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Pentose Phosphate Pathway - Ask4Biology

Pentose Phosphate Pathway

Pentose phosphate pathway (Figure 1) is also called hexose monophosphate shunt (HMP shunt). It occurs in cytosol. It is an alternative metabolism pathway of glucose. Pentose phosphate pathway neither produces nor consumes ATP. It produces two major products, NADPH and ribose. NADPH functions as biochemical reductant and for biosynthesis of fatty acids and steroids. Ribose is for biosynthesis of nucleotide which necessary for the formation of DNA and RNA.

Pentose phosphate pathway occurs in two phases:

  1. Oxidative phase
  2. Non-oxidative phase
Figure 1: Pentose Phosphate Pathway, Hexose Monophosphate Shunt, HMP Shunt
Pentose Phosphate Pathway
(Image extracted from Lippincott’s Illustrated Review Biochemistry)

Oxidative Phase

Oxidative phase is irreversible phases in pentose phosphate pathway. In oxidative phase, glucose 6-phosphate is oxidized to 6-phophogluconate by glucose 6-phosphate dehydrogenase along the formation of first molecule NADPH.

6-Phosphogluconolactone is hydrolyzed by 6-phosphogluconolactone hydrolase. The oxidative decarboxylation of the product, 6-phosphogluconate is catalyzed by 6-phosphogluconate dehydrogenase. Pentose sugar–phosphate (ribulose 5-phosphate), CO2 (from carbon 1 of glucose), and a second molecule of NADPH are produced.

This irreversible oxidative pathway is very important in liver, mammary glands and adipose which actively synthesis fatty acids. NADPH is also very important for red blood cells to keep glutathione reduced.

Non-oxidative Phase

Non-oxidative phase is reversible pathway. These reactions catalyze the interconversion of three-, four-, five-, six-, and seven-carbon sugars. Transketolase (which transfers two-carbon units) and transaldolase (which transfers three-carbon units) convert the ribulose 5-phosphate produced as an end product of the oxidative reactions to glyceraldehyde 3-phosphate and fructose 6-phosphate, which are intermediates of glycolysis.

In the condition where ribose is demanded, the nonoxidative reactions can provide the biosynthesis of ribose 5-phosphate from glyceraldehyde 3-phosphate and fructose 6-phosphate in the absence of the oxidative steps.

References:

  1. Champe, PC, Harvey, RA. 2007. Biochemistry. Lippincott’s Illustrated Review. 4th Edition. Lippincott Williams & Wilkins.
  2. Murray, RK, Bender, DA, Botham, KM, Kennelly, PJ, Rodwell, VW, Weil, PA. 2006. Harper’s Illustrated Biochemistry, 28e. Lange, McGraw Hill.

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