Glyoxylate Cycle

glyoxylate cycle: see Krebs cycle.

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glyoxylate cycle [glī′äk·sə‚lāt ‚sī·kəl]

(biochemistry)

A sequence of biochemical reactions related to respiration in germinating fatty seeds by which acetyl coenzyme A is converted to succinic acid and then to hexose.

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Glyoxylate Cycle 

a sequence of biochemical transformations of acetic acid in which glyoxylic acid (CHOCOOH) is an intermediate product. It is a variation of the Krebs cycle. It is observed in microorganisms growing in a medium that contains acetic acid as the sole carbon source, as well as in molds and in some plants.

The glyoxylate cycle starts with the condensation of oxalic acid with acetyl coenzyme A (acetyl-CoA) to give citric acid, which by way of cis-aconitic acid is transformed into isocitric acid. The latter decomposes into succinic and glyoxylic acids, which then converts to malic acid by condensation with another molecule of acetyl-CoA. These two reactions are catalyzed by the enzymes isocitratase and malate synthetase, which are characteristic of the glyoxylate cycle. As in the Krebs cycle, malic acid is converted into oxaloacetic acid. (See Figure 1.)

Figure 1. The glyoxylate cycle

The glyoxylate cycle may be regarded as a mechanism for the regeneration of Krebs-cycle intermediates. In the higher plants, malate synthetase and especially isocitratase are present in tissues that actively degrade fats. During the germination of the seeds of oleaginous plants, the glyoxylate cycle is responsible for the transformation of fats into carbohydrates. The occurrence of the glyoxylate cycle in animal tissues is still open to question. The formation of glyoxylic acid in animals occurs during the deamination of glycine in the presence of glycine oxidase.

REFERENCES

Biokhimiia rastenii. Edited by V. L. Kretovich. Moscow, 1968. (Translated from English.)
Kretovich, V. L. Osnovy biokhimii rastenii, 5th ed. Moscow, 1971.

G. A. SOLOV’EVA