Glyoxysomes

Glyoxysomes

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  Presented by Ajit KumarDansena M.Sc. Life Science(1st sem) Central University of Guajrat
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  Glyoxysomes
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  The Discovery ofGlyoxysomes: the Work of Harry Beevers(in 1961)  They analyzed the linear sucrose gradients of endosperm homogenates and showed that the glyoxylate cycle.  Enzymes were found in an organelle fraction that was not mitochondria .  Beevers and Breidenbach called these new organelles glyoxysomes. Beevers and their postdoctoral fellow was Bill Breidenbach.
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  Glyoxysomes are specializedperoxisomes found in plants (particularly in the fat storage tissues of germinating seeds) and also in filamentous fungi. glyoxysomes possess the key enzymes of glyoxylate cycle (isocitrate lyase and malate synthase). INTRODUCTION OF GLYOXYSOMES Glyoxysomes are found in contact with lipid bodies in cotyledons or endosperm where fatty acids are being converted to carbohydrate (sugars) during germination.
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   Thus, glyoxysomes(as all peroxisomes) contain enzymes that initiate the breakdown of fatty acids and additionally possess the enzymes to produce intermediate products for the synthesis of sugars by gluconeogenesis. The seedling uses these sugars synthesized from fats until it is mature enough to produce them by photosynthesis. Glyoxysomes also participate in photorespiration and nitrogen metabolism in root nodules.
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  Succinate produced inglyoxysomes is ultimately converted to sucrose in the cytosol. It is presumed that presence of glyoxysomes in senescent organs is in response to the mobilization of membrane lipids.
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   Glyoxysomes havethe following characteristics: (1) They have a single membrane. (2) They have high equilibrium density in sucrose gradient centrifugation. (3)Their matrix (internal content) is finely granular. CHARACTERISTICS OF GLYOXYSOMES
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  The Glyoxylate Cycle AnotherProcess Involving Glycolytic Enzymes and Metabolites
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  Glyoxylate cycle The glycosylatecycle a variation of the tri- carboxylic acid cycle, is an anaerobic pathway occurring in plant, bacteria, fungi and protists. The glyoxylate cycle centres on the conversion of acetyl-coA to succinate for the synthesis of carbohydrate. Glyoxylate cycle allows cell to naturalise simple carbon compound as a carbon source when complex source such as glucose is not available.
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  It is ananabolic pathway occurring in plants, and several microorganisms but not in animals. The pathway occurs only in glyoxysome. The enzymes common to the TCA cycle and the glyoxysomes are isoenzymes, one is specific to mitochondria and the other is to glyoxysomes. The glyoxylate cycle allows plants to use acetyl-CoA derived from β-oxidation of fatty acids for carbohydrate synthesis (use fats for the synthesis of carbohydrates).  Acetyl-CoA is totally oxidized to CO2
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  -oxidation occurs inmitochondria and peroxisomes in mammals, but exclusively in the peroxisome in plants and yeast.
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  Function This cycle allowsseeds to use lipid as a source of energy to form shoot and root during germination. The lipid stores of germinating seeds are used for the formation of the carbohydrates that fuel the growth and development of the organisms. Glyoxysomes also functions in photorespiration and nitrogen fixation.
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  Role in Gluconeogenesis Fattyacid from lipid are commonly used as an energy source by vertebrates as fatty acid degraded through beta oxidation into acetate molecule. This acetate, bound to the active thiol group of co-A, enters the citric acid cycle where it is fully oxidised to carbon dioxide.  This pathway thus allows cell to obtain energy from fat. To utilize acetate from fat for biosynthesis of carbohydrates, the glyoxylate cycle whose initial reaction are identical to the TCA cycle is used.
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  Summary  Pyruvate isconverted to acetyl-CoA by the action of pyruvate dehydrogenase complex, a huge enzyme complex.  Acetyl-CoA is converted to 2CO2 via the eight-step citric acid cycle, generating three NADH, one FADH2, and one ATP (by substrate-level phophorylation).  Intermediates of citric acid cycle are also used as biosynthetic precursors for many other biomolecules, including fatty acids, steroids, amino acids, heme, pyrimidines, and glucose.
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  Oxaloacetate can getreplenished from pyruvate, via a carboxylation reaction catalyzed by the biotin- containing pyruvate carboxylase.  Net conversion of fatty acids to glucose can occur in germinating seeds, few invertebrates and some bacteria via the glycoxylate cycle.  Cycle shares three steps with the citric acid cycle but bypasses the two decarboxylation steps, converting two molecules of acetyl-CoA to one succinate.