answer 10

Nonoxidative phase converts it into intermediates for glycolysis, glycogenesis, or PPP

Question 11

First reaction of the NONoxidative phase is catalyzed by an epimerase that converts what to what?

Answer 11

Ribose5-phosphate –> Xylulose 5-phosphate (ketose)

Question 12

Second step of the nonoxidative phase

Answer 12

Transketolase transfers a 2C fragment from Xylulose 5-phosphate to Ribose 5-phosphate

• Turned these 5C sugars to a 3Cand 7C sugar.
• Cofactor: thiamine pyrophosphate

Question 13

Thiamine pyrophosphate

Answer 13

derivative of vitamin B1; necessary cofactor for the second step of the nonoxidative phase - transketolase

Question 14

The third step of the PPP nonoxidative phase

Answer 14

Transaldolase transfers a 3C fragment from a 7C compound to the 3C Glyceraldehyde-3-phosphate –> fructose-6-phosphate

–> 7C sugar bcomes a 4C sugar

3C sugar becomes a 6C sugar (F6P)

Question 15

Final step of the PPP nonoxidative phase

Answer 15

Transketolase transfers a 2C fragment from a second molecule of Xylulose 5-phosphate to the 4C sugar

• Xylulose 5-phosphate -> Glyceraldehyde 3-phosphate (3C)
• 4C sugar -> Fructose 6-phosphate (6C)

Question 16

The two ___ and one ___ generated in the nonoxidative phase are glycolytic intermediates, which can be converted to ____ that can be used for ___

Answer 16

The two fructose-6-phosphate & one Glyceradehyde 3- phosphate can be converted to glucose 6-phosphate.

Can be for glycogen storage, glycolysis, gluconeogenesis, or feed back intothe PPP

Question 17

___ inhibits glucose 6-phosphate dehydrogenase

Answer 17

NADPH

Question 18

RBCs don’t need nucleotides (don’t have a nucleus), but does need a lot of NADPH to protect its membranes from oxidative damage.

So what does it do w glucose 6-phosphate from the nonoxidative phase?

Answer 18

Glucose-6-phosphate produced from the nonoxidative phase can reenter the PPP oxidative phase

Question 19

If a cell only needs Ribose 5-phosphate, but not NADPH, then

Answer 19

Oxidative phase is shut down by NADPH inhibiting G6P dehydrogenase

Nonoxidative phase is run in the reverse, using both fructose 6-phosphate and glyceraldehyde 3-phosphate to generate the phosphopentose

Question 20

Xylulose 5-phosphate as a regulator of carbohdyrate and lipid metabolism

Answer 20

1. Xylulose 5-phosphate activates phophoprotein phosphatease (PP2A), which will dephosphorylate the PFK-2/FBPase-2 enzyme,

–> activates PFK-2, inhibits FBPase-2

–> increased Fructose 2,6-bisphosphate activates glycolysis, inhibits gluconeogenesiss

–> NADPH from PPP and Acetyl-CoA from glycolysis stimulate FA synthesis.

Question 21

Glutathione exists in 2 states because of its sulfhydryl R-group. Describe them.

RBCs use NADPH to ___ glutathione.

Answer 21

Oxidized glutathione (GSSG) forms a disulfide bond between two glutathione molecules

Reduced glutathione molecules (2GSH) does not

RBCs use NADPH to reduce glutathione –> 2GSH preferred

Question 22

ROs are produced in the mitochondria through ionizing radiation, oxidizng rxns, sulfa an dantimalarial drugs, etc; can cause tissue damage

How do we reduce them?

Answer 22

1. Glutathione peroxidase & Glutathione reduces hydrogen peroxide (H₂O₂) to H₂O and O₂ –> 2GSH oxidized to GSSG
2. Glutathione reductase uses NADPH from PPP to reduce GSSG back to 2GSH

Question 23

Drug-induced hemolytic anemia is caused by

Answer 23

G6P dehydrogenase deficiency –> NADPH levels drop –> less GSH

Usually asymptomatic unless you have a sudden increase in ROS from infection, drugs, or favabeans –> damaged RBC membranes –> hemolytic crisis & anemia

Question 24

What are some sources of hemolytic crisis (yellow skin, mucus, and urine; rise of body temp; fatigue; rapid pulse; fast breathing)

Answer 24

• Oxidative/sulfa drugs
  
  • Ex) Quinine used protect against malaria, but causes G6PDH deficiency
• Fava beans
• Alcoholics have thiamine pyprophosphate deficiency

Question 25

Is the PPP more active when well-fed or starved?

Answer 25

Well fed, becuase [glucose-6-phosphate] is higher and NADPH is beign used for FA synthesis

Question 26

Why would RBCs be more sensitive than other tissues to defects in pyruvate kinase (PK)?

Answer 26

They don't have mitochondria, so they only get ATP from glycolysis. Need pyruvate kinase