Pentose Phosphate Pathway

Question 1

What is the pentose phosphate pathway?

Answer 1

Oxidation of glucose 6-phosphate to pentose phosphates

Question 2

What are the two phases of the pentose phosphate pathway?

Answer 2

1. The oxidative phase: involves oxidation reduction (dehydrogenases)
2. The nonoxidative phase: goal is to regenerate glucose 6-phosphate

Question 3

What are the two important products of the pentose phosphate pathway?

Answer 3

1. Ribose 5 phosphate: makes DNA and RNA
2. NADPH: useful if the cell is going to face oxidative damage caused by free radicals

Question 4

How many steps are in the oxidative phase of the pentose phosphate pathway

Answer 4

4 steps

Question 5

Describe the first step in oxidative phase of the pentose phosphate pathway

Answer 5

Oxidation of glucose 6-phosphate by glucose 6-phosphate dehydrogenase to form 6-phospho-glucono-δ-lactone
First molecule of NADPH is generated

Question 6

What is the role of NADP+ in the first step of the oxidative phase of the pentose phosphate pathway?

Answer 6

NADP+ acts as the electron acceptor
The pentose phosphate pathway is unable to proceed without NADP+ as the electron acceptor

Question 7

Describe the second step in oxidative phase of the pentose phosphate pathway

Answer 7

Lactonase hydrolyzes 6-phospho-glucono-δ-lactone to 6-phosphogluconate

Question 8

Describe the third step in oxidative phase of the pentose phosphate pathway

Answer 8

6-phosphogluconate undergoes oxidation and decarboxylation by 6-phosphogluconate dehydrogenase to form ketopentose ribulose 5-phosphate
Second molecule of NADPH = generated

Question 9

Describe the fourth step in oxidative phase of the pentose phosphate pathway

Answer 9

Phosphopentose isomerase converts ribulose 5-phosphate to its aldose isomer ribose 5-phosphate

Question 10

Describe the first step of the nonxidative phase of the pentose phosphate pathway

Answer 10

• Epimerization / Transketolase
• Ribulose 5-phosphate is epimerized to xylulosze 5-phosphate
• Part of xylulose 5-phospahte gets broken and stitched to ribose 5-phosphate in a transketolase reaction (TPP = a cofactor)
• The new products are glyceraldehyde 3-phosphate and sedoheptulose 7-phosphate

Question 11

What is the significance of TPP

Answer 11

• TPP facilitates the making and breaking of carbon-carbon bonds by stabilizing the carbanion intermediate

Question 12

Describe the second step of the nonxidative phase of the pentose phosphate pathway

Answer 12

• Transaldolase
• A three carbon fragment is removed from sedoheptulose 7-phosphate and condensed with glyceraldehyde 3-phosphate
• The new products are erythrose 4-phosphate and xylulosze 5-phosphate
  *Carbanion intermediate is stabilized by Lysine (protonated Schiff base)

Question 13

Describe the third step of the nonxidative phase of the pentose phosphate pathway

Answer 13

• Transketolase (again!)
• erythrose 4-phosphate and xylulosze 5-phosphate combine
• The new products are fructose 6-phosphate and glyceraldehyde 3-phosphate
• Catalyzed by TPP

Question 14

How can NADPH regulate the use of glucose 6-phosphate in either the pentos phosphate pathway or glycolysis through feedback inhibition?

Answer 14

• Glucose 6-phosphate = partitioned between glycolysis and pentose phosphate pathway
• Whether glucose 6-phosphate ends up in the pentose phosphate pathway or glycolysis depends on what the cell need and the concentration of NADPH in the cytosol
• Too much NADPH (low NADP+) will stop the pentos phosphate pathway and glucose 6-phosphate will be used for glycolysis
• Low NADPH (high NADP+) will increase the flow of glucose 6-phosphate through the pentose phosphate pathway

Question 15

Why wouldn’t Pythagoras eat falafel?

Answer 15

• Pythagoras had a glucose-6-phosphate dehydrogenase deficiency which is needed in the first step of the Pentos phosphate pathway
• Falafel = made from fava beans
• Fava beans contain a molecule called divicine which reacts with oxygen to form superoxide radical which can then form hydrogen peroxide
• Hydrogen peroxide is typically converted to water by glutathione peroxidase (which uses reduced glutathione (GSH)) BUT if this does not happen hydroxyl free radicals will form and cause a lot of damage
• Glutathione reductase generates GSH from its oxidized form (GSSG)
• Glutathione reductase cannot function without NADPH (produced in the first step of the Pentos phosphate pathway)