Pentose Phosphate Pathway (PPP) Lecture

May 21, 2024

Pentose Phosphate Pathway (PPP) Lecture

Importance of PPP

  • Takes place in the cytoplasm
  • Two main reasons for importance:
    1. Generation of 5-carbon pentose sugars:
      • Used to produce DNA, RNA, ATP, NADH, FADH2, and coenzyme A
    2. Production of reducing agent NADPH:
      • Used in biosynthetic processes (e.g., fatty acids, cholesterol, nucleotides)
      • Involved in detoxification processes

Oxidative Phase of PPP

  • Focus of this lecture
  • Main activities:
    • Oxidation of glucose
    • Release of CO2
    • Generation of NADPH

Step-by-Step Breakdown of the Oxidative Phase

Step 1: Dehydrogenation

  • Enzyme: Glucose 6-phosphate dehydrogenase
  • Substrate: Glucose 6-phosphate
  • Reaction:
    • NADP+ acts as an electron acceptor
    • Two electrons and an H+ ion are extracted from glucose 6-phosphate
    • Generates NADPH and 6-phosphoglucono-δ-lactone
  • Significance:
    • Formation of NADPH (reducing agent)
    • Formation of an intramolecular ester bond (to be broken in step 2)

Step 2: Hydrolysis

  • Enzyme: Lactonase
  • Reaction: Opens the ring structure of 6-phosphoglucono-δ-lactone via hydrolysis
  • Product: 6-phosphogluconate
  • Preparation for next step: Molecule is now ready for decarboxylation

Step 3: Oxidative Decarboxylation

  • Enzyme: 6-phosphogluconate dehydrogenase
  • Reaction: Oxidation and decarboxylation of 6-phosphogluconate
    • Removal of carbon 1 (and its associated oxygens) to form CO2
    • Generation of NADPH and ribulose 5-phosphate
  • Outcome: Net production of 2 NADPH molecules from one glucose 6-phosphate

Step 4: Isomerization

  • Enzyme: Phosphopentose isomerase
  • Reaction: Converts ribulose 5-phosphate (ketose) into ribose 5-phosphate (aldose)

Summary of the Oxidative Phase

  • Four main steps:
    1. Oxidation-reduction reaction forming NADPH
    2. Hydrolysis opening the ring structure
    3. Oxidative decarboxylation forming another NADPH and releasing CO2
    4. Isomerization converting ribulose 5-phosphate to ribose 5-phosphate
  • Outcome: Generation of necessary molecules for cell functions and biosynthesis
  • Preparation for the non-oxidative phase in subsequent lectures