The Pentose Phosphate Pathway
High-Yield Summary
- The pentose phosphate pathway (PPP, hexose monophosphate shunt) branches off glycolysis at glucose-6-phosphate. Its purpose is NOT ATP — it makes NADPH and ribose-5-phosphate.
- Oxidative phase (irreversible): glucose-6-phosphate dehydrogenase (rate-limiting) oxidizes G6P, producing 2 NADPH total and ribulose-5-phosphate, which becomes ribose-5-phosphate for nucleotide synthesis.
- Non-oxidative phase (reversible): rearranges pentose sugars, can feed fructose-6-phosphate back into glycolysis.
- NADPH is an electron DONOR for biosynthesis/antioxidant defense (fatty acid/cholesterol synthesis, respiratory burst, maintaining reduced glutathione) — opposite role from NADH (electron acceptor feeding the ETC for ATP).
- Red blood cells depend almost entirely on the PPP for NADPH, given their high oxidative stress exposure.
Key Terms
- Glucose-6-phosphate dehydrogenase (G6PD)
- Rate-limiting, irreversible first enzyme of the PPP's oxidative phase; deficiency causes hemolytic anemia under oxidative stress.
- Ribose-5-phosphate
- Pentose sugar product used to build nucleotides (DNA/RNA).
- Reduced glutathione
- Cell's primary antioxidant defense against reactive oxygen species; requires NADPH to stay reduced.
- Respiratory burst
- NADPH-driven production of reactive oxygen species by white blood cells to destroy pathogens.
Two-Phase Pathway
- 1Glucose-6-phosphate diverted from glycolysis into the PPP
- 2Oxidative phase (irreversible): G6P → 6-phosphogluconate → ribulose-5-phosphate, producing 2 NADPH
- 3Ribulose-5-phosphate → ribose-5-phosphate (for nucleotide synthesis) or into the non-oxidative phase
- 4Non-oxidative phase (reversible): sugar rearrangement can regenerate fructose-6-phosphate to re-enter glycolysis
NADH vs. NADPH
| Feature | NADH vs. NADPH |
|---|---|
| Role | Electron acceptor / Electron donor |
| Typical fate | Feeds ETC → ATP / Powers biosynthesis and antioxidant defense |
| Source pathway | Glycolysis, citric acid cycle / Pentose phosphate pathway |
Common MCAT Trap
- The PPP does not produce ATP — don't tally its NADPH into an ATP-yield calculation the way you would NADH.
- NADPH and NADH are chemically similar but functionally opposite: NADPH is a donor for biosynthesis, NADH is an acceptor headed for the ETC.
- The oxidative phase is irreversible; the non-oxidative phase is reversible — direction of sugar flow can go either way in the latter.
Quick Recall
What are the two major products of the PPP?
What is the rate-limiting enzyme of the PPP?
Why do red blood cells depend heavily on the PPP?
Which phase of the PPP is reversible?