Pentose Phosphate Pathway (PPP)

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Function: The Pentose Phosphate Pathway (PPP) is vital for producing NADPH and ribose-5-phosphate, essential for fatty acid synthesis, nucleotide synthesis, and maintaining redox balance.

Location: This pathway occurs in the cytoplasm of cells.

Steps and Enzymes Involved:

  1. Oxidative Phase:

    • Step 1: Glucose-6-phosphate is oxidized by glucose-6-phosphate dehydrogenase (G6PD), producing 6-phosphoglucono-δ-lactone and NADPH.

      • Reaction: Glucose-6-phosphate + NADP⁺ → 6-phosphoglucono-δ-lactone + NADPH + H⁺

    • Step 2: 6-Phosphoglucono-δ-lactone is hydrolyzed by lactonase to form 6-phosphogluconate.

      • Reaction: 6-phosphoglucono-δ-lactone + H₂O → 6-phosphogluconate

    • Step 3: 6-Phosphogluconate undergoes oxidative decarboxylation by 6-phosphogluconate dehydrogenase to produce ribulose-5-phosphate, CO₂, and NADPH.

      • Reaction: 6-phosphogluconate + NADP⁺ → ribulose-5-phosphate + CO₂ + NADPH

  2. Non-Oxidative Phase:

    • Step 4: Ribulose-5-phosphate is isomerized to ribose-5-phosphate by phosphopentose isomerase.

      • Reaction: Ribulose-5-phosphate → Ribose-5-phosphate

    • Step 5: Alternatively, ribulose-5-phosphate is converted to xylulose-5-phosphate by phosphopentose epimerase.

      • Reaction: Ribulose-5-phosphate → Xylulose-5-phosphate

    • Step 6: Ribose-5-phosphate and xylulose-5-phosphate can enter a series of reversible reactions catalyzed by transketolase and transaldolase, producing glyceraldehyde-3-phosphate and fructose-6-phosphate.

      • Reactions:

        • Ribose-5-phosphate + Xylulose-5-phosphate → Sedoheptulose-7-phosphate + Glyceraldehyde-3-phosphate (catalyzed by transketolase)

        • Sedoheptulose-7-phosphate + Glyceraldehyde-3-phosphate → Fructose-6-phosphate + Erythrose-4-phosphate (catalyzed by transaldolase)

        • Erythrose-4-phosphate + Xylulose-5-phosphate → Fructose-6-phosphate + Glyceraldehyde-3-phosphate (catalyzed by transketolase)

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