Chapter 20: Pentose Phosphate Pathway

Pentose Phosphate Pathway Phase 1

Oxidative, nonreversible steps

1. 3 (glucose 6-phosphate) + 3(NADP⁺) → 3 (6-phosphogluconate) + 3(NADPH) [G6PD (glucose 6-phosphate dehydrogenase]
   
   

2. 3 (6-phosphogluconate) + 3 (NADP)→ 3 (ribulose 5-phosphate) + 3 (NADPH)

Pentose Phosphate Pathway Phase 2

Nonoxidative, reversible steps, continuation of step 2

• 3 (ribulose 5-phosphate) → ribose 5-phosphate + xylulose 5-phosphate

• Ribose 5-phosphate is essential for synthesis of nucleotides, DNA, RNA

Fructose Catabolism Step 1

• Fructose → fructose 1-phosphate

• Main enzyme is fructokinase

Fructose Catabolism Step 2

Fructose 1-phosphate → DHAP + glyceraldehyde

Fructose Catabolism Step 3

• Glyceraldehyde → glyceraldehyde 3-phosphate

• Either enter glycolysis or gluconeogenesis

Galactose Catabolism Step 1

• Galactose → galactose 1-phosphate

• Main enzyme is galactokinase

Galactose Catabolism Step 2

• Galactose 1-phosphate ⇔ glucose 1-phosphate

• Main enzyme is galactose 1-phosphate uridyl transferase

Aldose Reductase

Enzyme in blood that converts glucose to sorbitol

Glutamine

• Involves in synthesis of amino sugars

• Donates amino group to fructose 6-phosphate to form glucosamine 6-phosphate

Lactose Synthase

Enzyme that converts UDP galactose to lactose for mammary gland (breast milk production)