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:

[NAD+]/[NADH]=700/1

[NADP+]/[NADPH] = 1/71

Oxidative Branch

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

 

Phosphopentose Isomerase

 

 

Phosphopentose Epimerase

 

• 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