Pentose Phosphate Pathway

Abstract The elementary textbook treatment of the pentose phosphate pathway (PPP) describes its nature and occurrence in the cytoplasm of most cells. The reaction scheme is of two segments, oxidative and nonoxidative. The reactions of the oxidative segment are few and involve the decarboxylation of glucose 6‐phosphate (Glc 6‐P) to ribulose 5‐phosphate (Ru 5‐P) and CO2via 6‐phosphogluconolactone and 6‐phosphogluconate (6‐PG). Concomitant production of two moles of NADPH and H+for each mole of Glc 6‐P converted to Ru 5‐P. The nonoxidative reactions are depicted classically with an ordered reaction sequence of reversible steps for the interconversion of other pentose phosphate products that originate from Ru 5‐P by epimerase and isomerase enzymes. The reactions of the PPP also link the formation of sugar phosphate intermediates (glycolyl units) containing 3–7 carbon atoms that are generated by freely reversible equilibrium reactions catalyzed in fixed order by Transketolase and Tansaldolase. These reactions permit the following three biosynthetic PPP functions to emerge: cellular energetics, growth, and repair. Specifically via 1) the contribution by the oxidative segment of a high NADPH / NADP+redox potential that provideselectronsfor most reductive anabolic processes; 2) the formation of ribose 5‐phosphate (Rib 5‐P) for all nucleotide and nucleic acid biosynthesis, which is a demand that does not usually exceed 2% of all Glc metabolized by PPP activity; and (c) a storage pool of diverse phosphorylated glycolyl units that may be used directly or as signals for biosynthetic and energy‐yielding reactions by other pathways. Finally, selected reactions of the nonoxidative segment are also part of the most extensive synthetic and life‐sustaining event on the planet, namely the photosynthetic reductive path of CO2assimilation in all C‐3 plants.