Glyoxylate Cycle- Basic idea and biochemical cycle with importane

Glyoxylate Cycle- Basic idea and biochemical cycle with importane

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  GLYOXYLATE CYCLE Dr. NandadulalSannigrahi, Associate Professor, Department of Botany, Nistarini college, Purulia D.B. Road, Purulia, INDIA (W.B)
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  GLYOXYLATE CYCLE  Thelower group of plants including bacteria, fungi , and he higher plants used non-glucose source to synthesize glucose as respiratory substances to avoid some urgent emergence in the life cycle. The neoglucogenesis is one of the important biochemical cycles operate in the cellular level to address the emergent situations. The answer lies in the understanding of glyoxylate cycle.  Some questions may come to address the unique anaplerotic cycle  What is glyoxylate cycle?  Where does it occur?  Why is it Occur?  How does it occur?  Why animals can not do it but lower group of organisms like protists, fungi and plant can perform ?  What is its significance?  What is Glyoxysomes and its specialty?  Why animal and human can not perform glyoxylate cycle?
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  GLYOXYLATE CYCLE  Theglyoxylate cycle, a kind of Gluconeogenesis is a metabolic pathway found in plants, bacteria, fungi, and protists that allows for the synthesis of carbohydrates from two-carbon compounds like acetate (Acetyl Co-A) and fatty acids.  It is a variation of the citric acid cycle that bypasses the two decarboxylation steps of the citric acid cycle, retaining the carbon atoms to build larger molecules.  This anabolic pathway or anaplerotic pathway is crucial for organisms like germinating seeds, which use stored fat to produce glucose for energy. It occurs in glyoxysome and plays an important role during seedlings. During the seed germination plants converts store lipid molecule into carbohydrate using glyoxylate cycle.  During seed germination photosynthesis is not operating so role of glyoxylate cycle is crucial. It enables plant to use acetate as a carbon and energy source.  Carbohydrate synthesis: It provides a way to convert fats into glucose, which is essential for growth in organisms that can't make their own carbohydrates from fats.  Energy source: It allows organisms to use two-carbon compounds, such as acetate, as a sole source of carbon and energy  Anabolism: It is an anabolic pathway that builds new molecules, allowing for the synthesis of new biomass.
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  GLYOXYLATE CYCLE  Twoenzymes play an important role in the glyoxylate cycle i.e. Isocitrate lyase and malate synthase.  Smith & Gunsalus (1954) reports the enzyme isocitrate lyase in the extract of Pseudomonas species.  Kornberg and co-workers reported the glyoxylate cycle or glyoxylate bypass.  The glyoxylate cycle is observed in plants, archaea, bacteria, protists, fungi and nematodes. However the presence of glyoxylate cycle in animals including human beings remains controversial.  Glyoxylate cycle plays an important role in plants during seedlings. During the seed germination plants converts store lipid molecule in to carbohydrate using glyoxylate cycle. During seed germination photosynthesis is not operating so role of glyoxylate cycle is crucial.  The glyoxylate bypass allows plant seeds to stock up energy and carbon sources from fat. This stored fat is used for the generation of glucose during the germination.
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  GLYOXYLATE CYCLE  Inplant cells some organelles show morphological similarities to the peroxisomes but different from the peroxisomes enzymes.  In 1961 the scientist Beevers and Breidenbach demonstrated that enzymes for glyoxylate cycle were found in a separate organelle to convert lipid into glucose, which they named as glyoxysomes.  The glyoxylate cycle allows fungi and higher plants to convert stored fat into carbohydrates.  The glyoxysome are ovoid granules limited by single layered lipid bilayer. It consist amorphous crystalline matrix.  Matrix consist enzymes similar to peroxisomes, in addition of some enzymes required for glyoxylate cycle also present in core matrix. Glyoxysomes are synthesized by endoplasmic reticulum.  It is an ovoid shaped looks similar to the peroxisomes having diameter of 0.1 1μm  Glyoxysome mostly found in oil yielding seed.
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  GLYOXYLATE CYCLE
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  GLYOXALATE CYCLE  Thegermination of oil seed is accompanied by the breakdown of large amount of fat stored in endosperm.  During this process special organelle involved in fat metabolism called glyoxysome.  They contain the enzyme required for transformation of fats to carbohydrate.  The stored fat is converted to carbohydrate by way of glyoxylate cycle,  For glyoxylate cycle different types of enzymes are required namely isocitratelyase, malate synthatase, citrate synthatase, aconitase and malate dehydrogenase which present in glyoxysome matrix.  Fat storing bodies present in the cytoplasm are transported in glyoxysome.  In the matrix of glyoxysome fats undergoes β oxidation and produce acetyl CoA which runs the glyoxylate cycle.  Citrate converts into isocitrate on the acting of enzyme aconitase. Then the isocitrate lyase acting on isocitrate produce glyoxylate and succinate. Glyoxylate continue the glyoxylate cycle and succinate translocate in mitochondria, runs the TCA/ Krebs/ Citric acid cycle.  TCA cycle releases malate into cytoplasm and converted into OAA.  OAA forms the PEP which undergoes in reverse glycolysis to produce sugar molecule.
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  GLYOXYLATE CYCLE  Theglyoxylate cycle is a special variant of the tricarboxylic cycle (TCA) that allows utilization of two carbons compounds in the absence of glucose. The glyoxylate cycle is generally not present in human and animal tissue, and can only be found in plants, bacteria, fungi and protists.  As a shunt in the TCA cycle, the glyoxylate cycle shares three out of five metabolic enzymes with the cycle: malate dehydrogenase(EC 1.1.1.37), citrate synthase (EC 2.3.3.1) and aconitase (EC 4.2.1.3), by-passing the two decarboxylation steps catalyzed by isocitrate dehydrogenase (EC 1.1.1.41) and α-ketoglutarate dehydrogenase complex (EC 1.2.4.2, EC 2.3.1.61, EC 1.8.1.4).  Instead of converting isocitrate to α-ketoglutarate in TCA cycle, the glyoxylate cycle enzyme isocitrate lyase (EC 4.1.3.1) catalyzes the conversion of isocitrate (C6) into glyoxylate (C2) and succinate (C4). Subsequently, malate synthase (EC 2.3.3.9) catalyzes the condensation of glyoxylate with acetyl-CoA (C2) to produce malate (C4) and a free CoA molecule.  In this way, malate can be further oxidized into oxaloacetate, an important precursor for gluconeogenic biosynthesis of glucose and other sugars.
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  GLYOXYLATE CYCLE  Alternatively,malate that is generated from the alternative carbon utilization (e.g., fatty acids, ethanol and acetate, via acetyl-CoA) also replenishes the energy-producing TCA cycle.  In short, the glyoxylate cycle intermediate serve as a linkage between anabolic and catabolic fungal metabolism in the midst of glucose deprivation and enables assimilation of alternative carbon sources.  SITES OF OCCURRENCE:  Glyoxysomes and Mitochondrial Matrix- both are directly involved in this process.  STEPS TO BE FOLLOWED: A pathway related to the Citric Acid Cycle (CAC) is the glyoxylate pathway . This pathway, which overlaps all of the non-decarboxylation reactions of the CAC does not operate in animals, because they lack two enzymes necessary for the pathway – isocitrate lyase and malate synthase. Isocitrate lyase catalyzes the conversion of isocitrate into succinate and glyoxylate. Because of this, all six carbons of the CAC survive and do not end up as carbon dioxide.
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  GLYOXYLATE CYCLE
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  GLYOXYLATE CYCLE  Reactions1 and 2 -Glyoxysomal oxaloacetate is condensed with acetyl CoA to form citrate, which is isomerized to isocitrate as in the citric acid cycle. Since the glyoxysome contains no aconitase,  Reaction 2- presumably takes place in the cytosol.  Reaction 3- Glyoxysomal isocitrate lyase cleaves the isocitrate to succinate and glyoxylate (hence the cycle’s name).Isocyterate bypass the citric acid cycle,  Reaction -4-Malate synthase, a glyoxysomal enzyme, condenses glyoxylate with a second molecule of acetyl-CoA to form malate.  Malate enters into the Krebs’ cycle,  The cyclic pathway results in the conversion of 2 carbon fragments of Acetyl CoA to 4 carbon compound, succinate,  Succinate converts into malate and the malate in now follow the reverse pathway of glycolysis to synthesize glucose,  The glucose is now being utilized as respiratory pathway; a little bit of anaplerotic behavior is found to happen here in the quest for big molecule, glucose.
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  GLYOXYLATE CYCLE  Malateundergoes dehydrogenation in presence of malate dehyrogenase enzyme and form Oxaloacetate,  Oxaloacetate in presence of PEP carboxykinase produces phosphoenolpyruvate,  Phosphoenolpyruvate then transforms into Fructose6P , then glucose 6P and then glucose.  All the reaction from malate to glucose takes place in the cytosol.
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  THANKS TO VISITTHE CONTENT  References: i. Google for images, ii. Different open sources of information of WebPages, iii. Biochemistry- Lehninger, iv. Biomolecules & Cell Biology- Arun chandra Sahu, v. Fundamentals of Biochemistry- Jain, Jain, & Jain, vi. A Textbook of Microbiology- Chakraborty.  DISCLAIMER:  This presentation has been made to enrich open source of knowledge without any financial interest. The presenter acknowledges Google for images and other open sources of information to develop this PPT.