Pentose Phosphate Pathway

The early stages of the pathway carried out the reaction:

glucose 6-phosphate + 2 NADP+ + H2O = pentose 5-phosphate + 2 NADH + CO2 + 2 H+

where the pentose phosphate formed is interconvertible between ribose 5-phosphate, ribulose 5-phosphate and xylulose 5-phosphate.

The later stages of the pathway carried out the reaction:

6 pentose 5-phosphate + H2O = 5 glucose 6-phosphate + phosphate

If six molecules of glucose 6-phosphate undergo the early stages, and if the 6 molecules of pentose 5-phosphate formed are converted into 5 molecules of glucose 6-phosphate + phosphate in the later stages, then the net overall reaction will be:

glucose 6-phosphate + 12 NADP+ + 7 H2O = 12 NADPH + 12 H+ + 6 CO2 + phosphate

i.e. effectively complete oxidation of glucose 6-phosphate to CO2 with generation of NADPH for fat synthesis, etc. The scheme below shows how this conversion of six molecules of pentose phosphate into five molecules of glucose 6-phosphate is achieved.

© IUBMB 2003

The [C2] and [C3] units are transferred from a ketose to an aldose by EC, transketolase, and EC, transaldolase, respectively.

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pentose pathway (early stages)
pentose pathway (later stages)
EC transketolase (mechanism)
EC transaldolase (mechanism)
EC fructose-bisphosphatase
EC fructose-bisphosphate aldolase
EC triose-phosphate isomerase
EC glucose-6-phosphate isomerase