Glyoxylate pathway

The enzymes of the glyoxylate cycle in plants are contained in glyoxysomes, organelles devoted to this cycle. Yeast and algae carry out the glyoxylate cycle in the cytoplasm. The enzymes common to both the TCA and glyoxylate pathways exist as isozymes, with spatially and functionally distinct enzymes operating independently in the two cycles. 

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. 

Peroxisomes are rich in enzymes that produce and decompose hydrogen peroxide. They often make a major contribution to the oxidative metabolism of cells. In germinating oilseeds glyoxysomes, a type of peroxisome, contain enzymes that catalyze reactions of the biosynthetic "glyoxylate pathway" of metabolism.51 Organelles that resemble peroxisomes in appearance... 

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... 

Malate is formed in the glyoxylate pathway by reaction of glyoxylate with Acetyl-CoA. Indicate the chemical mechanism of the reactions involved and structure of an intermediate species. 

Figure 17-11 Some major biosynthetic pathways. Some key intermediates are enclosed in boxes and the 20 common amino acids of proteins are encircled. Key intermediates for each family are in shaded boxes or elipses. Green lines trace the reactions of the glyoxylate pathway and of glucogenesis.

Figure 17-16 The glyoxylate pathway. The green line traces the pathway of labeled carbon from fatty acids or acetyl-CoA into malate and other products.

The FADH2 and NADH produced feed directly into oxidative phosphorylation, while the acetyl CoA feeds into the citric acid cycle where further FADH2 and NADH are produced. In animals the acetyl CoA produced in 13-oxidation cannot be converted into pyruvate or oxaloacetate, and cannot therefore be used to make glucose. However, in plants two additional enzymes allow acetyl CoA to be converted into oxaloacetate via the glyoxylate pathway. 

Topic LI). Thus, animals cannot convert fatty acids into glucose. In contrast, plants have two additional enzymes, isocitrate lyase and malate synthase, that enable them to convert the carbon atoms of acetyl CoA into oxaloacetate. This is accomplished via the glyoxylate pathway, a route involving enzymes of both the mitochondrion and the glyoxysome, a specialized membranous plant organelle. 

Fig. 12.5. The two main processes identified that lead to the precipitation of calcium carbonate from an oxalate source in oxalotrophic bacteria, the formate and the glyoxylate pathways. TCA, tricarboxylic acid cycle GA, glyoxyhc acid cycle EPS, exopolysaccharides 1, oxalate decarboxylation into formate 2, formate dehydrogenation permease/transporters.

Figure 17.21. The Glyoxylate Pathway. The glyoxylate cycle allows plants and some microorganisms to grow on acetate because the cycle bypasses the decarboxylation steps of the citric acid cycle. The enzymes that permit the...

The Glyoxylate Pathways Biosynthesis of Glucose from Three-Carbon Compounds... 

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