Pentose phosphate pathway is a metabolic pathway that takes place in cytosol in most organisms, and in plastids in plants. Pentose phosphate pathway generates pentoses, ribose 5-phosphate, NADPH and erythrose 4-phosphate. Ribose 5-phosphate is required for synthesis of nucleic acids and nucleotides. Erythrose 4-phosphate is required for synthesis of aromatic amino acids. Glycolysis is also a part of pentose phosphate pathway. The first phase of the pathway is the oxidative phase, which generates NADPH; and second phase is non-oxidative phase, which is synthesis of 5-carbon sugers (pentoses). First, glucose-6-phosphate dehydrogenase catalyzes the conversion of beta-D-glucose 6-phosphate into 6-phosphonoglucono-D-lactone, reducing NADP to NADPH in the process. Second, 6-phosphogluconolactonase catalyzes the conversion of 6-phosphonoglucono-D-lactone into 6-phospho-D-gluconate. Third, 6-phosphogluconate dehydrogenase catalyzes the conversion of 6-phospho-D-gluconate into D-ribulose 5-phosphate, reducing NADP to NADPH in the process and finishing the oxidative phase of the pathway. Next, ribulose-phosphate 3-epimerase catalyzes the interconversion of D-ribulose 5-phosphate and D-xylulose 5-phosphate, requiring a divalent metal cation as a cofactor. Ribose-5-phosphate isomerase catalyzes the interconversion of D-ribose 5-phosphate and D-ribulose 5-phosphate. Transketolase catalyzes two different reactions: glyceraldehyde-3-phosphate and either fructose-6-phosphate or sedoheptulose-7-phosphate is interconverted into xylulose 5-phosphate and either erythrose-4-phosphate or ribose-5-phosphate respectively. It requires a divalent metal cation and thiamine diphosphate as cofactors. Transaldolase catalyzes the interconversion of D-sedoheptulose 7-phosphate and D-glyceraldehyde 3-phosphate into D-erythrose 4-phosphate and beta-D-fructose 6-phosphate. Xylulose 5-phosphate/phosphate translocator preferentially transports xylulose 5-phosphate (but will also transport ribulose 5-phosphate) from the cytosol and into the chloroplast in exchange for phosphate. Xylulose 5-phosphate supplies the plastidic pentose phosphate pathway with intermediates, especially when under high demand.

Pentose Phosphate Pathway References