Glyoxylate cycle enzymes

See also Glyoxylate Cycle, Glyoxylate Cycle Enzymes, Figure 14.20... 

Kondrashov EA, Koonin EV, MOTgunov IG, Einogenova TV, Kondrashova MN. Evolution of glyoxylate cycle enzymes in metazoa evidence of multiple horizontal transfer events and pseudogene formation. Biol Diiect. 2006 1 36. doi 10.1186/1745-6150-131. 

McLaughlin, J.C., Smith, S.M. (1994) Metabolic regulation of glyoxylate-cycle enzyme synthesis in detached cucumber cotyledons and protoplasts. Planta 195, 22-28 Turpin, D.H., Weger, H.G. (1990) Interactions between photosynthesis, respiration and nitrogen assimilation. 

Authentication of the overall reaction scheme shown in Fig. 4.11 comes from a variety of sources, notably experiments demonstrating correlations between the activities of the glyoxylate cycle enzymes and most convincingly in experiments in which the labelling pattern in sucrose synthesized by castor bean endosperm from specifically labelled acetate is as predicted by the scheme. 

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. 

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... 

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 ... 

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. 

A key enzyme in the glyoxylate cycle is isocitrate lyase, which cleaves isocitrate (Eq. 13-40) to succinate and glyoxylate. The latter is condensed with a second acetyl group by the action of malate synthase (Eq. 13-38). The L-malate formed in this reaction is dehydrogenated to the regenerating substrate oxalo-... 

Comparison of the glyoxylate cycle with the TCA cycle reveals that two of the five reactions of the glyoxylate cycle tire unique to this cycle, whereas the other three reactions are common to both cycles (fig. 13.13). In plant seedlings and many other eukaryotic organisms that possess this capability, the enzymes of the glyoxylate cycle are compartmentalized in specialized organelles called glyoxysomes. 

Glyoxylate cycle. A pathway that uses acetyl-CoA and two auxiliary enzymes to convert acetate into succinate and carbohydrates. 

Glyoxysome. An organelle containing key enzymes of the glyoxylate cycle. 

The glyoxylate and citric acid cycles have several reactions in common, but what two enzymes are unique to the glyoxylate cycle ... 

The formation of acetyl-CoA from pyruvate in animals is via the pyruvate dehydrogenase complex, which catalyzes the irreversible decarboxylation reaction. Carbohydrate is synthesized from oxaloacetate, which in turn is synthesized from pyruvate via pyruvate carboxylase. Since the pyruvate dehydrogenase reaction is irreversible, acetyl-CoA cannot be converted to pyruvate, and hence animals cannot realize a net gain of carbohydrate from acetyl-CoA. Because plants have a glyoxylate cycle and animals do not, plants synthesize one molecule of succinate and one molecule of malate from two molecules of acetyl-CoA and one of oxaloacetate. The malate is converted to oxaloacetate, which reacts with another molecule of acetyl-CoA and thereby continues the reactions of the glyoxylate cycle. The succinate is also converted to oxaloacetate via the enzymes of the citric acid cycle. Thus, one molecule of oxaloacetate is diverted to carbohydrate synthesis and, therefore, plants are able to achieve net synthesis of carbohydrate from acetyl-CoA. 

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...

Many aroma compounds in fruits and plant materials are derived from lipid metabolism. Fatty acid biosynthesis and degradation and their connections with glycolysis, gluconeogenesis, TCA cycle, glyoxylate cycle and terpene metabolism have been described by Lynen (2) and Stumpf ( ). During fatty acid biosynthesis in the cytoplasm acetyl-CoA is transformed into malonyl-CoA. The de novo synthesis of palmitic acid by palmitoyl-ACP synthetase involves the sequential addition of C2-units by a series of reactions which have been well characterized. Palmitoyl-ACP is transformed into stearoyl-ACP and oleoyl-CoA in chloroplasts and plastides. During B-oxi-dation in mitochondria and microsomes the fatty acids are bound to CoASH. The B-oxidation pathway shows a similar reaction sequence compared to that of de novo synthesis. B-Oxidation and de novo synthesis possess differences in activation, coenzymes, enzymes and the intermediates (SM+)-3-hydroxyacyl-S-CoA (B-oxidation) and (R)-(-)-3-hydroxyacyl-ACP (de novo synthesis). The key enzyme for de novo synthesis (acetyl-CoA carboxylase) is inhibited by palmitoyl-S-CoA and plays an important role in fatty acid metabolism. 

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