Glyoxylate cycles

Glyoxylate cycle A modification of the Krebs cycle, which occurs in some bacteria. Acetyl coenzyme A is generated directly from oxidation of fatty acids or other lipid compounds. 

FIGURE 20.28 The glyoxylate cycle. The first two steps are identical to TCA cycle reactions. The third step bypasses the C09-evolving steps of the TCA cycle to produce snc-cinate and glyoxylate. The malate synthase reaction forms malate from glyoxylate and another acetyl-CoA. The result is that one torn of the cycle consumes one oxaloacetate and two acetyl-CoA molecnles bnt produces two molecnles of oxaloacetate. The net for this cycle is one oxaloacetate from two acetyl-CoA molecnles. 

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 Glyoxylate Cycle Helps Plants Grow in the Dark... 

Glyoxysomes do not contain all the enzymes needed to run the glyoxylate cycle succinate dehydrogenase, fumarase, and malate dehydrogenase are absent. Consequently, glyoxysomes must cooperate with mitochondria to run their cycle (Figure 20.31). Succinate travels from the glyoxysomes to the mitochondria, where it is converted to oxaloacetate. Transamination to aspartate follows... 

Barrett, J., Ward, C.W. and Fairbairn, D. (1970) The glyoxylate cycle and the conversion of triglycerides to carbohydrates in developing eggs of Ascaris lumbricoides. Comparative Biochemistry and Physiology 35, 577-586. 

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... 

The Glyoxylate Cycle Produces Four-Carbon Compounds from Acetate... 

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 ... 

The Citric Acid and Glyoxylate Cycles Are Coordinately Regulated... 

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 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. 

In the glyoxylate cycle, the bypassing of the two decarboxylation steps of the citric acid... 

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