Ultimate explanation – Biological glyoxylate cycle

The glyoxylate cycle is the process of succinic acid, glyoxylic acid and malic acid formed in the glycoxysome after the oxidative decomposition of fatty acids in plant cells into acetyl CoA; this succinic acid can be used in the synthesis of sugars. There are no glyoxylates in animals and human cells, and it is impossible to convert fat into sugar. Plants and microorganisms have glyoxylic acid. Oil plant seeds (peanuts, canola, cottonseed, etc.) have a glyoxylate cycle capable of converting fat into sugar upon germination. Two key enzymes in the glyoxylate cycle, isocitrate lyase, and malate synthase, were also isolated from the rice scutellum.

Biological glyoxylate cycle ultimate reaction equation

2 acetyl-CoA+NAD++2H2O→succinic acid+2Coenzyme A+NADH+H+


Biological glyoxylate cycle reaction process description

The fatty acid is decomposed into acetyl CoA by β-oxidation. Under the action of citrate synthase, acetyl CoA and oxaloacetate are condensed into citric acid, and then aconitate is catalyzed by aconitase. Subsequently, isocitratelyase decomposes isocitrate into succinic acid and glyoxylic acid. Under the catalysis of malate synthetase, glyoxylic acid combines with acetyl CoA to form malic acid. Dehydrogenation of malate re-forms oxaloacetate, which can be condensed with acetyl CoA to form citric acid, thus forming a cycle.

The overall result is the formation of one molecule of succinic acid from two molecules of acetyl CoA. The reaction equation is as follows:
2 acetyl CoA+NAD+→succinic acid+2CoA+NADH+H+

Succinic acid is transferred from glyoxylic acid to mitochondria, where it is converted to fumaric acid, malic acid by a partial reaction of the tricarboxylic acid cycle, and oxaloacetate is formed. Then, oxaloacetate continues to enter the TCA cycle or is transferred to the cytoplasm, decarboxylated under the catalysis of PEP carboxykinase to form phosphoenolpyruvate (PEP), which is then converted to glucose 6-phosphate and sucrose by reversal of glycolysis.

During the germination of oilseeds, many glyoxylates appear in the cells. The stored fat is first hydrolyzed to glycerol and fatty acids, and then the fatty acids are oxidatively decomposed into acetyl CoA in glyoxylic acid and converted into sugars through the glyoxylate cycle until the seeds. When the stored fat is exhausted, the glyoxylate cycle activity disappears. Glyoxylate cycle does not occur when starch seeds are germinated. It can be seen that the glyoxylate cycle is a respiratory metabolic pathway unique to fat-rich oilseeds.

Later, in the process of studying the transformation of fat→sugar during the germination of castor seeds, the above-mentioned glyoxylate cycle was modified. First, the malic acid formed by the combination of glyoxylic acid and acetyl CoA does not dehydrogenate, but directly enters the cytoplasm and transforms into sucrose against the glycolysis pathway. Second, there is a “malic acid shuttle” between glyoxylic acid and mitochondria.
Through the “malic acid shuttle” and transamination reaction, the regeneration of NAD+ in glyoxylic acid and the continuous replenishment of OAA are solved, which is essential for ensuring the normal operation of GAC.

The physiological significance of biological glyoxylate cycle

1. The glyoxylate cycle realizes the conversion of fat to sugar and plays an important role in the growth and development of plants.
During the germination period of oil crop seeds, the glyoxylate cycle is very active. During this period, the lipids stored in the seeds form sugar through acetyl-CoA, which supplies the energy and carbon frame needed for the growth point in time to promote germination and growth.

2. The glyoxylate cycle increases the ability of organisms to utilize acetyl-CoA. As long as a very small amount of oxaloacetate is used as a primer, the glyoxylate cycle can continue to operate, continuously producing succinic acid, and replenishing the four carbon units for TCA.


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