2.4.3  Pentose phosphate pathway

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An alternative route for the breakdown of glucose-6-P is provided by the pentose phosphate pathway (Figure 2.21, right side). This pathway functions mainly to generate NADPH and precursors for various biosynthetic processes. These include ribose-5-P, which provides the ribosyl moiety of nucleotides and is a precursor for the biosynthesis of the purine skeleton, and erythrose-4-P, for the biosynthesis of aromatic compounds in the shikimic acid pathway. Glyceraldehyde 3-P and fructose-6-P formed in the pentose phosphate pathway may be metabolised further in the glycolytic pathway. Alternatively, fructose-6-P may be converted back to glucose-6-P by phosphohexose isomerase and recycled through the pentose phosphate pathway. The pentose phosphate pathway may account for between 15 and 30% of the hexose phosphate oxidised to glyceraldehyde 3-P and CO2.

As with glycolysis, reactions of the pentose phosphate pathway are catalysed by different sets of isoenzymes that occur either in the cytosol or in plastids. The reactions in the non-oxidative phase of the pentose phosphate pathway are readily reversible and also form part of the PCR cycle of chloroplasts.