Question

Write a sequence of balanced reactions (intermediates and enzymes) for the catabolism of six molecules of glucose-6-phosphate by the pentose phosphate pathway followed by conversion of ribulose-5-phosphate back to glucose-6- phosphate by gluconeogenesis. What is the net reaction (reactants in equilibrium with products) for this “cycle”?

Answer 1

The equation for catabolism of six molecules of glucose-6-phosphate (G6P) by the pentose phosphate pathway is:

6 G6P + 12 NADP⁺ + 6 H₂O → 6 Ribulose-5-phosphate + 12 NADPH + 12 H⁺ + 6 CO₂

(The intermediates are 6P gluconate and then converted to Ribulose-5-phosphate (Ru5p)).

Ribulose-5-phosphate can be converted to Glucose-6-Phosphate by transaldolase, transketolase, and gluconeogenesis:

6 Ribulose-5-phosphate + H₂O → 5 Glucose-6-phosphate + P_i.

The net equation is,

Glucose-6-Phosphate + 12 NADP⁺ + 7 H₂O → 12 NADPH + 12 H⁺ + 6 CO₂ + P_i.