“Entner–Doudoroff pathway”的意思、由来-开放百科全书

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{{citation needed|date=August 2015}} (the latter two being most widely used in the Bacteria).{{citation needed|date=August 2015}} The ED pathway was first reported by Michael Doudoroff and Nathan Entner from the Bacteriology Department at the University of California, Berkeley in 1952.^[1]{{primary source inline|date=August 2015}} 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.^[2] Use of the Entner–Duodoroff pathway among other plants such as mosses and ferns is also probably widespread, based on preliminary sequencing data analysis.^[2]

Organisms that use the Entner-Doudoroff pathway

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).^[3] Genera in which the pathway is prominent include Gram-negative,{{citation needed|date=August 2015}} as listed below, Gram-positive bacteria such as Enterococcus faecalis,^[4]{{full citation needed|date=August 2015}}{{page needed|date=August 2015}}{{better source|date=August 2015}} 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).^[5] Most organisms that use the pathway are aerobes, due to the low ATP yield per glucose molecule metabolised.{{citation needed|date=August 2015}}{{clarify|date=August 2015}}

To date there is evidence of at least one Eukaryote using the pathway, suggesting it may be more widespread than previously thought:

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.^[5]

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 (K_m 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 aldehyde groups of the triose sugars are oxidised, and inorganic phosphate is added to them, forming 1,3-bisphosphoglycerate.

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 (P_i) 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

This step is the enzymatic transfer of a phosphate group from 1,3-bisphosphoglycerate to ADP by phosphoglycerate kinase, forming ATP and 3-phosphoglycerate.

Conversion of 3-phosphoglycerate to 2-phosphoglycerate

Conversion of 2-phosphoglycerate to phosphoenol pyruvate

Enolase next converts 2-phosphoglycerate to phosphoenolpyruvate. This reaction is an elimination reaction involving an E1cB mechanism.

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.

References

1. ^{{cite journal |author1=Nathan Entner |author2=Michael Doudoroff |title= Glucose and gluconic acid oxidation of Pseudomonas saccharophila |year= 1952|journal= J. Biol. Chem.|volume= 196|issue= |pages= 853–862|url= http://www.jbc.org/content/196/2/853.full.pdf+html | accessdate= 3 August 2015}}
2. ^¹²Chen, 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.
3. ^Conway,T. (1992) "The Entner-Doudorodd pathway: history, physiology and molecular biology" Microbiology of Reviews 103(19; May), pp. 1-28, DOI , see
4. ^{{cite book |author=Willey |author2=Sherwood |author3=Woolverton |title=Prescott's Principles of Microbiology}}{{full citation needed|date=August 2015}}{{page needed|date=August 2015}}
5. ^¹Bräsen C.; D. Esser; B. Rauch & B. Siebers (2014) "Carbohydrate metabolism in Archaea: current insights into unusual enzymes and pathways and their regulation," Microbiol. Mol. Biol. Rev. 78(1; March), pp. 89-175, DOI 10.1128/MMBR.00041-13, see {{cite web |url=https://mmbr.asm.org/content/78/1/89.full.pdf+html |title=Archived copy |accessdate=2015-08-04 |deadurl=yes |archiveurl=https://web.archive.org/web/20151122004330/http://mmbr.asm.org/content/78/1/89.full.pdf+html |archivedate=2015-11-22 |df= }} or [https://www.ncbi.nlm.nih.gov/pubmed/24600042], accessed 3 August 2015.
6. ^{{cite journal |author1=Michael P. Stephenson |author2=Frank A. Jackson |author3=Edwin A. Dawes |title=Further Observations on Carbohydrate Metabolism and its Regulation in Azotobacter beijerinckii |journal=Journal of General Microbiology |year=1978 |volume=109 |pages=89–96 |doi=10.1099/00221287 |url=http://mic.sgmjournals.org/cgi/content/abstract/109/1/89 |issue=1}}
7. ^Kuykendall, L. David; John M. Young; Esperanza Martínez-Romero; Allen Kerr & Hiroyuka Sawada (2006) Genus I. Rhizobium Frank 1889, 389^AL [Order VI. Rhizobiales ord. nov., Family I Rhizobiaceae Conn 1938, 321^AL (L. David Kuykendall, Ed.)], pp. 324-339, in Bergey's Manual® of Systematic Bacteriology, Vol. 2 The Proteobacteria, Part 3 The Alpha-, Beta-, Delta-, and Epsilonproteobacteria, (Don J. Brenner, Noel R. Krieg, James T. Staley, Vol. Eds., George M. Garrity, Ed.-in-Chief), New York, NY, USA: Springer Science & Business, {{ISBN|0387241450}}, [https://books.google.com/books?isbn=0387241450], accessed 3 August 2015.
8. ^{{cite journal|vauthors=Arthur LO, Nakamura LK, Julian G, Bulla LA | title=Carbohydrate catabolism of selected strains in the genus Agrobacterium. | journal=Appl Microbiol | year= 1975 | volume= 30 | issue= 5 | pages= 731–7 | pmid=128316 | doi= | pmc=187263 | url= }}
9. ^¹{{cite journal|vauthors=Peekhaus N, Conway T | title=What's for dinner?: Entner-Doudoroff metabolism in Escherichia coli. | journal=J Bacteriol | year= 1998 | volume= 180 | issue= 14 | pages= 3495–502 | pmid=9657988 | doi= | pmc=107313 | url= }}
10. ^{{cite journal|author1=Goddard J.L. |author2=J.R. Sokatch| title=2-Ketogluconate fermentation by Streptococcus faecalis. | journal=J. Bacteriol. | year= 1964 | volume= 87 | issue= | pages= 844–851 | pmid=14137623 | doi= | pmc=277103 | url= }}
11. ^{{cite journal|author1=Lu, G.T. |author2=J.R. Xie |author3=L. Chen |author4=J.R. Hu |author5=S.Q. An |author6=H.Z. Su | title=Glyceraldehyde-3-phosphate dehydrogenase of Xanthomonas campestris pv. campestris is required for extracellular polysaccharide production and full virulence. | journal=Microbiology | year= 2009 | volume= 155 | issue= 5 | pages= 1602–1612 | pmid=19372163 | doi=10.1099/mic.0.023762-0 | pmc= | url=https://www.ncbi.nlm.nih.gov/entrez/eutils/elink.fcgi?dbfrom=pubmed&tool=sumsearch.org/cite&retmode=ref&cmd=prlinks&id=19372163 |display-authors=etal}}