Entner–Doudoroff pathway

The Entner- Doudoroff pathway or ED pathway ( according to the discoverers Nathan Entner Doudoroff and Michael, also known as 2-keto -3- deoxy-6 - phosphogluconate pathway or KDPG - way ) is a way of degradation of sugars in living things for the purpose of energy production. It comes only in some bacteria and - in modified form - even in archaea ago. The ED pathway is an alternative to the Embden - Meyerhof - Parnas pathway ( glycolysis), which is taken by most other living things. The energy yield in the form of adenosine triphosphate (ATP ) is only 1 ATP, while it is 2 ATP in the Embden - Meyerhof - Parnas pathway.

Biochemistry

ED pathway in bacteria

In bacteria, D -glucose is first activated to glucose -6-phosphate with ATP consumption in the pathway. By a glucose -6-phosphate dehydrogenase (EC 1.1.1.49 ) that is oxidized to 6- Phosphoglucono - δ - lactone, thereby NADPH is formed. The lactone is then hydrolyzed by a 6- Phosphoglucolactonase ( EC 3.1.1.31 ) to 6 -phosphogluconate. These steps correspond to the initial reaction in the oxidative part of the pentose phosphate pathway. 6-phosphogluconate is then transferred by a Phosphogluconatdehydratase ( EC 4.2.1.12 ) in the characteristic of the metabolic pathway of the compound 2 -keto -3- deoxy-6 -phosphogluconate ( KDPG ). A KDPG aldolase ( EC 4.1.2.14 ) cleaves this into pyruvate and glyceraldehyde -3-phosphate in. The latter will eventually also transferred in the course of glycolysis to pyruvate, two molecules of ATP and one molecule of NADH are formed.

In the net balance sheet of one molecule of D -glucose, two molecules of pyruvate and water, and each formed one molecule of NADPH, NADH and ATP:

Sugar transport into the cell

The transport of D-glucose is not made through the phosphoenolpyruvate -dependent sugar phosphotransferase systems of many bacteria, but an H Symportsystem. Thus, no PEP molecule is per -transporting molecule consumed glucose and can be used for anabolism.

As the only prokaryote Zymomonas mobilis glucose, however, can take up by facilitated diffusion into the cell. He lives in places where the concentration of sugars is particularly high, for example in the sugary sap of agaves.

Modified forms in archaea and other species

Sugar -degrading archaea, and many other organisms use not only modified EMP pathways to the ED pathway for the degradation of glucose, although this compared to bacteria shows some differences. Have been identified in archaea three ED - metabolic pathways, in which glucose is always at the beginning, and is not glucose -6-phosphate reduced. They are explained below.

Non - phosphorylating ED pathway

In ( hyper) thermoacidophilic archaea as Sulfolobus solfataricus, and Thermoplasma acidophilum Thermoproteus tenax a variant of the ED pathway has been discovered in the KDPG not formed. Here, glucose is first to 2-keto -3 -deoxy- gluconate (KDG ), not to KDPG, implemented, thereby arise as in the bacterial NADH and NADPH ED pathway ( see illustration). Thus, there will be no phosphorylation. KDG is cleaved by a specific aldolase to pyruvate and glyceraldehyde. The latter is oxidized to glycerate (GA ), which is either a NAD (P ) -, or a ferredoxin - dependent dehydrogenase catalyzing. Glycerate is converted by a kinase to 2 -phosphoglycerate (2- PG) consumption of ATP, and then to 3 -phosphoglycerate (3- PG) isomierisiert. Through the glycolytic enzymes described above then arises out of it Pyrvuat.

Since this pathway takes place no net ATP - profit, it is called a non- phosphorylating ED pathway.

Halbphosphorylierender ED pathway

Some Clostridienarten and halophilic archaea such as Halobacterium saccharovorum and Halobacterium halobium use an ED variant, which is called semi ( semi) phosphorylating. Here, glucose is converted to KDG as in non- phosphorylating ED pathway ( see illustration). KDG is phosphorylated prior to aldol cleavage KDPG by KDG kinase, one molecule of ATP is invested. Subsequently, the aldol cleavage follows Pyrvuat and glycerol -3-phosphate (GAP ). This is then followed by reaction in glycolysis to pyruvate, depending on the result either of enzymes NADPH, NADH or even reduced ferredoxin as a reducing agent. In this metabolic pathway, one molecule of ATP and two molecules of reducing equivalents are generated as a whole.

Branched ED pathway

The Crenarchaeota Sulfolobus and Thermoproteus apparently do the same to non- and semi- phosphorylating ED pathway, which is called a branched ED pathway. But there are two special features. Firstly catalyzes a bifunctional KDG / KDPG kinase the aldol cleavage of KDG or KDPG. On the other hand, the GAP generated in the aldol cleavage of KDPG is not oxidized by a classical GAPDH. Instead, it is oxidized directly by a non -phosphorylating Glycerinaldehyddehydrogenase ( GAPN ) to 3 -phosphoglycerate. Since at this step no ATP is produced by substrate chain as in the classical glycolysis, no net ATP gain in branched ED pathway takes place.

Importance

A significant number of bacteria does not have the full features of the enzymes for the (classical) Embden - Meyerhof - Parnas pathway of glycolysis, for example, lacks the phosphofructokinase -1. This enzyme catalyzes the initial reactions of glycolysis. Therefore, they are dependent on Entner- Doudoroff pathway to metabolize glucose can. Other bacteria, such as Escherichia coli use both this path as well as the classic form of glycolysis. The Entner- Doudoroff pathway allows namely the metabolism of gluconate or other, related organic acids that can not enter the glycolysis.

The Entner- Doudoroff path is taken also for example in the alcoholic fermentation of the bacterium Zymomonas mobilis. The pyruvate formed is here decarboxylated to acetaldehyde and this reduced to the cleaved from glucose -6-phosphate and glyceraldehyde -3-phosphate to ethanol, hydrogen. Industrially this fermentation is used for the production of pulque. The rate of fermentation and the product yields are significantly higher than for the alcoholic fermentation by yeasts that degrade glucose via the Embden - Meyerhof - Parnas pathway.

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