Entner Doudoroff Pathway: overview, diagram, steps, products

What Is The Entner-Doudoroff Pathway?

The Entner-Doudoroff Pathway is an alternative glycolytic pathway found in some bacteria and archaea converting glucose to pyruvate while endowing ATP and NADPH. Unlike the much more common Embden-Meyerhof-Parnas, EMP, pathway (traditional glycolysis), this pathway involves unique enzymatic machinery and intermediates, including 2-keto-3-deoxy-6-phosphogluconate, or KDPG. The ED Pathway is one of the most interesting routes of metabolism in microbial ecology because it shows a very interesting way of producing energy and biosynthesis under certain conditions.

Steps of the Entner–Doudoroff Pathway (with Enzymes)

The steps of the Entner Duodoroff Pathway are given below:

Step 1 — Glucose → Gluconolactone → 6-Phosphogluconate

Enzyme:

• Glucose dehydrogenase

• Gluconolactonase

Mechanism:

• Gluconolactone is hydrolysed by gluconolactonase to form 6-phosphogluconate.

Step 2 — 6-Phosphogluconate → KDPG

Enzyme:

• 6-Phosphogluconate dehydratase

• KDPG aldolase

Mechanism:

• 6-Phosphogluconate to 2-Keto-3-deoxy-6-phosphogluconate (KDPG):

• 6-phosphogluconate is dehydrated by 6-phosphogluconate dehydratase to form KDPG, an intermediate in the pathway.

Step 3 — KDPG → Pyruvate + G3P

• Enzyme: KDPG aldolase

• Mechanism: KDPG is split by KDPG aldolase to form one molecule of pyruvate and one molecule of glyceraldehyde-3-phosphate (G3P).

Step 4 — G3P → Pyruvate (via glycolysis steps)

• Through EMP pathway from G3P onward

• Produces: 1 NADH, 1 ATP

ATP and Reducing Power Yield (ED vs Glycolysis)

The energy yield of ED Pathway

The glycolysis yields 2 ATP, ED yields 1 ATP.

Comparison – ED vs EMP (Glycolysis) vs PPP

The differences between the pathway is included in the table:

Enzymes Involved

The key enzymes involved are:

Glucose Dehydrogenase:

• Function: It is used to catalyse the reaction where glucose is oxidised.

• Importance: It is important because this enzyme ensures that the pathway does not halt because the compound gluconolactone is not stable.

KDPG Aldolase :

• Function: The enzyme is responsible for cleaving KDPG to pyruvate and glyceraldehyde-3-phosphate (G3P).

• Importance: The enzyme gives the two products emerging on the exit of the pathway and therefore emerging products are available for more metabolism. Most significantly since pyruvate is part of the cycle produced it means the energy demands of the cell and the energy cycle via respiration can be met

Coenzymes Involved

The coenzymes involved are:

NADP+

• Role: Accept electrons from glucose oxidation to gluconolactone and maintains the balance of reducing and oxidising agents in the cell, and provides reducing equivalents to the biosynthetic pathways.

• Function: The reduction of NADP+ to NADPH is the first reaction in this pathway. It represents a very important initial reaction for which adequate reducing power is required for anabolic reactions, for instance, the synthesis of fatty acids and nucleotides.

NAD+

• Role: Although an early coenzyme, NADP↓ is more prominent than NAD+ as a glycolytic coenzyme, because NAD+ could function very well with glycolysis at a more advanced stage of nutrient depletion

• Function: NAD+ is reduced to NADH in glycolysis and all catabolic reactions

Importance of Entner Duodoroff Pathway

The importance of Entner Duodoroff Pathway are:

• The NADH proceeds through the electron transport chain to form ATP.

• Links the Entner-Doudoroff pathway to the rest of the cell's catabolism.

• Produces NADPH for biosynthesis of fatty acids and nucleotides

• Important for metabolic engineering, for example, bioethanol producing in Zymomonas

• Helps to classify bacteria biochemically.

Entner Duodoroff Pathway NEET MCQs (With Answers & Explanations)

Important topics for NEET are:

• Steps of Entner Duodoroff Pathways

• Enzymes Involved

Practice Questions for NEET

Q1. The organism which lacks phosphofructokinase and can not degrade glucose by the glycolytic process is

1. Nostoc fluorescens

2. Opuntia lindeneri

3. Pseudomonas saccharophila

4. None of these

Correct answer: 3) Pseudomonas saccharophila

Explanation:

Archaea is an organism devoid of phosphofructokinase (PFK) and unable to break down glucose through the glycolytic pathway, particularly methanogens like Methanococcus jannaschii. These beings depend on different metabolic routes for energy generation, such as the Entner-Doudoroff pathway or altered ones rather than traditional glycolysis.

PFK is a crucial enzyme in glycolysis, transforming fructose-6-phosphate into fructose-1,6-bisphosphate. Its non-existence signifies that these organisms adopt alternative carbohydrate-processing strategies outside typical glycolysis.

Hence, the correct answer is option 3) Pseudomonas saccharophila.

Q2. In eukaryotes, __________is the main enzyme for replication.:

1. DNA polymerase

2. RNA polymerase

3. DNA ligase

4. None of the above

Correct answer: 1) DNA polymerase

Explanation:

DNA polymerase, in eukaryotes, DNA polymerase ? has 3’→5’ exonuclease activity for proofreading which is the main enzyme for replication. RNA polymerase is the primary function of DNA polymerase is to synthesize primers. The smaller subunit has a primase activity and the largest subunit has polymerization activity. DNA ligase is the main function is DNA repair and removes primers for Okazaki fragments from the lagging strand.

Hence, the correct answer is option 1) DNA polymerase.

Q3. In the Entner-Doudoroff (ED) pathway. glucose is phosphorylated to glucose -6-phosphate by which of the following enzymes:

1. Hexokinase

2. Hydrolase

3. Aldolase

4. Glucose-6-phosphate dehydrogenase

Correct answer: 1) Glucose-6-phosphate dehydrogenase

Explanation:

In the Entner-Doudoroff pathway, glucose is first phosphorylated to glucose-6-phosphate by the enzyme hexokinase (or glucokinase, depending on the species), as in the case of the first step of glycolysis. Then, unlike glycolysis, the ED pathway is continued with the conversion of glucose-6-phosphate to 2-keto-3-deoxy-6-phosphogluconate, which later splits into two 3-carbon molecules. This pathway is an alternative to glycolysis. It is used by certain bacteria and archaea in the metabolism of glucose, yielding less ATP and NADPH than the classic glycolysis pathway.

Hence, the correct answer is option 1) Hexokinase is the correct answer.

Also Read:

• MCQs on Entner Duodoroff Pathway

• Metabolic Fate of Pyruvate

• Lactic Acid Fermentation

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