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The Entner–Doudoroff pathway (ED pathway) describes a pathway—a series of enzyme-catalyzed chemical reactions—that are active in bacterial primary metabolism, a pathway that catabolizes glucose to pyruvic acid using enzymes distinct either from those used in glycolysis or the pentose phosphate pathway (the latter two being most widely used in the Bacteria). The ED pathway was first reported by Michael Doudoroff and Nathan Entner from the Bacteriology Department at the University of California, Berkeley in 1952.[non-primary source needed] Recent work on the Entner–Duodoroff pathway has shown that its use is not restricted to prokaryotes as was previously thought. Specifically, there is direct evidence that Hordeum vulgare uses the Entner–Doudoroff pathway. Use of the Entner–Duodoroff pathway among other plants such as mosses and ferns is also probably widespread, based on preliminary sequencing data analysis.
Distinct features of the Entner–Doudoroff pathway are that it:
- Uses 6-phosphogluconate dehydratase and 2-keto-3-deoxyphosphogluconate aldolase to create pyruvate from glucose.
- Has a net yield of 1 ATP for every one glucose molecule processed, as well as 1 NADH and 1 NADPH. In comparison, glycolysis has a net yield of 2 ATP molecules and 2 NADH molecules for every one glucose molecule metabolised.
- 1 Organisms that use the Entner-Doudoroff pathway
- 2 Enzymes catalysing Entner Doudoroff Pathway (ED Pathway)
- 2.1 Conversion of glucose to glucose-6-phosphate
- 2.2 Conversion of glucose-6-phosphate to 6-phosphoglucanolactone
- 2.3 Conversion of 6-phosphoglucanolactone to 6-Phosphogluconic acid
- 2.4 Conversion of 6-Phosphogluconic acid to 2-keto-3-deoxy-6-phosphoglucanate
- 2.5 Conversion of 2-keto-3-deoxy-6-phosphoglucanate to Pyruvate and Glyceraldehyde-3-phosphate
- 2.6 Conversion of Glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate
- 2.7 Conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate
- 2.8 Conversion of 3-phosphoglycerate to 2-phosphoglycerate
- 2.9 Conversion of 2-phosphoglycerate to phosphoenol pyruvate
- 2.10 Conversion of phosphoenol pyruvate to pyruvate
- 2.11 References
- 2.12 Further reading
Organisms that use the Entner-Doudoroff pathway
This section needs expansion with: the further known species that use the ED or its variants, based on the reviews provided, and other modern secondary sources. You can help by adding to it. (August 2015)
This section relies too much on references to primary sources. (August 2015) (Learn how and when to remove this template message)
There are several bacteria that use the Entner–Doudoroff pathway for metabolism of glucose and are unable to catabolize via glycolysis (e.g., therefore lacking essential glycolytic enzymes such as phosphofructokinase as seen in Pseudomonas). Genera in which the pathway is prominent include Gram-negative, as listed below, Gram-positive bacteria such as Enterococcus faecalis,[full citation needed][page needed][better source needed] as well as several in the Archaea, the second distinct branch of the prokaryotes (and the "third domain of life", after the prokaryotic Eubacteria and the eukaryotes). Most organisms that use the pathway are aerobes, due to the low ATP yield per glucose molecule metabolised.[clarification needed]
Examples of bacteria using the pathway are:
- Pseudomonas, a genus of Gram-negative bacteria
- Azotobacter, a genus of Gram-negative bacteria
- Rhizobium, a plant root-associated and plant differentiation-active genus of Gram-negative bacteria
- Agrobacterium, a plant pathogen (oncogenic) genus of Gram-negative bacteria, also of biotechnologic use
- Escherichia coli, a Gram-negative bacterium
- Enterococcus faecalis, a Gram-positive bacterium
- Zymomonas mobilis, a Gram-negative facultative anaerobe
- Xanthomonas campestris, a Gram negative bacterium which uses this pathway as main pathway for providing energy.
To date there is evidence of at least one Eukaryote using the pathway, suggesting it may be more widespread than previously thought:
- Hordeum vulgare, barley uses the Entner-Duodoroff pathway.
- Phaeodactylum tricornutum diatom model species presents functional phosphogluconate dehydratase and dehoxyphosphogluconate aldolase genes in its genome 
The Entner-Doudoroff pathway is present in many species of Archaea (caveat, see following), whose metabolisms "resemble... in [their] complexity those of Bacteria and lower Eukarya", and often include both this pathway and the Embden-Meyerhof-Parnas pathway of glycolysis, except most often as unique, modified variants.
Enzymes catalysing Entner Doudoroff Pathway (ED Pathway)
Conversion of glucose to glucose-6-phosphate
The first step in ED is phosphorylation of glucose by a family of enzymes called hexokinases to form glucose 6-phosphate (G6P). This reaction consumes ATP, but it acts to keep the glucose concentration low, promoting continuous transport of glucose into the cell through the plasma membrane transporters. In addition, it blocks the glucose from leaking out – the cell lacks transporters for G6P, and free diffusion out of the cell is prevented due to the charged nature of G6P. Glucose may alternatively be formed from the phosphorolysis or hydrolysis of intracellular starch or glycogen.
In animals, an isozyme of hexokinase called glucokinase is also used in the liver, which has a much lower affinity for glucose (Km in the vicinity of normal glycemia), and differs in regulatory properties. The different substrate affinity and alternate regulation of this enzyme are a reflection of the role of the liver in maintaining blood sugar levels.
Conversion of glucose-6-phosphate to 6-phosphoglucanolactone
The G6P is then converted to 6-phosphoglucanolactone in the presence of enzyme glucose-6-phosphate dehydrogenase( an oxido-reductase) with the presence of co-enzyme nicotinamide adenine dinucleotide phosphate (NADP+) which will be reduced to nicotineamide adenine dinucleotide phosphate hydrogen along with a free hydrogen atom H+
Conversion of 6-phosphoglucanolactone to 6-Phosphogluconic acid
The 6PGL is converted into 6-phosphogluconic acid in the presence of enzyme hydrolase.
Conversion of 6-Phosphogluconic acid to 2-keto-3-deoxy-6-phosphoglucanate
The 6-phosphogluconic acid is converted to 2-keto-3-deoxy-6-phosphogluconate(KDPG) in the presence of enzyme 6-phosphogluconate dehydratase in which water molecule is released to the surroundings.
Conversion of 2-keto-3-deoxy-6-phosphoglucanate to Pyruvate and Glyceraldehyde-3-phosphate
The KDPG is then converted into pyruvate or glyceraldehyde-3-phosphate in the presence of enzyme KDPG aldolase. when KDPG is converted into pyruvate, the ED pathway for that pyruvate ends here and then the pyruvate goes into further metabolic pathways (TCA cycle,ETC cycle etc..).
The other product (Glyceraldehyde-3-phosphate) is further converted by entering into the glycolysis pathway and at last get converted into pyruvate for further metabolism.
Conversion of Glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate
The G3P is converted to 1,3-bisphosphoglycerate int the presence of enzyme Glyceraldehyde-3-phosphate dehydrogenase( an oxido-recductase).
The hydrogen is used to reduce two molecules of NAD+, a hydrogen carrier, to give NADH + H+ for each triose.
Hydrogen atom balance and charge balance are both maintained because the phosphate (Pi) group actually exists in the form of a hydrogen phosphate anion (HPO42−), which dissociates to contribute the extra H+ ion and gives a net charge of -3 on both sides.
Conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate
Conversion of 3-phosphoglycerate to 2-phosphoglycerate
Conversion of 2-phosphoglycerate to phosphoenol pyruvate
Cofactors: 2 Mg2+: one "conformational" ion to coordinate with the carboxylate group of the substrate, and one "catalytic" ion that participates in the dehydration
Conversion of phosphoenol pyruvate to pyruvate
A final substrate-level phosphorylation now forms a molecule of pyruvate and a molecule of ATP by means of the enzyme pyruvate kinase. This serves as an additional regulatory step, similar to the phosphoglycerate kinase step.
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- Chen, Xi, et al. "The Entner–Doudoroff pathway is an overlooked glycolytic route in cyanobacteria and plants." Proceedings of the National Academy of Sciences (2016): 201521916.
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