Pentose Phosphate Pathway

Where does this reaction occur?

Cytosol

What is the overall reaction?

G6P + 2NADP → CO2 + R5P + 2NADPH

What are the 2 branches of this pathway?

Oxidative & Non-Oxidative Branch

What is the Non-Oxidative Branch used for?

It can be used to recycle excess pentose-P into glycolytic intermediates OR to produce pentose-P when NADPH is NOT needed.

Step 1 (Oxidative Branch)

Glucose-6-Phosphate + NADP → 6-Phosphogluconolactone + NADPH + H

Enzyme: Glucose-6-Phosphate Dehydrogenase

Step 2 (Oxidative Branch)

6-Phosphogluconolactone + H2O → 6-Phosphogluconate + H

Enzyme: Lactonase

Step 3 (Oxidative Branch)

6-Phosphogluconate + NADP → Ribulose-5-Phosphate + NADPH + CO2

Enzyme: 5-Phosphogluconate Dehydrogenase

Step 4 (Oxidative Branch)

Ribulose-5-Phosphate ↔ Ribose-5-Phosphate

Enzyme: Phosphopentose Isomerase

Step 5 (Non-Oxidative Branch)

Ribulose-5-Phosphate ↔ Xylulose-5-Phosphate

Enzyme: Phosphopentose Epimerase

Step 6 (Non-Oxidative Branch)

Xylulose-5-Phosphate + Ribose-5-Phosphate ↔ Glyceraldehyde-3-Phosphate + Sedoheptulose-7-Phosphate

Enzyme: Transketolase

Xylulose-5-Phosphate becomes Glyceraldehyde-3-Phosphate 

Ribose-5-Phosphate becomes Sedoheptulose-7-Phosphate

Step 7 (Non-Oxidative Branch)

Glyceraldehyde-3-Phosphate + Sedoheptulose-7-Phosphate ↔ Fructose-6-Phosphate + Erythrose-4-Phosphate

Enzyme: Transaldolase

Glyceraldehyde-3-Phosphate becomes Fructose-6-Phosphate

Sedoheptulose-7-Phosphate becomes Erythrose-4-Phosphate

Step 8 (Non-Oxidative Branch)

Erythrose-4-Phosphate + Xylulose-5-Phosphate ↔ Fructose-6-Phosphate + Glyceraldehyde-3-Phosphate

Enzyme: Transketolase

Erythrose-4-Phosphate becomes Fructose-6-Phosphate

 Xylulose-5-Phosphate becomes Glyceraldehyde-3-Phosphate