Pentose phosphate pathway

Pentose phosphate pathway (also called HMP shunt or Hexose monophosphate shunt) is an alternative pathway for glucose metabolism. It does not lead to generation of any ATP. But this pathway has two major functions: (A) generation of NADPH for synthesis of fatty acid and steroid, and (B) formation of ribose for nucleotide and nucleic acid synthesis.

Pentose phosphate pathway occurs in the cytoplasm of cells. It can be divided into two phases: an irreversible oxidative phase that generates NADPH, and a reversible nonoxidative phase that provides ribose precursors for nucleotide synthesis.

In the oxidative phase of pentose phosphate pathway, glucose 6-phosphate undergoes dehydrogenation and decarboxylation to form ribulose 5-phosphate, carbon dioxide, and two molecules of NADPH. This phase is particularly important in adrenal cortex that is active in the NADPH dependent synthesis of steroid, in liver and lactating mammary gland that are active in the biosynthesis of fatty acid, and in red blood cell that require NADPH to keep glutathione reduced (reduced glutathione removes harmful H2O2 from red blood cell and prevent hemolysis).

The nonoxidative phase of pentose phosphate pathway occurs in all cells synthesizing nucleotide and nucleic acid. In this phase, ribulose 5-phosphate is converted either to ribose 5-phosphate that is the precursor of ribose required for nucleotide and nucleic acid synthesis or to fructose 6-phosphate and glyceraldehyde 3-phosphate that are the intermediates of glycolysis.

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