Pentose Phosphate Pathway

The pentose phosphate pathway is an important pathway for generating NADPH (for biosynthetic reactions) and pentose sugars (for nucleotide biosynthesis). It operates exclusively in the cytosol. Be aware that in contrast to pathways, such as glycolysis (with a linear sequence of reactions) or the citric acid cycle (with a circular sequence of reactions), the pentose phosphate pathway has several possible "branches" that can be taken to allow it to supply the cell with different products as needed. The primary products of the pathway include NADPH (from the oxidative reactions), pentoses (used in nucleotide synthesis), and miscellaneous other sugar phosphates.

A variation of the pentose phosphate pathway called the Calvin cycle is used by plants to fix CO 2 in photosynthesis.

Oxidative Phase (Generates NADPH)

> Glucose-6-Phosphate + NADP+ <=> 6-Phosphogluconolactone + NADPH (catalyzed by Glucose-6-Phosphate Dehydrogenase)

> 6-Phosphogluconolactone + H 2O <=> 6-Phosphogluconate + H+ (catalyzed by Lactonase)

> 6-Phosphogluconate + NADP+ <=> Ribulose-5-Phosphate + CO 2 + NADPH (catalyzed by 6-Phosphogluconate Dehydrogenase)

Nonoxidative Phase (note how products of each reaction are shuffled - the pathway does not lead to a single end product)

> Ribulose-5-Phosphate <=> Ribose-5-Phosphate (catalyzed by Phosphopentose Isomerase)

> Ribulose-5-Phosphate <=> Xylulose-5-Phosphate (catalyzed by Phosphopentose epimerase)

> Xylulose-5-Phosphate + Ribose-5-phosphate <=> Glyceraldehyde-3-phosphate + Sedoheptulose-7-Phosphate(catalyzed by Transketolase)

> Sedoheptulose-7-Phosphate + Glyceraldehyde-3-Phosphate <=> Erythrose-4-Phosphate + Fructose-6-Phosphate (catalyzed by Transaldolase)

> Xylulose-5-phosphate + Erythrose-4-Phosphate <=> Glyceraldehyde-3-phosphate + Fructose-6-phosphate (catalyzed by Transketolase)

See also: Enzymes of the Pentose Phosphate Pathway, Intermediates of the Pentose Phosphate Pathway,

INTERNET LINK: Pentose Phosphate Pathway