Pentose Phosphate Pathway


The pentose phosphate pathway (also called phosphogluconate pathway, or hexose monophosphate shunt or the pentose phosphate shunt) is a process that serves to generate NADPH and the synthesis of pentose (5-carbon) sugars. It is responsible for the generation of a substantial fraction of the cytoplasmic NADPH required for biosynthetic reactions, and for the generation of ribose 5-phosphate for nucleotide synthesis. Although the pentose phosphate pathway and glycolysis are distinct, they involve three common intermediates: glucose 6-phosphate, glyceraldehyde 3-phosphate, and fructose 6-phosphate, so the two pathways are interconnected. Dietary pentose sugars derived from the digestion of nucleic acids may be metabolized through the pentose phosphate pathway, and the carbon skeletons of dietary carbohydrates may be converted into glycolytic/gluconeogenic intermediates. Located exclusively in the cytoplasm, the pathway is one of the three main ways the body creates molecules with reducing power, accounting for approximately 60% of NADPH production in humans. The pentose phosphate pathway consists of a total of eight reactions:1) Conversion glucose 6-phosphate to D-glucono-1,5-lactone 6-phosphate, with the formation of NADPH; 2) Conversion of D-glucono-1,5-lactone 6-phosphate to 6-phospho-D-gluconate; 3) Conversion of 6-phospho-D-gluconate to ribulose 5-phosphate, with the formation of NADPH; 4) Conversion of ribulose 5-phosphate to xylulose 5-phosphate; 5) Conversion of ribulose 5-phosphate to ribose 5-phosphate; 6) Rearrangement of ribose 5-phosphate and xylulose 5-phosphate to form sedoheptulose 7-phosphate and glyceraldehyde 3-phosphate; 7) Rearrangement of sedoheptulose 7-phosphate and glyceraldehyde 3-phosphate to form erythrose 4-phosphate and fructose 6-phosphate; and 8) Rearrangement of xylulose 5-phosphate and erythrose 4-phosphate to form glyceraldehyde 3-phosphate and fructose-6-phosphate. There are two distinct phases in the pathway, the oxidative branch and the non-oxidative branch. The oxidative branch involves reactions 1-3 and generates NADPH and pentose 5-phosphate. The non-oxidative branch of the pathway, which involves reactions 4-8, converts pentose 5-phosphate to other 5-carbon sugars. The overall pathway can operate to generate only NADPH. The reactions of the non-oxidative branch can operate to generate net amounts of ribose 5-phosphate with no production of NADPH.

Pathway legend Zoom in

Pathway Image

Pathway legend Zoom in

References

  1. Lehninger, A.L. (2005) Lehninger principles of biochemistry (4 th ed.). New York: W.H Freeman.
  2. Salway, J.G. (2004) Metabolism at a glance (3 rd ed.). Alden, Mass. : Blackwell Pub.