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THE PENTOSE PHOSHATE PATHWAY

E. coli employs the Pentose Phosphate (PP) pathway primarily to generate carbon intermediates and NADPH needed for cell biosynthesis.

The overall PP pathway reaction is:

1 glucose-6-phosphate + 2 NADP+
1 fructose-6-phosphate + 1 glyceraldehyde-3 phosphate + 1 CO2 + 2NADPH + 2 H+

KEY CONCEPTS

  • E. coli cells acquire glucose from its surroundings by the glucose-specific PTS uptake system called PTS.
  • The uptake product, glucose-6-phosphate, is then oxidized and rearranged by the pentose phosphate pathway (PP pathway) to form fructose-6-phosphate, glyceraldehyde-3-phosphate and CO2.
  • The PP pathway generates reducing equivalents in form of two molecules of NADPH per molecule of glucose-6-phosphate consumed.
  • A number of PP pathway intermediates plus the end products serve as precursors for other cell biosynthesis reactions.

OPERATION OF THE PENTOSE PHOSPHATE PATHWAY

To metabolize glucose by the Pentose Phosphate (PP) pathway, glucose is first taken up into the cell by the PTS sugar uptake system (E. coli PTS glc). The product, glucose-6 phosphate is then rearranged in a seemingly complex set of reactions to form three end products, fructose-6-phosphate, glyceraldehyde-3-phosphate and CO2. Energy is also preserved in the form of 2 molecules of NADPH.

DETAILS OF THE PENTOSE PHOSPHATE PATHWAY

I. Active transport of glucose across the cytoplasmic membrane

Glucose is transported into E. coli cells by the glucose-specific PTS system (named PTS glc). During this process glucose is converted to glucose-6-phosphate. The general operation of PTS-type transport systems is described in the Nutrient Uptake section of the Student Portal. PTS

II. The three “oxidative phase” reactions.

Glucose-6-phosphate is oxidized and then de-carboxylated to form ribulose-5-phosphate plus CO2. The four electrons and four protons extracted from the sugar backbone are transferred onto two molecules of 2 NADP+ to generate 2 NADPH + H+. This conserves energy released during the oxidative phase.

  1. Glucose-6-phosphate dehydrogenase (Zwf)       
  2. 6-Phosphogluconolactonase (Pgl)       
  3. 6-Phospho-gluconate dehydrogenase, decarboxylating (Gnd)       

III. The five “non-oxidative phase” reactions.

As indicated by the name, no oxidation reactions occur during the “non-oxidation” phase of the PP pathway. Rather, a series of molecule rearrangements generate C3, C5, C6 and C7 sugars. Several of the pathway intermediates may be siphoned away and used in other cell pathways. The end products formed are one molecule each of fructose-6-phosphate, glyceraledhyde-3-posphate and CO2. No cellular energy is harvested in this “non-oxidation” phase.

  1. Ribulose-5-phosphate 3-epimerase (Rpe)       
  2. Ribulose-5-phosphate isomerase (RpiA and RpiB)       
  3. Transketolase (TktA and TktB)       
  4. Transaldolase (TalA and TalB)       
  5. Transketolase (TktA and TktB) (same enzymes as in step 6)       

IV. Dual roles of PP pathway in E. coli cell metabolism.

E. coli uses the PP pathway mainly for anabolic purposes. NADPH is the primary reductant in all biosynthesis pathways. Ribose-5-phosphate is used in large amounts to make nucleosides for RNA and DNA synthesis. Likewise, frucose-6-phosphate is the precursor for the N-acetylglucosamine and N-acetylmuramic acid molecules that make up the backbone of the peptidoglycan-containing cell wall. The four carbon sugar, erythrose-4-phosphate is a precursor for synthesis of the three aromatic amino acids (tryptophan, tyrosine, and phenylalanine). It is also used to synthesize quinones and certain iron sequestration molecules.

When gluconate is present as carbon source, E. coli uses this pathway for catabolism generating ATP via substrate level phosphorylation and converting NADPH to NADH, which enters the respiratory chain as electron donor. However, E. coli’s main pathway for sugar oxidation is the glycolysis pathway (Embden Meyerhof Parnas pathway). E. coli uses yet a third pathway, the Entner-Doudoroff pathway, for metabolizing certain sugar acids such as glucuronate and galacturonate. Other bacteria lack the glycolysis pathway and employ either the PP or the Entner-Doudoroff pathway exclusively for sugar catabolism.

Summary

  • Glucose enters the cell via a substrate specific uptake system called PTSglc.
  • The pentose phosphate pathway reactions preserve reducing power in the form of two molecules of NADPH.
  • NADPH is used for cell biosynthetic reactions or it may be converted to NADH if needed.
  • Pathway end products are fructose-6-phosphate, glyceraldehyde-3-phosphate, and CO2.
  • The eight pathway reactions are divided into the “oxidative” phase and “non-oxidative” phase reactions.
  • Several pathway intermediates can be used for cell biosynthesis processes.

 

Credits:

Authored by Robert Gunsalus and Imke Schröder
©The Escherichia coli Student Portal

This project acknowledges support from:
NIH Grant Award GM077678 to SRI, International
Peter Karp and coworkers at EcoCyc.org
The UCLA Department of MIMG