Entner–Doudoroff pathway – definition, Steps.

Entner–Doudoroff pathway is an alternative pathway of Glycolysis. This pathway is found in Gram-negative bacteria, certain Gram-positive bacteria, and archaea.

Entner–Doudoroff pathway Definition

• Entner–Doudoroff pathway is an alternative metabolic pathway of glycolysis where glucose is converted into Pyruvate with the help of a series of reactions and enzymes.
• Those bacteria are unable to perform glycolysis due to the absence of glycolytic enzymes they perform ED pathway to break down the glucose molecule into pyruvate. 
• This pathway was first reported by Entner and Doudoroff (1952) and MacGee and Doudoroff (1954) in the bacterium Pseudomonas saccharophila.
• Recent evidence showed that this pathway also found in cyanobacteria, ferns, algae, mosses, and plants.
• In this pathway, two unique enzymes are involved such as 2-keto-deoxy-6-phosphogluconate (KDPG) aldolase and 6-phosphogluconate dehydratase aldolase.
• In ED pathway 1 ATP is formed per glucose molecule and as well as 1 NADH and 1 NADPH.

Entner–Doudoroff pathway containing Organisms

• Those bacteria unable to metabolism glucose by using glycolysis follow this alternative or ED pathway to break the glucose into pyruvate. For example, Pseudomonas lacks the essential glycolytic enzyme phosphofructokinase, that’s why they follow the ED pathway to form pyruvate from Glucose.
• Only the aerobic and facultative anaerobes use the Entner–Doudoroff pathway and anaerobes use glycolysis due to its low energy yield.

Some example of bacteria those contain Entner–Doudoroff pathway are Pseudomonas, Azotobacter, Rhizobium, Agrobacterium, Escherichia coli, Enterococcus faecalis, Xanthomonas campestris, Zymomonas mobilis, Enterococcus faecalis. This pathway also found in Hordeum vulgare, Phaeodactylum tricornutum.

Entner–Doudoroff pathway Procedure

1. In the first step, glucose is converted into glucose 6 phosphate in the presence of enzyme hexokinase and Cofactors Mg2+. In this reaction, one molecule of ATP is consumed.
2. The glucose 6 phosphate is converted into 6-phosphoglucono 𝛿 lactone in presence of enzyme glucose 6 phosphate dehydrogenase. In this reaction, one NADP is reduced to NADPH.
3. The 6-phosphoglucono 𝛿 lactone is converted into 6-phosphogluconate  in presence of enzyme lactonase. In this reaction, one molecule of H2O is required.
4. The 6-phosphogluconate  is converted into 2-keto-3-deoxy-6-phosphoglucanate (KDPG) in presence of enzyme 6-phosphogluconate dehydrogenase. From this reaction, one H2O is released.
5. Now, KDPG is split into pyruvate and glyceraldehyde 3 phosphate in presence of enzyme KDPG aldolase. The pyruvate from this step enters into further metabolic pathways such as TCA cycle, ETC cycle, etc.
6. The glyceraldehyde 3 phosphate enters into the Glycolysis pathway and converts into pyruvate. The conversation of  glyceraldehyde 3 phosphate to pyruvate is followed by the following steps;
   1. glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate
   2. 1,3-bisphosphoglycerate to 3-phosphoglycerate
   3. 3-phosphoglycerate to 2-phosphoglycerate
   4. 2-phosphoglycerate to phosphoenol pyruvate
   5.  phosphoenol pyruvate to pyruvate