The pentose phosphate pathway is a metabolic pathway parallel to glycolysis to form ribose-5-phosphate, which can be used to synthesize nucleotides.
The net equation of the pentose phosphate pathway is: Glucose 6-Phosphate + 2 NADP+ + H2O ↔ Ribose 5-Phosphate + 2 NADPH + CO2 + 2H+. The pathway occurs in the cytoplasm in an oxidate phase and a non-oxidative phase.
Oxidative Phase
The purpose of the oxidative phase is to produce NADPH which can be used in reductive reactions as a reducing agent. Fatty acid synthesis is a good example of using reduced NADPH. Key enzymes are glucose-6-phosphate dehydrogenase, which converts glucose-6-phosphate to 6-phosphogluconolactone and 6-phosphogluconate dehydrogenase, which converts 6-phosphogluconolactone to ribulose-5-phosphate.
Non-oxidative Phase
The purpose of the non-oxidative phase is to produce ribose-5-phosphate which can be used to synthesize DNA/RNA nucleotides. Furthermore, the non-oxidative phase allows us to interconvert between glycolytic intermediates, essentially allowing us to regulate the “flux” through glycolysis and other metabolic pathways.
Practice Questions
Khan Academy
Glucose-6-phosphate dehydrogenase and the most important function of the pentose phosphate pathway
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MCAT Official Prep (AAMC)
Sample Test C/P Section Passage 4 Question 21
Section Bank B/B Section Question 86
Section Bank B/B Section Question 89
Section Bank B/B Section Question 98
Key Points
• The pentose phosphate pathway is an alternative path for the oxidation of glucose
• The net equation for the pentose phosphate pathway is: Glucose 6-Phosphate + 2 NADP+ + H2O ↔ Ribose 5-Phosphate + 2 NADPH + CO2 + 2H+
• The rate-limiting enzyme for the pentose phosphate pathway is glucose-6-phosphate dehydrogenase
• Glucose-6-phosphate dehydrogenase is activated by NADP+ and inhibited by NADPH and insulin
Key Terms
oxidation: is the breakdown of a molecule as it loses at least one of its electrons
NADPH: nicotinamide adenine dinucleotide phosphate (NADP) carrying electrons and bonded with a hydrogen (H) ion; the reduced form of NADP+.
glycolysis: the cellular degradation of the simple sugar glucose to yield pyruvic acid and ATP as an energy source.