Pentose Phosphate Pathway

The pentose phosphate pathway (PPP) is a cytosolic pathway of glucose metabolism that branches from glucose-6-phosphate and operates alongside glycolysis. Its major products include NADPH, which provides reducing power for cellular reactions, and ribose-5-phosphate, a five-carbon sugar phosphate used in nucleotide synthesis. The pathway is organized into an oxidative phase and a nonoxidative phase .^1,2

In the oxidative phase, glucose-6-phosphate is converted through a series of reactions into ribulose-5-phosphate while carbon dioxide and NADPH are produced. Each glucose-6-phosphate passing through this phase generates two NADPH molecules. The first committed reaction is catalyzed by glucose-6-phosphate dehydrogenase (G6PD) .^1,2

The nonoxidative phase consists largely of reversible reactions that rearrange sugar phosphates containing different numbers of carbon atoms. These reactions can contribute to the production of ribose-5-phosphate when cells require material for nucleotide synthesis or convert pentose phosphates into intermediates shared with glycolysis, including fructose-6-phosphate and glyceraldehyde-3-phosphate. This flexibility helps cells adjust pathway activity according to their relative requirements for NADPH and ribose-5-phosphate .^1,2

The PPP differs from glycolysis in its primary metabolic functions. Glycolysis converts glucose toward pyruvate while producing ATP and NADH, whereas the PPP is particularly important for generating NADPH and pentose sugars. NADPH produced by the PPP supports biosynthetic reactions and cellular redox systems that help protect against oxidative stress .^1,2