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Pentose Phosphate Pathway

During respiration breakdown of glucose in cytosol occurs both by glycolysis (about 2/3) as well as by oxidative pentose phosphate pathway (about 1/3). Pentose phosphate pathway was described by Warburg, Dickens and Lipmann (1938). Hence, it is also called Warburg-Dickens-Lipmann pathway. It takes place in cytoplasm of mature plant cells. It is an alternate way for breakdown of glucose (Figure 14.15).
Pentose Phosphate Pathway img 1

It is also known as Hexosemonophosphate shunt (HMP Shunt) or Direct Oxidative Pathway. It consists of two phases, oxidative phase and non-oxidative phase. The oxidative events convert six molecules of six carbon Glucose-6-phosphate to 6 molecules of five carbon sugar Ribulose-5 phosphate with loss of 6CO2 molecules and generation of 12 NADPH + H+ (not NADH).

The remaining reactions known as non-oxidative pathway, convert Ribulose-5-phosphate molecules to various intermediates such as Ribose-5-phosphate(5C), Xylulose-5-phosphate(5C), Glyceraldehyde-3 phosphate(3C), Sedoheptulose-7-Phosphate (7C), and Erythrose-4-phosphate (4C). Finally, five molecules of glucose-6-phosphate is regene-rated (Figure 14.16). The overall reaction is:

6 × Glucose-6-Phosphate + 12NADP+ + 6H2O
↓
5 × Glucose-6-Phosphate + 6CO2 + Pi + 12NADPH + 12H+

Pentose Phosphate Pathway img 2
The net result of complete oxidation of one glucose-6-phosphate yield 6CO2 and 12NADPH + H+. The oxidative pentose phosphate pathway is controlled by glucose-6-phosphate dehydrogenase enzyme which is inhibited by high ratio of NADPH to NADP+. Significance of pentose phosphate pathway.

  1. HMP shunt is associated with the generation of two important products, NADPH and pentose sugars, which play a vital role in anabolic reactions.
  2. Coenzyme NADPH generated is used for reductive biosynthesis and counter damaging the effects of oxygen free radicals
  3. Ribose-5-phosphate and its derivatives are used in the synthesis of DNA, RNA, ATP, NAD+, FAD and Coenzyme A.
  4. Erythrose is used for synthesis of anthocyanin, lignin and other aromatic compounds.
  5. It plays a role on fixation of CO2 in photosynthesis through RUBP.