Entner-Doudoroff way
Glc-6-P = glucose-6-phosphate
6-P-glucono-δ-lactone = 6-phosphoglucono-δ-lactone
6-P-gluconate = 6-phosphogluconate
KDPG = 2-keto-3-deoxy-6 -phosphogluconate
GAP = glyceraldehyde-3-phosphate
1,3-bPG = 1,3-bisphosphoglycerate
3-PG = 3-phosphoglycerate
2-PG = 2-phosphoglycerate
PEP = phosphoenolpyruvate
Pyr = pyruvate
The Entner-Doudoroff path or ED path (after the discoverers Nathan Entner and Michael Doudoroff ; also called 2-keto-3-deoxy-6-phosphogluconate path or KDPG path ) is a pathway for the breakdown of sugars in living beings for the purpose of energy generation. It occurs exclusively in some bacteria and - in a modified form - also in archaea . The ED path is an alternative to the Embden-Meyerhof-Parnas-Weg ( glycolysis ), which is used by most other living beings. The energy yield in the form of adenosine triphosphate (ATP) is only 1 ATP, while it is 2 ATP in the Embden-Meyerhof-Parnas-Weg.
biochemistry
ED pathway in bacteria
In bacteria, the metabolic pathway D - glucose is first activated to glucose-6-phosphate with consumption of ATP. A glucose-6-phosphate dehydrogenase ( EC 1.1.1.49 ) oxidizes this to 6-phosphoglucono-δ-lactone, thereby forming NADPH . The lactone is then hydrolyzed to 6-phosphogluconate by a 6-phosphoglucolactonase ( EC 3.1.1.31 ). These steps correspond to the initial reaction in the oxidative part of the pentose phosphate pathway . 6-phosphogluconate is then converted by a phosphogluconate dehydratase ( EC 4.2.1.12 ) into the compound 2-keto-3-deoxy-6- phosphogluconate ( KDPG ), which is characteristic of the metabolic pathway . A KDPG aldolase ( EC 4.1.2.14 ) splits this into pyruvate and into glyceraldehyde-3-phosphate . The latter is finally also converted into pyruvate in the course of glycolysis, with two molecules of ATP and one molecule of NADH being formed.
In the net balance, one molecule of D- glucose produces two molecules of pyruvate and water, and one molecule each of NADPH, NADH and ATP:
Sugar transport into the cell
The D- glucose is not transported via the phosphoenolpyruvate-dependent sugar phosphotransferase system of many bacteria, but via an H + symport system. This means that no PEP molecule is consumed per transporting molecule of glucose and can be used for anabolism .
In contrast , Zymomonas mobilis is the only prokaryote that can absorb glucose through easier diffusion into the cell. He lives in places where the concentration of sugar is particularly high, for example in the sugary juice of agaves .
Modified forms in archaea and other species
Sugar-degrading archaea, and many other organisms, use not only modified EMP pathways but also the ED pathway to break down glucose, although this shows some differences compared to bacteria. Three different ED metabolic pathways have been identified in archaea, in which initially glucose and not glucose-6-phosphate is reduced. They are explained below.
Non-phosphorylating ED pathway
In (hyper) thermoacidophilic archaea such as Sulfolobus solfataricus , Thermoplasma acidophilum and Thermoproteus tenax , a variant of the ED pathway was discovered that does not produce KDPG. Here, glucose is first converted to 2-keto-3-deoxy-gluconate (KDG), not to KDPG; as in the bacterial ED pathway, NADH and NADPH are formed (see figure). There is therefore no phosphorylation. KDG is split into pyruvate and glyceraldehyde by a specific aldolase . The latter is oxidized to glycerate (GA), which catalyzes either an NAD (P) + or a ferredoxin- dependent dehydrogenase. Glycerate is converted to 2-phosphoglycerate (2-PG) by a kinase with consumption of ATP, and then isomerized to 3-phosphoglycerate (3-PG). The glycolytic enzymes described above then produce pyrvuate.
Since there is no net gain of ATP in this metabolic pathway, it is called a non-phosphorylating ED pathway.
Semi-phosphorylating ED pathway
Some clostridial species as well as halophilic archaea such as Halobacterium saccharovorum and Halobacterium halobium use a variant of ED, which is called semi (semi) phosphorylating. As in the non-phosphorylating ED pathway, glucose is converted to KDG (see figure). KDG is phosphorylated by a KDG kinase prior to aldol cleavage to KDPG, with one molecule of ATP being invested. This is followed by aldol cleavage to pyrvuate and glycerol-3-phosphate (GAP). This is then converted into pyruvate in the glycolysis process, depending on the enzyme composition, either NADPH, NADH or reduced ferredoxin are produced as a reducing agent. This metabolic pathway generates a total of one molecule of ATP and two molecules of reduction equivalents.
Branched ED path
The Crenarchaeota Sulfolobus and Thermoproteus apparently operate the non- and the semi-phosphorylating ED pathway simultaneously, which is known as a branched ED pathway. But there are two special features. On the one hand, a bifunctional KDG / KDPG kinase catalyzes the aldol cleavage of KDG or KDPG. On the other hand, the GAP generated by the aldol cleavage of KDPG is not oxidized by a classic GAPDH. Instead, it is oxidized directly to 3-phosphoglycerate by a non-phosphorylating glyceraldehyde dehydrogenase (GAPN) . Since no ATP is generated in this step as in classical glycolysis by substrate chain phosphorylation , there is no net ATP gain in the branched ED path.
meaning
A considerable number of bacteria do not have the complete set of enzymes for the (classic) Embden-Meyerhof-Parnas pathway of glycolysis, for example phosphofructokinase-1 is missing . This enzyme catalyzes one of the initial glycolysis reactions. Therefore, they rely on the Entner-Doudoroff route to metabolize glucose. Other bacteria, such as B. Escherichia coli , use this route as well as the classic form of glycolysis. The Entner-Doudoroff path allows the metabolism of gluconate or other related organic acids that cannot enter into glycolysis.
The Entner-Doudoroff path is also used, for example, in the alcoholic fermentation of the bacterium Zymomonas mobilis . The pyruvate formed is decarboxylated to acetaldehyde and this is reduced to ethanol with the hydrogen split off from glucose-6-phosphate and glyceraldehyde-3-phosphate . This fermentation is used industrially for the production of pulque . The speed of fermentation and the product yield are significantly higher than with alcoholic fermentation using yeasts , which break down sugar via the Embden-Meyerhof-Parnas path.
literature
- Georg Fuchs (ed.), Hans. G. Schlegel (Author): General Microbiology . Thieme Verlag, Stuttgart 8th edition 2007, ISBN 3-13-444608-1 , p. 204ff.
- Katharina Munk (Ed.): Pocket textbook Biology: Microbiology . Thieme Verlag, Stuttgart 2008, ISBN 978-3-13-144861-3 , pp. 352f.
Individual evidence
- ↑ Entner, N. and Doudoroff, M. (1952): Glucose and gluconic acid oxidation of Pseudomonas saccharophila . In: J Biol Chem . 196 (2); 853-862; PMID 12981024 ; PDF (free full text access).
- ↑ Garabed Antranikian: Applied Microbiology . Springer, Berlin 2006; ISBN 978-3-540-24083-9 ; P. 51.
- ↑ Garabed Antranikian: Applied Microbiology . Springer, Berlin 2006; ISBN 978-3-540-24083-9 ; P. 45.
- ↑ a b c Reher, M. et al . (2010): The nonphosphorylative Entner-Doudoroff pathway in the thermoacidophilic euryarchaeon Picrophilus torridus involves a novel 2-keto-3-deoxygluconate-specific aldolase . In: J Bacteriol . 192 (4); 964-974; PMID 20023024 .
- ↑ Ahmed, H. et al .: The semi-phosphorylative Entner-Doudoroff pathway in hyperthermophilic archaea: a re-evaluation. In: The Biochemical journal. Volume 390, September 2005, pp. 529-540, doi: 10.1042 / BJ20041711 , PMID 15869466 , PMC 1198933 (free full text).
- ^ H. Robert Horton, Laurence A. Moran, K. Gray Scrimgeour, Marc D. Perry, J. David Rawn and Carsten Biele (translators): Biochemie . Pearson Studies; 4th updated edition 2008; ISBN 978-3-8273-7312-0 ; P. 474ff.