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The Entner-Doudoroff pathways are routes of glucose metabolism alternative to the Embden-Meyerhof pathway. All of the Entner-Doudoroff pathways generate less ATP from a given amount of glucose than the Embden-Meyerhof pathway does.

There are three known variations of the Entner-Doudoroff pathway. All three involve the thematic steps of

1) oxidation of glucose to a δ-lactone, yielding NADPH,

2) hydrolysis of this lactone to yield a sugar acid,

3) dehydration of this sugar acid to yield a 2-keto-3-deoxy-gluconate, and

4) aldol cleavage of this 2-keto-3-deoxy-gluconate to yield pyruvate and a glyceraldehyde.

The differences between these variations involve the points at which the substrate is phosphorylated.

As in the Embden-Meyerhof pathway, the processes of the Entner-Doudoroff pathways also take place in the cytosol.

Phosphorylative Entner-Doudoroff pathway
This version of the pathway is the one exhibited in Escherichia coli, and is known as the classical Entner-Doudoroff pathway. This pathway is also used by many other bacteria of interest, such as Pseudomonas aeruginosa and Vibrio cholerae.

In the phosphorylative Entner-Doudoroff pathway, phosphorylation occurs before oxidation. Glucose is first phosphorylated to glucose-6-phosphate, which then undergoes oxidation to yield 6-phosphogluconolactone. These two steps are shared with the oxidative branch of the pentose phosphate pathway.

6-phosphogluconolactone then undergoes hydrolysis, giving 6-phosphogluconate. 6-phosphogluconate is dehydrated to yield 2-keto-3-deoxy-6-phosphogluconate (KDPG). KDPG is then cleaved to release glyceraldehyde-3-phosphate and pyruvate.

This gives the following net reaction:

After the further conversion of glyceraldehyde-3-phosphate to pyruvate, the overall reaction yields only 1 ATP per glucose molecule and is thus less energetically favorable than oxidation of glucose via the Embden-Meyerhof pathway. In E. coli, it is solely responsible for the metabolism of sugar acids, including the uronic acids, which enter via KDPG.

Semi-phosphorylative Entner-Doudoroff pathway
The semi-phosphorylative Entner-Doudoroff pathway is used mostly by certain halophilic archaea, and also by some bacteria. The only difference of the semi-phosphorylative pathway from the phosphorylative pathway is that the phosphorylation occurs after the dehydration step, not at the start. The net reaction of the semi-phosphorylative Entner-Doudoroff pathway is identical to that of the phosphorylative pathway; therefore, the semi-phosphorylative pathway is energetically equivalent to the phosphorylative pathway, also generating 1 ATP per glucose molecule.

Non-phosphorylative Entner-Doudoroff pathway
This variation is used by certain thermophilic archaea of genuses Sulfulobus, Thermoplasma, and Thermoproteus, and often coexists with the semi-phosphorylative pathway. No substrate-level phosphorylation occurs before cleavage. As a result, there is no immediate generation of ATP from the oxidation of glucose to pyruvate through this pathway; the net reaction is

Cells that utilize this pathway to support growth must therefore obtain their energy from respiration.

The generated glyceraldehyde is then oxidized to glycerate and phosphorylated to 2-phosphoglycerate, where this pathway rejoins the Embden-Meyerhof.

It has been suggested that the non-phosphorylative Entner-Doudoroff pathway is completely reversible, and may only run in reverse. When run in reverse, the non-phosphorylative Entner-Doudoroff pathway becomes a gluconeogenetic pathway, and would function in relation to the semi-phosphorylative pathway as gluconeogenesis does to the Embden-Meyerhof pathway.

References and Citations
General:

Madigan, Martinko, and Parker. Brock Biology of Microorganisms - 9th ed. QR41.2, B77 2000

Entner-Doudoroff pathway I (phosphorylative). Metacyc Encyclopedia of Metabolic Pathways. Retrieved 11 November 2007.

Entner-Doudoroff pathway II (non-phsophorylative). Ibid.

Entner-Doudoroff pathway III (semi-phosphorylative). Ibid.

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