Question

In the oxidative branch of the pentose phosphate pathway, how much NADPH is generated in the conversion of 12 mmoles of glucose-6-phophate into 12 mmoles of Ribose-5-Phosphate? How many mmoles of G6P could be generated from the resulting R5P via the non-oxidative branch of gluconeogenesis?   

Answer 1

In the pentose phosphate pathway, the reaction for conversion of glucose-6-phophate into ribose-5-phosphate is as follows -

Glucose-6-Phosphate + 2NADP⁺ + H₂O ? Ribose-5-Phosphate + 2NADPH + 2H⁺ + CO₂

For 12 mmoles, the reaction will look as follows -

12 mmoles Glucose-6-Phosphate + 24 mmoles NADP⁺ + 12 mmoles H₂O ? 12 mmoles Ribose-5-Phosphate + 24 mmoles NADPH + 24 mmoles H⁺ + 12 mmoles CO₂

So, this will generate 24 mmoles of NADPH.

From the non-oxidative branch, Ribose-5-Phosphate generates the following -

Ribose-5-Phosphate ? Glyceraldehyde-3-Phosphate + Fructose-6-Phosphate

From this,

Fructose-6-Phosphate ? Glucose-6-Phosphate

So from 12 mmoles Ribose-5-Phosphate, 12 mmoles of  Glucose-6-Phosphate can be generated.