Pentose Phosphate Pathway of Glucose Oxidation

• The pentose phosphate pathway (phosphogluconate pathway, hexose monophosphate pathway) is a pathway that oxidizes glucose 6-phosphate, producing pentose phosphates and NADPH.
• Cells that use the pentose phosphate pathway:

  • Rapidly dividing cells use ribose 5-phosphate to make RNA, DNA, and coenzymes.
  • Tissues that carry out extensive fatty acid synthesis (liver, adipose, lactating mammary gland) require the NADPH provided by this pathway
  • Tissues that actively synthesize cholesterol and steroid hormones (liver, adrenal glands, gonads) require the NADPH provided by this pathway.
• Which pathway would be the most important to produce sugars required in high concentration in advance mitosis?

  • Pentose phosphate, oxidative phase
  • Gluconeogenesis
  • Lactate fermentation
  • Pentose phosphate, nonoxidative phase
  • None would contribute the specific sugars required in advance of mitosis

  Pentose phosphate, oxidative phase. The oxidative phase produced ribose phosphates needed for doubling genomic DNA.
• The oxidative phase produced NADPH and pentose phosphates.
• Glucose 6-phosphate dehydrogenase (G6PD) catalyzes the oxidation of glucose 6-phosphate to 6-phosphoglucono-delta-lactone using NADP+ as the electron acceptor.
• Glucose 6-phosphate dehydrogenase catalyzes the conversion of glucose 6-phosphate to 6-phosphoglucono-delta-lactone.

  • Uses Mg2+
  • NADP+ becomes NADPH and H+
• Lactonase catalyzes the hydrolysis of lactone to free acid 6-phosphogluconate.

  • Requires Mg2+ and H2O
• 6-phosphogluconate dehydrogenase catalyzes the oxidation and decarboxylation of 6-phosphogluconate to form ribulose 5-phosphate and NADPH.

  • Requires Mg2+
  • Produces CO2
• Phosphopentose isomerase converts ribulose 5-phosphate to its aldolase isomer, ribose 5-phosphate.
• Which pathway leads to a loss of carbon, which is undesirable in organisms that cannot fix carbon?

  • Glycolysis
  • Gluconeogenesis
  • Pentose phosphate, oxidative phase
  • Pentose phosphate, nonoxidative phase
  • None of the answers are correct

  Pentose phosphate, oxidative phase. The 6-phosphogluconate dehydrogenase reaction oxidatively decarboxylates 6-phosphogluconate to form the ketopentose ribulose 5-phosphate.
• The overall equation for the pentose phosphate pathway is:

  • Glucose 6-phosphate + 2 NADP+ + H2O --> ribulose 5-phosphate + CO2 + 2 NADPH + 2 H+
• The nonoxidative phase recycles pentose phosphates to glucose 6-phosphate.
• Ribulose 5-phosphate epimerase epimerizes ribulose 5-phosphate to xylulose 5-phosphate
• The conversion of pentose phosphates to glucose 6-phosphate begins the oxidative cycle again.
• Transketolase catalyzes the transfer of a two carbon fragment from a ketose donor to an aldose acceptor.
• The first transketolase reaction yields sedoheptulose 7-phosphate.
• Transaldolase catalyzes the condensation of a three carbon fragment from sedoheptulose 7-phosphate and glyceraldehyde 3-phosphate, forming fructose 6-phosphate and the tetrose erythrose 4-phosphate.
• Transketolase and transalsolase:

  • Play key roles during the oxidative phase of the pentose phosphate pathway
  • Convert glucose 6-phosphate to ribulose 5-phosphate and reduce NADP+ to NADPH
  • Catalyze the interconversion of 3-, 4-, 5-, 6-, and 7-carbon sugars
  • Are located in the mitochondria


  Catalyze the interconversion of 3-, 4-, 5-, 6-, and 7-carbon sugars. The first transketolase reaction converts the five carbon ribose 5-phosphate and xylulose 5-phosphate to the seven carbon sedoheptulose 7-phosphate and the three carbon glyceraldehyde 3-phosphate.
• The second transketolase reaction forms fructose 6-phosphate and glyceraldehyde 3-phosphate from erythrose 4-phosphate and xylulose 5-phosphate.

  • Uses TPP
• Thiamine pyrophosphate (TPP) stabilizes a two carbon carbanion in the transketolase reaction.
• Protonated Schiff base stabilizes the carbanion in the transaldolase reaction.
• The first and third steps of the oxidative pentose phosphate pathway are essentially irreversible in the cell.
• The nonoxidative pentose phosphate pathway reactions are readily reversible.
• The reductive pentose phosphate pathway converts hexose phosphates to pentose phosphates

  • Central to the photosynthetic assimilation of CO2 by plants
  • Essentially the reversal of the nonoxidative reactions of the pentose phosphate pathway
• Glucose 6-phosphate is partitioned between glycolysis and the pentose phosphate pathway.
• The relative concentrations of NADP+ and NADPH determine whether glucose 6-phosphate enters glycolysis or the pentose phosphate pathway.
• Which outcome would be an inappropriate biochemical response to low blood sugar?

  • Increase in gluconeogenesis
  • Increase in pentose phosphate, nonoxidative phase
  • Increase in formation of glycogen
  • Inhibition of glycolysis
  • Increase in pentose phosphate, oxidative phase

  Increase in pentose phosphate, oxidative phase. The oxidative phase converts glucose 6-phosphate to ribulose 5-phosphate. Therefore, increasing metabolite flow through this pathway would decrease the total glucose stores.
• Thiamine deficiency causes Beriberi and Wernicke-Korsakoff Syndrome.
• Thiamine is the precursor to the cofactor thiamine pyrophosphate (TPP).
• Beriberi is characterized by swelling, pain, paralysis, and ultimately, without treatment, death.
• Wernicke-Korsakoff syndrome is characterized by problems with voluntary movements. It is more common among heavy drinkers due to decreased intestinal absorption of thiamine.
• What should individuals avoid if they have a transketolase mutation the results in an enzyme with a lowered affinity for thiamine pyrophosphate (TPP)?

  • Heavy alcohol consumption
  • Foods rich in vitamin B1
  • Fava beans
  • Antimalarial drugs

  Heavy alcohol consumption.