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.

The PP phosphate pathway consists of eight reactions where the first three make up the “oxidative phase” used to harvest cell energy in form of NADPH. The remaining five pathway reactions make up the “non-oxidative phase” where a number of sugar rearrangements occur to generate various 3, 4, 5, 6, and 7-carbon compounds.

During the “oxidative phase”, the six carbon sugar, glucose-6-phosphate is first oxidized and then de-carboxylated to give the five carbon sugar, ribulose-5-phosphate plus CO2. Energy released by two of the oxidative phase reactions is “harvested” to generate 2 molecules of NADPH. This key cellular cofactor is then recycled back to it’s oxidized form (NADP) by energy consuming reactions in other pathways.

In the PP pathway “non-oxidative phase”, ribulose-5-phosphate undergoes a series of molecular rearrangements to form the three and seven carbon sugar intermediates, glyceradehyde-3-phospate and sedoheptulose-7-phosphate. These are subsequently converted to the four and six carbon sugars, erythrose-4-phosphate and fructose-6-phoshate. Finally, another pathway end product, glyceraldehyde-3- phosphate is formed.

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.

This “preparing” reaction rearranges the five carbon sugar substrate to form a structurally related sugar, D-xylulose-5-phosphate, needed for the transketolase reaction.

During this companion “preparing” reaction, an isomerase rearranges the five carbon substrate to the related sugar, D-ribose-5-phosphate. E coli has two isoenzymes encoded by the rpiA and rpiB genes that perform the reaction. Of both, RpiA is the major enzyme.

In this key PP pathway reaction, the two newly “prepared” 5-carbon sugars (D-ribose-5-phosphae and D-xylulose-5-phosphate) are inter-converted into the 3-carbon and 7-carbon products, glyceradehyde-3-phospate and sedoheptulose-7-phosphate. These products are important molecules used in other cell pathways. Two E. coli has two isoenzymes encoded by the tktA and tktB genes that catalyze this ketol group transfer reaction; TktA is the major enzyme.

The trans-aldolase enzyme interconverts the 3-carbon and the 7-carbon products of the prior pathway step to the 4-carbon and the 6-carbon products, erythrose-4-phosphate and fructose-6-phosphate. The transaldolase products provide several more molecules needed for other cell pathways. E. coli contains two transaldolase isoenzymes encoded by talA and talB.

The same transketolase enzymes described above in step 6 now performs a related ketol group re-arrangement to generate fructose-6-phosphate and glyceraldehyce-3-phosphate.

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.

In the first of the two oxidation steps, two electrons are removed from the 6-carbon sugar substrate and transferred onto NADP+ to form of NADPH + H+.

This enzyme adds a water molecule to the 6-carbon sugar to open the ring. The product is now ready for the subsequent oxidation reaction.

This enzyme extracts two additional electrons from the sugar backbone and cleaves a terminal carbon-carbon bond to release CO2. Energy released by the oxidation reaction is harvested by the formation of a second molecule of NADPH + H+.

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