The pentose phosphate pathway (PPP), also called the hexose monophosphate shunt, occurs in the cytosol. It is a major source of the body's NADPH, a biochemical reductant. It also produces ribose 5-phosphate, used in the synthesis of nucleotides.
Glucose 6-phosphate, the first metabolite of glycolysis, enters the PPP, producing one molecule of C02 and 2 molecules of NADPH.
The PPP consists of two irreversible oxidative reactions followed by a series of reversible interconversions. No ATP is directly consumed or generated.
Glucose 6-phosphate is catalyzed by glucose 6-phosphate dehydrogenase (G6PD) in an irreversible reaction that is specific for NADP+ and its coenzyme. The PPP is regulated primarily by G6PD, and NADPH is a potent competitive inhibitor of the enzyme.
The second oxidative reaction produces a pentose sugar phosphate, ribulose 5-phosphate, and C02. This pentose can return to glycolysis through glyceraldehyde 3-phosphate or can be routed to nucleotide biosynthesis.