Question

The pentose phosphate pathway contains two dehydrogenases. What are they? Write the reactions catalyzed by these two enzymes. Comparing to the dehydrogenase in the glycolysis pathway, what is the difference of these two dehydrogenases in terms of cofactor utilization.

Answer 1

The pentose phosphate pathway is a parallel pathway to glycolysis that yields sugars that conform DNA and RNA. The two dehydrogenases involved in this pathway are:

1) Glucose-6-phosphate dehydrogenase that catalyzes the following reaction:

D-glucose 6-phosphate + NADP+ ⇌ 6-phospho-D-glucono-1,5-lactone + NADPH + H+

and 2) phosphogluconate dehydrogenase that catalyzes the following reaction:

6-phospho-D-gluconate + NADP+ ⇌ D-ribulose 5-phosphate + CO2 + NADPH

In glycolysis there is one dehydrogenase enzyme: glyceraldehyde-3-phosphate dehydrogenase (GAPDH). This enzyme catalyzes the 6th step of the pathway:

Glyceraldehyde 3-phosphate + NAD+ + Pi⇌D-1,2-bisphosphoglycerate + NADH + H+

Both dehydrogenase enzymes in the pentose phosphate pathway use NADP+/NADPH as cofactor, while the dehydrogenase in the glycolysis pathway uses NAD+/NADH. The main difference between these cofactors is a phosphate group (NAD+ vs. NADP+ and NADP vs. NADPH).