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Nonoxidative Phase of Pentose Phosphate Pathway
The first phase of the pentose phosphate pathway generates two NADPH molecules and a pentose sugar called ribose 5-phosphate. The NADPH molecules are important reductant agents that are used in fatty acid biosynthesis, nucleotide biosynthesis, cholesterol biosynthesis, neurotransmitter biosynthesis and various detoxification processes. When needed, the ribose 5-phosphate can be used to create DNA, RNA and nucleotide-bases molecules such as ATP, NADH, FAD and CoA. But when the cell needs NADPH more than ribose 5-phosphate, the cell can convert the ribose sugar into specific glycolytic intermediates via the nonoxidative phase. This is the second phase of the pentose phosphate pathway. The overall net reaction of this phase transforms three ribose 5-phosphate molecules into two fructose 6-phosphate molecules and one glyceraldehyde 3-phosphate. These glycolytic intermediates can now be transformed back into glucose 6-phosphate, which in turn can be oxidized to form additional NADPH molecules. The nonoxidative phase links the glycolytic pathway to the pentose phosphate pathway and it allows our cells a way to break down ribose molecules ingested into our body. This phase uses several important enzymes, including phosphopentose isomerase, phosphopentose epimerase, transketolase and transaldolase.
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