If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
|Superclasses:||Generation of Precursor Metabolites and Energy|
Expected Taxonomic Range: Spermatophyta
Oil is a major carbon reserve in most plant seeds. Conversion of carbohydrates to oil generally starts by glycolysis (either mitochondrial or cytosolic). Pyruvate, the end product of glycolysis, is further decarboxylated to acetyl-CoA, the precursor for fatty acid and oil biosynthesis. In the process, one-third of carbon is lost as CO2. An elegant experiment illustrated the existence of a Rubisco shunt [Schwender04]. Comparing to glycolysis, the Rubisco shunt results in an increase of carbon conversion efficiency, yielding 20% more acetyl-CoA with 40% less carbon loss. In oilseed rape, the Rubisco shunt is responsible for the production of 37% -75% of phosphoglycerate in the developing embryo.
There are three stages in the Rubisco shunt.
Stage I is composed of the non-oxidative pentose phosphate pathway (in reverse direction of what is shown in pentose phosphate pathway (non-oxidative branch)), where a series of re-arrangements of the carbon skeleton convert the six-carbon sugar β-D-fructofuranose 6-phosphate to the five-carbon sugar D-ribulose 5-phosphate, which is further converted to D-ribulose-1,5-bisphosphate. The net change is:
Stage II is catalyzed by the carboxylase activity of Rubisco, which fixes one molecule of CO2 with one molecule of D-ribulose-1,5-bisphosphate, yielding two molecules three-carbon 3-phospho-D-glycerate. The net change is:
In stage III 3-phospho-D-glycerate is converted to pyruvate via the later steps of glycolysis. Pyruvate is further metabolized to acetyl-CoA which fuels fatty acid biosynthesis (fatty acid biosynthesis initiation I) and seed oil (triacylglycerol biosynthesis) accumulation. The net change is:
Overall, the net carbon stoichiometry via the Rubisco shunt is:
Comparing to glycolysis (5 β-D-fructofuranose 6-phosphate = 10 acetyl-CoA + 10 CO2), the Rubisco shunt allows the conversion of carbohydrate into 20% more acetyl-CoA than glycolysis, and has 40% less carbon loss as CO2.
Note that there is no active Calvin-Benson-Bassham cycle following the Rubisco step in developing seeds. This allows the continuation of Rubisco shunt to stage III.
The whole pathway is located in the plastid.
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Comi01: Comi GP, Fortunato F, Lucchiari S, Bordoni A, Prelle A, Jann S, Keller A, Ciscato P, Galbiati S, Chiveri L, Torrente Y, Scarlato G, Bresolin N (2001). "Beta-enolase deficiency, a new metabolic myopathy of distal glycolysis." Ann Neurol 50(2);202-7. PMID: 11506403
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David70: David J, Wiesmeyer H (1970). "Regulation of ribose metabolism in Escherichia coli. II. Evidence for two ribose-5-phosphate isomerase activities." Biochim Biophys Acta 208(1);56-67. PMID: 4909663
deZwaan75: de Zwaan A, Holwerda DA, Addink AD (1975). "The influence of divalent cations on allosteric behaviour of muscle pyruvate kinase from the sea mussel Mytilus edulis L." Comp Biochem Physiol B 52(4);469-72. PMID: 1203
Essenberg75: Essenberg MK, Cooper RA (1975). "Two ribose-5-phosphate isomerases from Escherichia coli K12: partial characterisation of the enzymes and consideration of their possible physiological roles." Eur J Biochem 55(2);323-32. PMID: 1104357
Ezaki99: Ezaki S, Maeda N, Kishimoto T, Atomi H, Imanaka T (1999). "Presence of a structurally novel type ribulose-bisphosphate carboxylase/oxygenase in the hyperthermophilic archaeon, Pyrococcus kodakaraensis KOD1." J Biol Chem 274(8);5078-82. PMID: 9988755
Foster10: Foster JM, Davis PJ, Raverdy S, Sibley MH, Raleigh EA, Kumar S, Carlow CK (2010). "Evolution of bacterial phosphoglycerate mutases: non-homologous isofunctional enzymes undergoing gene losses, gains and lateral transfers." PLoS One 5(10);e13576. PMID: 21187861
Grana89: Grana X, Urena J, Ludevid D, Carreras J, Climent F (1989). "Purification, characterization and immunological properties of 2,3-bisphosphoglycerate-independent phosphoglycerate mutase from maize (Zea mays) seeds." Eur J Biochem 186(1-2);149-53. PMID: 2480892
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