(b) Generates NADPH (reductive biosynthesis).

Cytosol of a liver cell of a well-fed rat:

[NAD⁺]/[NADH]=700/1

[NADP⁺]/[NADPH] = 1/71

Glucose 6-phosphate + 2 NADP⁺ + H₂0 --->

Ribulose 5-phosphate + 2 NADPH + 2 H⁺ + CO₂

G6P Dehydrogenase Lactonase 3PG Dehydrogenase

• The first step is rate limiting and essentially irreversible.

• It is regulated by the NADP⁺/NADPH ratio (substrate competition).

ribulose 5-P <-> xylulose 5-P <-> ribose 5-P

Phosphopentose Isomerase

Phosphopentose Epimerase

• Cellular pools of C5 sugars rapidly interconvert.

2 Xylulose 5-phosphate + Ribose 5-phospate --->

2 Fructose 6-phosphate + Glyceraldehyde 3-phospate

1. Ribose 5-phosphate is required, but not NADPH.

5 Glucose 6-phosphate + ATP --->

6 Ribose 5-phosphate + ADP + H⁺

2. NADPH is required, but not ribose.

Glucose 6-phosphate + 12 NADP⁺ + 7 H₂O --->

6 CO₂ + 12 NADPH + 12 H⁺ + P_i