Glyoxylate cycle intermediates

D-Isocitrate is a citric acid cycle (and glyoxylate cycle) intermediate produced as a result of action of the enzyme aconitase on citrate. Isocitrate is converted to... 

See also Isocitrate Lyase, Glyoxylate Cycle, Glyoxylate Cycle Intermediates, Figure 14.20, Malate Synthase... 

See also Glyoxylate Cycle Intermediates, Glyoxylate Cycle Reactions, Figure 14.20, Citric Acid Cycle Pathway... 

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 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. 

Eastmond, P.J. Graham, I. A. (2001) Re-examining the role of the glyoxylate cycle in oilseeds. Trends Plant Sci. 6, 72-77. Intermediate-level review of studies of the glyoxylate cycle in Arabidopsis. 

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. 

OAA is an intermediate in several important pathways, including gluconeogenesis, citric acid cycle, glyoxylate cycle, urea cycle, and amino acid metabolism (see here). 

Plants and some bacteria contain two enzymes (isocitrate lyase and malate synthase) that enable them to synthesize sugars by using the glyoxylate cycle, a variant form of the citric acid cycle. Notice in Figure 14.20 that the glyoxylate cycle uses some of the same enzymes as the citric acid cycle, but that the steps in which decarboxylations occur are bypassed. One of the intermediates in the bypass is glyoxylate, which gives the cycle its name. 

See also Glyoxylate Cycle, Aconitase, Citrate, Isocitrate Dehydrogenase, ct-Ketoglutarate, Fluorocitrate, Citric Acid Cycle Intermediates, Figure 14.3, Table 14.1... 

Succinate is an intermediate of the citric acid cycle (and the glyoxylate cycle) produced by action of the enzyme succinyl-CoA synthetase on succinyl-CoA. Succinate is converted to fumarate by action of the enzyme succinate dehydrogenase (with formation of FADH2)... 

L-malate is an intermediate in the citric acid cycle, urea cycle, amino acid metabolism, the glyoxylate cycle, and shuttles across membranes of the cell (Figure 18.31). 

The citric acid cycle is a source of starting materials for the biosynthesis of many important biomolecules, but the supply of the starting materials that are components of the cycle must be replenished if the cycle is to continue operating. See the Biochemical Connections box on page 569. In particular, the oxaloacetate in an organism must be maintained at a level sufficient to allow acetyl-CoA to enter the cycle. A reaction that replenishes a citric acid cycle intermediate is called an anaplerotic reaction. In some organisms, acetyl-CoA can be converted to oxaloacetate and other citric acid cycle intermediates by the glyoxylate cycle (Section 19.6), but mammals cannot do this. In mammals. 

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