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Pentose phosphate pathway or the phosphogluconate pathway or the hexose monophosphate shunt is a biochemical pathway that occurs in the cytosol of the cells. This pathway is similar to the glycolysis in the sense that it is also involved in glucose oxidation. The main products of this pathway are NADPH and Pentoses.
Though glucose oxidation occurs in this pathway, it is an anabolic pathway rather than catabolic. This is because the products formed from the pentose phosphate pathways are used in different biosynthetic pathways. For example, NADPH formed is used in several biosynthetic pathways (biosynthesis of fatty acids, cholesterol, and steroid hormone) as a reducing agent and Ribose-5-Phosphate formed is used as a precursor for the nucleotide biosynthesis and carbon dioxide fixation in plants.
Pentose phosphate pathway occurs in tissues involved in lipid biosyntheses, such as the liver, mammary gland, adipose tissue, adrenal cortex, and plastids of plants. Pentose phosphate pathway comprises two phases oxidative phase and non-oxidative phase.
In the oxidative phase, Glucose-6-Phosphate enters the pathway. Glucose 6 Phosphate Dehydrogenase catalyzes the conversion of Glucose-6-Phosphate into 6-Phosphoglucono-∂-Lactone. This step is the committed step of the pentose phosphate pathway and the enzyme is an NADP+ dependent enzyme that reduces NADP+ into NADPH (forming NADPH).
6-Phosphoglucono-∂-Lactone is then converted into 6-Phosphogluconate catalyzed by 6-Phosphoglucono lactonase. In the next step, 6-Phosphogluconate is converted into Ribulose-5-Phosphate by 6-Phosphogluconate Dehydrogenase; another NADP+ dependent enzyme that produces NADPH from NADP+.
In the non-oxidative phase, 3 molecules of Ribulose-5-Phosphate are converted into 2 molecules of Fructose-6-Phosphate and Glyceraldehydes-3-Phosphate in multistep reactions. The first Ribulose-5-Phosphate is converted into Ribose-5-Phosphate catalyzed by Ribulose-5-Phosphate isomerase or Xylulose-5-Phosphate catalyzed by Ribulose-5-Phosphate epimerase. However, the relative amount of Ribose-5-Phosphate and Xylulose-5-Phosphate produced from Ribulose-5-Phosphate depends on the current need of the cell.
In rapidly dividing cells, Ribose-5-Phosphate is produced significantly more than Xylulose-5-Phosphate as it is the biosynthetic precursor of the nucleotide biosynthesis used in DNA synthesis and in normal condition Xylulose-5-Phosphate is produced more than Ribose-5-Phosphate to be converted into glycolytic intermediates. Two enzymes Transketolase and Transaldolase, catalyze the different steps to convert Ribose-5-Phosphate and Xylulose-5-Phosphate into final products Fructose-6-Phosphate and Glyceraldehydes-3-Phosphate.
Transketolase catalyzes the transfer of the C2 units. It requires Thiamine Pyrophosphate (TPP) as a co-factor and catalyzes the transfer of the C2 unit from Xylulose-5-Phosphate to Ribose-5-Phosphate producing Glyceraldehydes-3-Phosphate and Sedoheptulose-7-Phosphate.
Similarly, it catalyzes the transfer of the C2 unit from Xylulose-5-Phosphate to Rrythrose-4-Phosphate forming Fructose-6-Phosphate and Glyceraldehydes-3-Phosphate. Transaldolase catalyzes the transfer of C3 units from Sedoheptulose-7-Phosphate to Glyceraldehydes-3-Phosphate yielding Erytrose-4-Phosphate and Fructose-6-Phosphate.