Pentose-Phosphate Cycle

a complex enzymatic pathway in living cells that carries out the direct aerobic oxidation of phosphorylated glucose into CO₂ and H₂O; the oxidation process is accompanied by the accumulation of reduced nicotinamide adenine dinucleotide phosphate (NADPH), an important coenzyme.

The pentose-phosphate cycle consists of the oxidative decarboxylation of glucose-6-phosphate, by which the primary carbon atom is split off from glucose, and the nonoxidative conversions of pentose phosphates with the formation of the original glucose-6-phosphate:

6 glucose-6-phosphate + 12NADP =
6CO₂ + 12NADPH + 12H⁺ +
+ 5 glucose-6-phosphate + H₃PO₄

Many intermediate products of the pentose-phosphate cycle can take part in other processes within the organism, for instance, glycolysis and photosynthesis.

The enzymes of the pentose-phosphate cycle are present in microorganisms and in the tissues of animals and plants. In terms of the quantity of glucose catabolized, the pathway generally plays a minor role that varies from one organism to another and depends on the type and functional state of the tissue involved. In mammals the activity of the pentose-phosphate cycle is high in the liver, adrenal glands, and embryonic tissue and in the mammary glands during lactation. The significance of this metabolic pathway also consists in its role as a supplier of NADPH, which is essential for the biosynthesis of many substances, including fatty acids, cholesterol, and purines; it also supplies pentose phosphates, which are components of nucleic acids and several coenzymes.

In humans a congenital insufficiency of certain enzymes that are essential to the pentose-phosphate cycle can result in hemolytic anemia. The activity of the pathway is normally regulated by glutathione and by insulin and other hormones that affect carbohydrate metabolism.