Glyoxylate cycle aconitase

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

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 Citric Acid Cycle, Glyoxylate Cycle Reactions, Stereospecificity of Aconitase, Enzymes of the Citric Acid Cycle, Table 14.1... 

Fig. 2. (A) The glyoxylate cycle as a bypass of the TCA cycle (after Komberg and Krebs, 1957). (B) The glyoxylate cycle as it functions in the glyoxysome, showing the production of succinate from 2 mol of acetyl-CoA. The five steps constituting the cycle are catalyzed by the following enzymes (1) citrate synthetase, (2) aconitase, (3) isocitrate lyase, (4) malate synthetase, (5) malate dehydrogenase.

IsocHricacid HOOC-CH2-CH(COOH)-CHOH-COOH, a monohydroxy tricarboxylic acid, an isomer of citric acid, which is widely distributed in the plant kingdom and occurs in free form especially in plants of the stone-crop family (Crassulaceae), and in fruits. The salts of I. a., isocitrates, are important metaboli-cally as intermediates in the Tricarboxylic acid cycle (see), where they are formed from citrate by the enzyme aconitase, then oxidized to 2-oxoglutarate. In the Glyoxylate cycle (see), isocitrate is cleaved to succinate and glyoxylate. 

Aconitic acid involved in the TCA and glyoxylate cycles and the acid commonly occurring in nature has the c/5-configuration. The trans- om x has also been isolated from some plant materials - for example, sugarcane Saccharum offi-cinarum) juice (17), tomato (Lycopersicon esculentum) (56) or moss (Bryophyta) (34). However, some of the occurrences might be artifacts of the isolation procedures, for an interconversion between two isomers of aconitic acid has been reported (14, 81). In both cycles, cw-aconitic acid is formed upon dehydration of citric acid catalyzed by aconitase (aconitate hydratase) which also catalyzes the rehydration of cw-aconitate to isocitric acid. 

Fig. 9. Pathway duplication the methyl citrate cycle and the glyoxylate shunt. A pathway for acetate metabolism in E. coli that uses the glyoxylate shunt is depicted on the right. Part of the methyl citrate cycle, a pathway for propionate metabolism, is depicted on the left. The pathways are analogous furthermore, three of the four steps are catalyzed by homologous enzymes. PrpE (propionyl-CoA synthase) is homologous to AcsA (acetyl-CoA synthase). PrpC (2-methyl-citrate synthase) is homologous to GltA (citrate synthase). PrpB (2-methyl-isocitrate lyase) is homologous to AceA (isocitrate lyase). The third step in the methyl citrate cycle has been suggested to be catalyzed by PrpD the second half of the reaction (the hydration) can be catalyzed by aconitase.

From the fact that the glyoxylate and TCA cycles have several enzymes (citrate synthetase, aconitase, malate dehydrogenase) in common it appeared axiomatic that they must operate together in the same intracellular compartment, the mitochondrion. Early experiments with crude particulate pellets containing mitochondria gave only partial support to this view since, although malate synthetase was present in such preparations, most of the isocitrate lyase was present in the supernatant fraction (Yamamoto and Beevers, 1961 Marcus and Velasco, 1960). 

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