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.

The [C₂] and [C₃] units are transferred from a ketose to an aldose by EC 2.2.1.1, transketolase, and EC 2.2.1.2, transaldolase, respectively.

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enzymes homepage.
pentose pathway (early stages)
pentose pathway (later stages)
EC 2.2.1.1 transketolase (mechanism)
EC 2.2.1.2 transaldolase (mechanism)
EC 3.1.3.11 fructose-bisphosphatase
EC 4.1.2.13 fructose-bisphosphate aldolase
EC 5.3.1.1 triose-phosphate isomerase
EC 5.3.1.9 glucose-6-phosphate isomerase