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MetNet - plant pathway - superpathway of cytosolic glycolysis (plants), pyruvate dehydrogenase and TCA cycle
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Pathway details: superpathway of cytosolic glycolysis (plants), pyruvate dehydrogenase and TCA cycle


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  Pathway was created on Tue Jul 30, 2013.
 Contributed by aracyc:
Supporting evidence for this pathway in Arabidopsis: This pathway is on the list of Ubiquitous Plant Pathways, which includes pathways that are thought to be present in all or most land plants. [more info] Summary from MetaCyc: The glycolysis pathway represented here is also known as the 'Embden-Meyerhof-Parnas pathway'. First identified in yeast cells and mammalian tissue, it is now also seen as the 'central' metabolic pathway in plants, and can be found, if at least in part, in all organisms. Glycolysis has evolved as a catabolic anaerobic pathway that fulfills two essential functions: i) it oxidizes hexoses to generate ATP, reductants and pyruvate, and ii) it is an amphibolic pathway (pathway that involves both catabolism and anabolism) because it can reversibly produce hexoses from various low-molecular weight molecules. The ultimate purpose of the pathway is to convert carbon in its reduced form in storage carbohydrates (e.g. glycogen, and simpler carbohydrates such as sucrose, and ). In plants, glycolysis is the predominant pathway fueling respiration (see ) because, unlike animal mitochondria, plant mitochondria rarely respire fatty acids. In plants, this pathway occurs in two different subcellular locations: the cytosol and plastids, which are the sites of sucrose degradation III and , respectively. Whereas the plastidic glycolysis pathway (see glycolysis I (plastidic)) is identical to the conventional microbial glycolysis, the cytosolic pathway (i.e. this pathway) is slightly modified. These pathways can interact with one another though the action of highly selective transporters present in the inner plastid envelope |CITS:[Emes93]|. In chloroplasts in the dark, as well as in plastids of non-photosynthetic tissues, the primary function of glycolysis I (plastidic) is the degradation of to generate carbon skeletons, reductants and ATP for anabolic pathways such as that of fatty acid biosynthesis initiation I via acetyl-CoA biosynthesis (from pyruvate). In the cytosol, the same products are generated from the degradation of sucrose. The pathway starts with β-D-glucose-6-phosphate, which is made available from a variety of sources (in the plant plastids, it results from ). The first committed step of glycolysis is the reversible conversion of β-D-glucose-6-phosphate into D-fructose-6-phosphate by an hexose phosphate isomerase, which changes the pyranose configuration of glucose into the furanose configuration of fructose. The second step is catalyzed by a phosphofructokinase in the presence of ATP; this step is irreversible (in the cytosol of plants, see sucrose degradation III, an alternative enzyme uses diphosphate instead of ATP). The third step is catalyzed by an aldolase which cleaves fructose-1,6-bisphosphate into interconvertable (they are keto-enol tautomers) two three-carbon fragments: D-glyceraldehyde-3-phosphate and dihydroxyacetone phosphate. The interconversion of these tautomers is facilitated by a triose phosphate isomerase. The following reaction, which adds one phosphate residue to D-glyceraldehyde-3-phosphate to form 1,3-diphosphateglycerate, is freely reversible and requires NAD+ and phosphate. The next step releases one molecule of ATP during the conversion of 1,3-diphosphateglycerate into 3-phosphoglycerate by a Mg2+-dependent glyceraldehyde-3-phosphate kinase. The next step requires little energy change and leads to the reversible transfer of a phosphate group from the 3- to the 2-hydroxyl group of glycerate, leading to the formation of 2-phosphoglycerate. The removal of a molecule of water by an enolase in the presence of Mg2+ converts 2-phosphoglycerate into phosphoenolpyruvate. The final step of glycolysis involves the ketolization of phosphoenolpyruvate to pyruvate by a pyruvate kinase, leading to the release of a molecule of ATP. In the cytosol, there are two alternative reactions to the plastidic pathway: EC 2.7.1.90 and EC 1.2.1.9. The former represents an alternative to EC 2.7.1.11; this reaction utilizes diphosphate rather than ATP to convert D-fructose-6-phosphate to fructose-1,6-bisphosphate. The plant cytosol lacks soluble inorganic alkaline pyrophosphatases (PPiases) and, consequently, contains higher concentrations of diphosphate (up to 0.3 mM |CITS:[WEINER87]|). It has been proposed that the utilization of pyrophosphate rather than that of ATP in glycolysis is favored under nutritional Pi deprivation or oxygen deficiency |CITS:[DAVIES93],. The latter reaction, EC 1.2.1.9, provides a bypass between D-glyceraldehyde-3-phosphate and 3-phosphoglycerate, which allows for their conversion without phosphorylation by a non-phosphorylating glyceraldehyde 3-P dehydrogenase ,. This reaction produces NADPH but not ATP. The overall process, which requires the initial investment of one molecule of ATP to produce fructose-1,6-bisphosphate, yields four molecules of ATP and two molecules of NADH for every molecule of hexose fed to the system. The fate of the final product of glycolysis, pyruvate, depends on the state of the oxygen supply and can be consumed, for example, by the via the formation acetyl-CoA in aerobic conditions, or degraded to ethanol or lactate under anaerobic conditions (see sucrose degradation to ethanol and lactate (anaerobic)).
  Parts of this pathway occur in:   plastid stroma     mitochondrion     cytosol     nucleus     plastid     apoplast     mitochondrial matrix     peroxisome   multiple locations  


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metabolite [54]
protein complex [44]
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