Glyoxylate cycle malate synthetase

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.

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). 

Fig. 73. The glyoxylate cycle. The key enzymes are encircled I = Isocitratase Isocitrate lyase, 2 = malate synthetase.

We have said that the glyoxylate cycle is /orw Z/y a variant of the citric acid cycle. This statement needs further underpinning Beevers was able to show that both of the key enzymes, isocitratase and malate synthetase, are localized in special organelles, the glyoxysomes, which can be distinguished from the mitochondria. Here again we come across the phenomenon of compartmentalization —this time between such closely related processes as the citric acid cycle and the glyoxylate cycle. 

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