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The Pentose Phosphate Pathway

Tier 4 · #28 of 33 · lower yield · topic 3.7 — every high-yield fact from the book chapter, exam-ready.

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  1. Front
    What two products does the pentose phosphate pathway (PPP) generate that make it essential to every cell?
    Back
    NADPH and ribose-5-phosphate
  2. Front
    How much ATP does the pentose phosphate pathway produce?
    Back
    None
  3. Front
    At which glycolytic intermediate does the PPP branch off?
    Back
    Glucose-6-phosphate
  4. Front
    In which cellular compartment does the PPP run?
    Back
    The cytosol
  5. Front
    What is the alternative name for the pentose phosphate pathway ?
    Back
    The hexose monophosphate shunt
  6. Front
    Which phase of the PPP is irreversible and generates NADPH ?
    Back
    The oxidative phase
  7. Front
    Which phase of the PPP is reversible and rearranges carbon skeletons?
    Back
    The non-oxidative phase
  8. Front
    What is the committed, rate-limiting enzyme of the entire PPP ?
    Back
    Glucose-6-phosphate dehydrogenase (G6PD)
  9. Front
    What ratio regulates G6PD activity (rather than ATP or AMP as in glycolysis)?
    Back
    The NADPH/NADP+ ratio
  10. Front
    G6PD is feedback-inhibited by which of its own products?
    Back
    NADPH
  11. Front
    Per glucose-6-phosphate oxidized, the oxidative phase yields ___ NADPH , ___ ribose-5-phosphate , and ___ CO2.
    Back
    2 NADPH , 1 ribose-5-phosphate , 1 CO2
  12. Front
    What product does G6PD form from glucose-6-phosphate in step 1 of the oxidative phase?
    Back
    6-phosphogluconolactone
  13. Front
    Which enzyme catalyzes step 2 of the oxidative phase (hydrolysis of the lactone ring)?
    Back
    6-phosphogluconolactonase
  14. Front
    What is the planted exam distractor for the step-2 enzyme 6-phosphogluconolactonase , and which pathway does it actually belong to?
    Back
    Phosphohexose isomerase — a glycolysis enzyme
  15. Front
    Which oxidative-phase enzyme oxidatively decarboxylates 6-phosphogluconate , releasing CO2 and the second NADPH ?
    Back
    6-phosphogluconate dehydrogenase
  16. Front
    What 5-carbon sugar is the direct product of 6-phosphogluconate dehydrogenase ?
    Back
    Ribulose-5-phosphate
  17. Front
    Which enzyme converts ribulose-5-phosphate to the ribose-5-phosphate needed for nucleotide synthesis?
    Back
    Ribulose-5-phosphate isomerase
  18. Front
    At which step of the oxidative phase does the carbon count drop from six to five (carbon lost as CO2)?
    Back
    Step 3 ( 6-phosphogluconate dehydrogenase , the decarboxylation step)
  19. Front
    What are the two key enzymes of the non-oxidative phase that reshuffle carbon skeletons?
    Back
    Transketolase and transaldolase
  20. Front
    Which cofactor does transketolase require?
    Back
    Thiamine pyrophosphate (TPP)
  21. Front
    Which two glycolytic intermediates do transketolase and transaldolase produce when shunting excess pentose back into glycolysis?
    Back
    Fructose-6-phosphate and glyceraldehyde-3-phosphate
  22. Front
    How can a cell make ribose-5-phosphate without producing any NADPH ?
    Back
    By running the non-oxidative phase in reverse ( transketolase / transaldolase pulling fructose-6-phosphate and glyceraldehyde-3-phosphate backward)
  23. Front
    NADPH production is exclusive to which phase of the PPP and ___ occur in reverse.
    Back
    The oxidative phase ; cannot
  24. Front
    How many NADPH does each elongation cycle of fatty acid synthase consume?
    Back
    Two
  25. Front
    How many NADPH are required to build one palmitate from eight acetyl-CoA units?
    Back
    14 NADPH
  26. Front
    Which three biosynthetic processes absolutely require NADPH (per exam question 18)?
    Back
    Fatty acid synthesis, cholesterol synthesis, and steroid hormone synthesis
  27. Front
    Does beta-oxidation require NADPH , and what reduced cofactors does it produce instead?
    Back
    No — it produces FADH2 and NADH
  28. Front
    Which enzyme regenerates reduced glutathione (GSH) from GSSG using NADPH ?
    Back
    Glutathione reductase
  29. Front
    What is GSSG ?
    Back
    Glutathione disulfide (the oxidized form of glutathione)
  30. Front
    Which enzyme uses GSH to neutralize H2O2?
    Back
    Glutathione peroxidase
  31. Front
    Why do erythrocytes depend entirely on the PPP for reducing power against oxidative stress?
    Back
    They have no mitochondria , so the PPP is their only source of NADPH
  32. Front
    Which enzyme requiring NADPH converts ribonucleotides to deoxyribonucleotides?
    Back
    Ribonucleotide reductase
  33. Front
    Which NADPH -dependent liver enzymes oxidize drugs and xenobiotics in detoxification?
    Back
    Cytochrome P450 enzymes
  34. Front
    What is the most common human enzyme deficiency worldwide (~400 million people)?
    Back
    G6PD deficiency
  35. Front
    What is the inheritance pattern of G6PD deficiency ?
    Back
    X-linked (males predominantly affected)
  36. Front
    Precipitation of oxidized hemoglobin in G6PD-deficient red cells forms what characteristic inclusion?
    Back
    Heinz bodies
  37. Front
    What is the hematologic consequence of G6PD deficiency when oxidative stress hits?
    Back
    Hemolytic anemia
  38. Front
    Ingestion of which food classically triggers hemolysis in G6PD deficiency ( favism )?
    Back
    Fava beans
  39. Front
    Name three drugs that can trigger hemolysis in G6PD deficiency .
    Back
    Primaquine, dapsone, nitrofurantoin
  40. Front
    Are G6PD-deficient patients typically anemic at rest?
    Back
    No — they are asymptomatic until an oxidative trigger ; hemolysis is episodic and oxidant-provoked
  41. Front
    How long after the oxidative trigger does hemolysis (dark urine, jaundice , anemia) appear in G6PD deficiency ?
    Back
    1 to 3 days
  42. Front
    G6PD deficiency confers protection against which infection, explaining its geographic distribution?
    Back
    Malaria ( Plasmodium falciparum )
  43. Front
    Besides glycolysis and the PPP , what two fates can glucose-6-phosphate be directed toward?
    Back
    Glycogen synthesis (storage) and gluconeogenesis
  44. Front
    Why is PPP activity relatively low in skeletal muscle ?
    Back
    Muscle has high ATP demand but minimal biosynthetic output
  45. Front
    Which steroid-producing tissue has high PPP activity for hormone synthesis?
    Back
    The adrenal cortex
  46. Front
    Why is the PPP described as a self-regulating, demand-driven pathway rather than hormonally toggled?
    Back
    It is controlled by the NADPH/NADP+ ratio at G6PD — as NADPH is consumed, NADP+ rises and relieves product inhibition, increasing flux automatically
  47. Front
    A thiamine (TPP) deficiency impairs which phase of the PPP , and via which enzyme?
    Back
    The non-oxidative phase , via transketolase (which requires thiamine pyrophosphate )
  48. Front
    Thiamine deficiency that impairs the PPP 's non-oxidative phase is associated with which two named clinical syndromes?
    Back
    Beriberi and Wernicke's encephalopathy
  49. Front
    Beyond steroid synthesis in the adrenal cortex and antioxidant defense in erythrocytes , name the two high- PPP -activity tissues that run it for fatty acid synthesis.
    Back
    Liver and adipose tissue
  50. Front
    Which two metabolic roles drive the liver 's high PPP activity?
    Back
    Fatty acid/cholesterol synthesis and drug (xenobiotic) detoxification
  51. Front
    Which tissue relies on high PPP activity primarily for fatty acid synthesis (not steroid synthesis or antioxidant defense)?
    Back
    Adipose tissue
  52. Front
    Does the insulin/glucagon axis directly toggle the PPP the way it governs glycolysis and gluconeogenesis?
    Back
    No — the PPP is not directly regulated by insulin/glucagon ; it is controlled by the NADPH/NADP+ ratio at G6PD
  53. Front
    Cloze: The PPP is a ___-regulated, demand-driven pathway whose flux is set by the ___ ratio rather than by a hormonal toggle.
    Back
    self ; NADPH/NADP+
  54. Front
    Order the integrated antioxidant chain in erythrocytes : G6PD / PPP → ___ → ___ → GSH → ___ → neutralizes H2O2.
    Back
    NADPH → glutathione reductase → GSH → glutathione peroxidase
  55. Front
    In the PPP antioxidant chain, what does NADPH directly power, and what does that enzyme regenerate?
    Back
    Glutathione reductase , which regenerates GSH from GSSG
  56. Front
    Cloze: GSH neutralizes H2O2 through the enzyme ___, becoming oxidized to ___ in the process.
    Back
    glutathione peroxidase ; GSSG
  57. Front
    Trace the G6PD-deficiency hemolysis cascade in order: no G6PD → no ___ → no ___ regeneration → ___ accumulate → oxidize Hb/membrane → Heinz bodies + hemolysis.
    Back
    NADPH → GSH → ROS
  58. Front
    In G6PD deficiency , what is the immediate biochemical consequence of losing G6PD that ultimately causes hemolysis?
    Back
    No NADPH from the PPP , so glutathione reductase cannot regenerate GSH and ROS accumulate
  59. Front
    Why are erythrocytes uniquely vulnerable in the G6PD-deficiency cascade while most other cells tolerate it?
    Back
    RBCs have no mitochondria , so the PPP is their only source of NADPH ; other cells get NADPH from mitochondrial sources
  60. Front
    In the G6PD-deficiency cascade, what do the accumulated ROS oxidize, producing the precipitated-hemoglobin inclusion seen on smear?
    Back
    Hemoglobin (and membrane proteins) , forming Heinz bodies
  61. Front
    Which oxidized cofactor does the PPP oxidative phase reduce — NADP+ or NAD+ ?
    Back
    NADP+ (reduced to NADPH ); the PPP uses NADP+ , not NAD+/NADH
  62. Front
    Besides liver , adipose , adrenal cortex , and erythrocytes , which cell population has high PPP activity — and for what purpose?
    Back
    Rapidly dividing cells — for nucleotide ( ribose-5-phosphate ) biosynthesis
  63. Front
    Besides fava beans and oxidant drugs, what is the third classic trigger of hemolysis in G6PD deficiency ?
    Back
    Infection

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