Glyoxylate cycle

The glyoxylate cycle is a metabolic pathway occurring in plants and several microorganisms.

The glyoxylate cycle allows these organisms to use acetyl CoAs for the synthesis of carbohydrates, a task which vertebrates, including humans, cannot perform.

When fatty acids are consumed by vertebrates they are degraded to many copies of a small 2-carbon-transporting compound: acetyl CoA, which enters the citric acid cycle, where it is fully oxidized to carbon dioxide, which is released into the environment. This pathway allows the cell to obtain energy from fatty acids.

The two initial stages of this cycle are identical to those of the citric acid cycle: acetate → citrate → isocitrate. The next step, however, is different: isocitrate is cleaved into succinate and glyoxylate (the latter gives the cycle its name). Succinate can be channeled directly into the citric acid cycle and eventually form oxaloacetate. Glyoxylate condenses with acetyl-CoA, yielding malate. Both malate and oxaloacetate can be converted into phosphoenolpyruvate and enter gluconeogenesis. The net result of the glyoxylate cycle is therefore the production of glucose from acetyl CoA.

In plants the glyoxylate cycle occurs in special peroxisomes which are called glyoxysomes. Vertebrates cannot perform the cycle because they lack its two key enzymes: isocitrate lyase and malate synthase.