II. The Pentose Phosphate Pathway
(a) Produces the five carbon sugar ribose (DNA).
(b) Generates NADPH (reductive biosynthesis).
Cytosol of a liver cell of a well-fed rat:
[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).
Nonoxidatative Branch Part I
ribulose 5-P <-> xylulose 5-P <-> ribose 5-P
Cellular pools of C5 sugars rapidly interconvert.
Nonoxidatative Branch Part II
2 Xylulose 5-phosphate + Ribose 5-phospate --->
2 Fructose 6-phosphate + Glyceraldehyde 3-phospate
Configurations of the Pentose Phosphate Pathway
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+ + Pi