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13.5 - Pentose Phosphate Pathway, Galactose + Fructose Metabolism/Pathologies

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  1. Front
    Pentose phosphate pathway (PPP)
    Back
    A metabolic pathway distinct from glycolysis that occurs in the cytosol
  2. Front
    Location of PPP
    Back
    Cytoplasm
  3. Front
    Tissue distribution of PPP
    Back
    Occurs in all tissues with varying intensity
  4. Front
    Connection of PPP to glycolysis
    Back
    PPP is a separate but connected pathway to glycolysis
  5. Front
    Main products of PPP
    Back
    NADPH and ribose-5-phosphate
  6. Front
    Function of ribose-5-phosphate
    Back
    Used for nucleotide synthesis
  7. Front
    Function of NADPH
    Back
    Provides reducing power for biosynthetic and protective processes
  8. Front
    Uses of NADPH
    Back
    Fatty acid synthesis, steroid synthesis, glutathione reduction, deoxyribonucleotide formation, and detoxification
  9. Front
    Role of NADPH in glutathione reduction
    Back
    Maintains reduced glutathione for antioxidant protection
  10. Front
    Role of NADPH in nucleotide metabolism
    Back
    Converts ribonucleotides to deoxyribonucleotides
  11. Front
    Role of NADPH in detoxification
    Back
    Supports cytochrome P450 reactions
  12. Front
    PPP activity in liver
    Back
    Approximately 30 percent of glucose goes through PPP
  13. Front
    Phases of PPP
    Back
    Oxidative phase and non-oxidative phase
  14. Front
    Oxidative phase of PPP
    Back
    Produces NADPH and is irreversible
  15. Front
    Non-oxidative phase of PPP
    Back
    Rearranges sugars and is reversible
  16. Front
    Tissues with high PPP activity
    Back
    Adrenal gland, liver, testes, ovaries, and red blood cells
  17. Front
    PPP role in adrenal gland
    Back
    Supports steroid synthesis
  18. Front
    PPP role in liver
    Back
    Supports fatty acid synthesis, cholesterol synthesis, detoxification, and glutathione maintenance
  19. Front
    PPP role in testes and ovaries
    Back
    Supports steroid synthesis
  20. Front
    PPP role in red blood cells
    Back
    Maintains glutathione for protection against oxidative stress
  21. Front
    Reason RBCs depend on PPP
    Back
    They rely on NADPH to maintain reduced glutathione
  22. Front
    Link between PPP and glycolysis
    Back
    PPP intermediates can enter or leave glycolysis
  23. Front
    Types of reactions in PPP
    Back
    Irreversible oxidative reactions and reversible non-oxidative reactions
  24. Front
    Oxidative phase reactions
    Back
    Oxidation and decarboxylation reactions
  25. Front
    Regulation of oxidative phase
    Back
    Inhibited by high NADPH to NADP⁺ ratio
  26. Front
    Key regulatory principle of PPP
    Back
    High NADPH slows PPP and low NADPH activates PPP
  27. Front
    First enzyme of oxidative phase
    Back
    Glucose-6-phosphate dehydrogenase
  28. Front
    Reaction of glucose-6-phosphate dehydrogenase
    Back
    Glucose-6-phosphate → 6-phosphoglucono-δ-lactone with NADP⁺ reduced to NADPH
  29. Front
    Second enzyme of oxidative phase
    Back
    Lactonase
  30. Front
    Reaction of lactonase
    Back
    6-phosphoglucono-δ-lactone → 6-phosphogluconate
  31. Front
    Third enzyme of oxidative phase
    Back
    6-phosphogluconate dehydrogenase
  32. Front
    Reaction of 6-phosphogluconate dehydrogenase
    Back
    6-phosphogluconate → ribulose-5-phosphate with NADP⁺ reduced to NADPH
  33. Front
    Overall output of oxidative phase
    Back
    NADPH and ribulose-5-phosphate
  34. Front
    Non-oxidative phase of PPP
    Back
    Reversible phase of the pentose phosphate pathway that rearranges sugars rather than oxidizing them
  35. Front
    Main function of the non-oxidative phase of PPP
    Back
    Interconverts sugar phosphates to match cellular needs for ribose-5-phosphate and glycolytic intermediates
  36. Front
    Reversibility of the non-oxidative phase
    Back
    The reactions are reversible
  37. Front
    Oxidation in the non-oxidative phase
    Back
    No oxidation occurs
  38. Front
    Decarboxylation in the non-oxidative phase
    Back
    No decarboxylation occurs
  39. Front
    Main types of reactions in the non-oxidative phase
    Back
    Isomerization, epimerization, and carbon-unit transfer reactions
  40. Front
    Starting sugar of the non-oxidative phase
    Back
    Ribulose-5-phosphate
  41. Front
    Isomerization step of the non-oxidative phase
    Back
    Ribulose-5-phosphate is converted to ribose-5-phosphate
  42. Front
    Enzyme for ribulose-5-phosphate to ribose-5-phosphate
    Back
    Phosphopentose isomerase
  43. Front
    Epimerization step of the non-oxidative phase
    Back
    Ribulose-5-phosphate is converted to xylulose-5-phosphate
  44. Front
    Enzyme for ribulose-5-phosphate to xylulose-5-phosphate
    Back
    Phosphopentose epimerase
  45. Front
    Transketolase
    Back
    Enzyme that transfers 2-carbon units in the non-oxidative phase
  46. Front
    Transaldolase
    Back
    Enzyme that transfers 3-carbon units in the non-oxidative phase
  47. Front
    Cofactor required by transketolase
    Back
    TPP (thiamine pyrophosphate)
  48. Front
    Vitamin required for transketolase
    Back
    Thiamine (vitamin B1)
  49. Front
    Cofactor required by transketolase
    Back
    TPP (thiamine pyrophosphate)
  50. Front
    Vitamin required for transketolase activity
    Back
    Thiamine (vitamin B1)
  51. Front
    Effect of thiamine deficiency on PPP
    Back
    Decreases transketolase activity
  52. Front
    First transketolase reaction
    Back
    Xylulose-5-phosphate + ribose-5-phosphate → glyceraldehyde-3-phosphate + sedoheptulose-7-phosphate
  53. Front
    Products of the first transketolase reaction
    Back
    Glyceraldehyde-3-phosphate and sedoheptulose-7-phosphate
  54. Front
    Transaldolase reaction
    Back
    Sedoheptulose-7-phosphate + glyceraldehyde-3-phosphate → fructose-6-phosphate + erythrose-4-phosphate
  55. Front
    Products of the transaldolase reaction
    Back
    Fructose-6-phosphate and erythrose-4-phosphate
  56. Front
    Second transketolase reaction
    Back
    Xylulose-5-phosphate + erythrose-4-phosphate → fructose-6-phosphate + glyceraldehyde-3-phosphate
  57. Front
    Products of the second transketolase reaction
    Back
    Fructose-6-phosphate and glyceraldehyde-3-phosphate
  58. Front
    Overall products of the non-oxidative phase
    Back
    Fructose-6-phosphate and glyceraldehyde-3-phosphate can be produced from pentose phosphates
  59. Front
    Glycolytic link of the non-oxidative phase
    Back
    Fructose-6-phosphate and glyceraldehyde-3-phosphate feed into glycolysis
  60. Front
    Role of ribose-5-phosphate in the non-oxidative phase
    Back
    Can be produced for nucleotide synthesis or converted into other sugars
  61. Front
    Role of xylulose-5-phosphate in the non-oxidative phase
    Back
    Acts as a ketose donor in transketolase reactions
  62. Front
    Role of sedoheptulose-7-phosphate
    Back
    Seven-carbon intermediate formed during sugar rearrangement
  63. Front
    Role of erythrose-4-phosphate
    Back
    Four-carbon intermediate formed during sugar rearrangement
  64. Front
    Why the non-oxidative phase is important
    Back
    It allows PPP to supply either ribose-5-phosphate or glycolytic intermediates depending on cellular demand
  65. Front
    Cellular demand handled by the non-oxidative phase
    Back
    Need for more ribose-5-phosphate than NADPH, equal need for both, or more NADPH than ribose-5-phosphate
  66. Front
    Key feature of the non-oxidative phase
    Back
    Sugar phosphates are rearranged without direct production of NADPH
  67. Front
    Interconversion of PPP sugars
    Back
    Ribose-5-phosphate and xylulose-5-phosphate can form glycolytic intermediates
  68. Front
    Glycolytic intermediates formed from PPP
    Back
    Fructose-6-phosphate and glyceraldehyde-3-phosphate
  69. Front
    Connection of PPP to glycolysis
    Back
    PPP can feed into glycolysis via F6P and GAP
  70. Front
    Flexibility of PPP
    Back
    Adapts to cellular needs for NADPH and ribose-5-phosphate
  71. Front
    PPP scenario 1
    Back
    More ribose-5-phosphate needed than NADPH
  72. Front
    PPP scenario 2
    Back
    Equal need for NADPH and ribose-5-phosphate
  73. Front
    PPP scenario 3
    Back
    More NADPH needed than ribose-5-phosphate
  74. Front
    Adaptive function of PPP
    Back
    Adjusts metabolic flow based on demand for biosynthesis or reducing power
  75. Front
    Metabolism of other sugars
    Back
    Non-glucose sugars can be converted into glycolysis intermediates
  76. Front
    Interconversion of carbohydrate metabolism
    Back
    Glycolysis intermediates can also be used to synthesize other sugars
  77. Front
    Galactose metabolism overview
    Back
    Galactose is converted into glucose intermediates for entry into glycolysis
  78. Front
    First step of galactose metabolism
    Back
    Galactose → galactose-1-phosphate
  79. Front
    Enzyme of first galactose step
    Back
    Galactokinase
  80. Front
    Second step of galactose metabolism
    Back
    Galactose-1-phosphate reacts with UDP-glucose to form glucose-1-phosphate and UDP-galactose
  81. Front
    Enzyme of second galactose step
    Back
    Uridyltransferase
  82. Front
    Third step of galactose metabolism
    Back
    UDP-galactose is converted to UDP-glucose
  83. Front
    Enzyme of third galactose step
    Back
    Epimerase
  84. Front
    Entry of galactose into glycolysis
    Back
    Converted to glucose-1-phosphate and then enters glycolytic pathways
  85. Front
    Uridyltransferase deficiency
    Back
    Leads to galactosemia and galactosuria
  86. Front
    Galactosemia
    Back
    Disorder caused by impaired galactose metabolism
  87. Front
    Galactosuria
    Back
    Presence of galactose in urine
  88. Front
    Effect of galactose metabolism defect
    Back
    Galactitol accumulation causes cataracts
  89. Front
    Effect of galactose-1-phosphate accumulation
    Back
    Causes brain and liver damage
  90. Front
    Treatment of galactosemia
    Back
    Removal of galactose and lactose from diet
  91. Front
    Fructose metabolism overview
    Back
    Fructose is converted into glycolysis intermediates mainly in the liver
  92. Front
    Minor pathway of fructose metabolism
    Back
    Hexokinase can phosphorylate fructose with low affinity
  93. Front
    Major pathway of fructose metabolism
    Back
    Occurs in liver via fructokinase
  94. Front
    First step of major fructose pathway
    Back
    Fructose → fructose-1-phosphate
  95. Front
    Enzyme of first fructose step
    Back
    Fructokinase
  96. Front
    Second step of fructose metabolism
    Back
    Fructose-1-phosphate is cleaved by aldolase B
  97. Front
    Entry of fructose into glycolysis
    Back
    Occurs after conversion to intermediates by aldolase B
  98. Front
    Hereditary fructose intolerance
    Back
    Genetic disorder caused by aldolase B deficiency
  99. Front
    Effects of aldolase B deficiency
    Back
    Increased fructose-1-phosphate, decreased phosphate and ATP, hypoglycemia, and liver and kidney damage
  100. Front
    Cause of ATP depletion in fructose intolerance
    Back
    Accumulation of fructose-1-phosphate traps phosphate
  101. Front
    Clinical features of fructose intolerance
    Back
    Fructosemia, fructosuria, hypoglycemia, and organ damage
  102. Front
    Treatment of fructose intolerance
    Back
    Removal of fructose from diet
  103. Front
    PPP summary
    Back
    Occurs in cytosol and produces NADPH and ribose-5-phosphate
  104. Front
    PPP phases summary
    Back
    Oxidative phase produces NADPH and is irreversible, non-oxidative phase rearranges sugars and is reversible
  105. Front
    Key PPP enzymes summary
    Back
    Transketolase requires TPP and transaldolase transfers carbon units
  106. Front
    PPP regulation summary
    Back
    Inhibited by high NADPH levels
  107. Front
    Galactose metabolism summary
    Back
    Galactose is converted to glucose intermediates via galactose-1-phosphate
  108. Front
    Key enzyme in galactose metabolism
    Back
    Uridyltransferase
  109. Front
    Disease of galactose metabolism
    Back
    Galactosemia due to uridyltransferase deficiency
  110. Front
    Fructose metabolism summary
    Back
    Fructose is converted to fructose-1-phosphate by fructokinase and processed by aldolase B
  111. Front
    Key enzyme in fructose metabolism
    Back
    Aldolase B
  112. Front
    Disease of fructose metabolism
    Back
    Hereditary fructose intolerance due to aldolase B deficiency

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