Pentose Phosphate Pathway

(Cytoplasm, Branches from G6P, Produces NADPH and ribose-5-phosphate) Steps 1-3 Oxidative, Steps 4-7 Non-Oxidative

Step 1

Glucose-6-phosphate is converted to 6-phosphogluconolactone by glucose-6-phosphate dehydrogenase (G6PD), producing 1 NADPH. It is the rate-limiting step, inhibited by NADPH, and is irreversible.

Step 2

6-phosphogluconolactone is converted to 6-phosphogluconate by lactonase via hydrolysis, which is reversible.

Step 3

6-phosphogluconate is converted to ribulose-5-phosphate by 6-phosphogluconate dehydrogenase, producing 1 NADPH and releasing CO₂, and is irreversible.

Step 4

Ribulose-5-phosphate is converted to either ribose-5-phosphate or xylulose-5-phosphate by isomerase or epimerase respectively. Ribose-5-phosphate serves as the precursor for nucleotide synthesis, and both reactions are reversible.

Step 5

In the first transketolase reaction, a 2-carbon unit is transferred from xylulose-5-phosphate to ribose-5-phosphate, producing G3P and sedoheptulose-7-phosphate. It requires the TPP cofactor and is reversible.

Step 6

In the transaldolase reaction, a 3-carbon unit is transferred from sedoheptulose-7-phosphate to G3P, producing erythrose-4-phosphate and fructose-6-phosphate, and is reversible.

Step 7

In the second transketolase reaction, a 2-carbon unit is transferred from xylulose-5-phosphate to erythrose-4-phosphate, producing G3P and fructose-6-phosphate. The fructose-6-phosphate can re-enter glycolysis, and the reaction is reversible.