Glyoxylic acid cycle

Using the so-called glyoxylic acid cycle, plants and bacteria are able to convert acetyl-CoA into succinate, which then enters the tricarboxylic acid cycle. For these organisms, fat degradation therefore functions as an anaplerotic process. In plants, this pathway is located in special organelles, the glyoxysomes. 

Secondary products derived from intermediates of the tricarboxylic and glyoxylic acid cycles (D 5)... 

Secondary products derived from the acids of the tricarboxylic acid, and glyoxylic acid cycles are built in microorganisms, plants, and animals. 

The simplest 2-oxocarboxyHc acid is glyoxyhc acid (8-68), which occurs in the immature fruits of many plants. Glyoxylic acid is produced as the main intermediate of the glyoxylic acid cycle (ongoing in plants and microorganisms) and is also formed by other biochemical reactions, such as degradation of purine bases. 

Foods also contain various oxodicarboxylic and oxotricarboxylic acids that arise predominantly in the citric acid or glyoxylic acid cycles. A common acid is oxaloacetic (2-oxosuccinic) acid (8-68). 

Rotte C, Stejskal F, Zhu G, Keithly JS, Martin W (2001) Pyruvate NADP+ oxidoreductase from the mitochondrion of Euglena gracilis and from the apicomplexan Cryptosporidium parvum a biochemical relic linking pyruvate metabolism in mitochondriate and amitochondriate protists. Mol Biol Evol 18 710-720 Schnarrenberger C, Martin W (2002) Evolution of the enzymes of the citric acid cycle and the glyoxylate cycle of higher plants. A case study of endosymbiotic gene transfer. Eur J Biochem 269 868-883... 

In plants, certain invertebrates, and some microorganisms (including E. coli and yeast) acetate can serve both as an energy-rich fuel and as a source of phosphoenolpyruvate for carbohydrate synthesis. In these organisms, enzymes of the glyoxylate cycle catalyze the net conversion of acetate to succinate or other four-carbon intermediates of the citric acid cycle ... 

Some bacteria, including E. coli, have the full complement of enzymes for the glyoxylate and citric acid cycles in the cytosol and can therefore grow on acetate as their sole source of carbon and energy. The phosphoprotein phosphatase that activates isocitrate dehydrogenase is stimulated by intermediates of the citric acid cycle and glycolysis and by indicators of reduced cellular energy supply (Fig. 16-23). The same metabolites inhibit the protein kinase activity of the bifunctional polypeptide. Thus, the accumulation of intermediates of... 

FIGURE 16-22 Relationship between the glyoxylate and citric acid cycles. The reactions of the glyoxylate cycle (in glyoxysomes) proceed simultaneously with, and mesh with, those of the citric acid cycle (in mitochondria), as intermediates pass between these compartments. The conversion of succinate to oxaloacetate is catalyzed by citric acid cycle enzymes. The oxidation of fatty acids to acetyl-CoA is described in Chapter 17 the synthesis of hexoses from oxaloacetate is described in Chapter 20. 

The same intermediates of glycolysis and the citric acid cycle that activate isocitrate dehydrogenase are allosteric inhibitors of isocitrate lyase. When energy-yielding metabolism is sufficiently fast to keep the concentrations of glycolytic and citric acid cycle intermediates low, isocitrate dehydrogenase is inactivated, the inhibition of isocitrate lyase is relieved, and isocitrate flows into the glyoxylate pathway, to be used in the biosynthesis of carbohydrates, amino acids, and other cellular components. 

The glyoxylate cycle is active in the germinating seeds of some plants and in certain microorganisms that can live on acetate as the sole carbon source. In plants, the pathway takes place in glyoxysomes in seedlings. It involves several citric acid cycle enzymes and two additional enzymes isocitrate lyase and malate synthase. 

The partitioning of isocitrate between the citric acid cycle and the glyoxylate cycle is controlled at the level of isocitrate dehydrogenase, which is regulated by reversible phosphoiylation. 

W. H. Holms (Control of Flux through the Citric Acid Cycle and the Glyoxylate Bypass in Escherichia coli), and R. N. Perham et al. (a-Keto Acid Dehydrogenase Complexes). 

Some bacteria use a "dicarboxylic acid cycle" to oxidize glyoxylate OHC-COO to C02. The regenerating substrate for this cycle is acetyl-CoA. It is synthesized from glyoxylate by a complex pathway that begins with conversion of two molecules of glyoxylate to tartronic semialdehyde ... 

E. coli enzyme,107 108 causing isocitrate to build up in the citric acid cycle (Fig. 10-6) and to be diverted into the glyoxylate pathway, which is depicted in Fig. 17-16. In this instance, it appears likely that the negative charge of the added phospho group causes electrostatic repulsion of the substrate isocitrate. In agreement with this concept, mutation of Ser 113 of this enzyme to Asp (mutant S113 D) also inactivates the enzyme.109... 

Figure 17-6 The dicarboxylic acid cycle for oxidation of glyoxylate to carbon dioxide. The pathway for synthesis of the regenerating substrate Carbohydrate synthesis...

An acetyl-CoA-glyoxylate cycle, which catalyzes oxidation of acetyl groups to glyoxylate, can also be constructed from isocitrate lyase and citric acid cycle... 

As mentioned in Section 4, glyoxylate can be converted to oxaloacetate by condensation with acetyl-CoA (Fig. 17-16) and the oxaloacetate can be decarboxylated to pyruvate. This sequence of reactions resembles that of the conversion of oxaloacetate to 2-oxoglutarate in the citric acid cycle (Fig. 17-4). Doth... 

Certain microorganisms have a modification of this cycle in which isocitric acid is cleaved to succinic acid and glyoxylic acid. The latter acid is condensed with acetyl-CoA to form malic acid. In this modification (the glvoxvlic acid cvcle), oxalsuccinic acid and alpha-ketoglularic acid are not involved. This is sometimes referred to as the glyoxylate shunt pathway. 

Summarize in the simplest words the portion of the tricarboxylic acid cycle that is bypassed by the glyoxylate cycle ... 

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