Glyoxylic acid or oxoacetic acid is an organic compound. It is a colourless solid that occurs naturally and is useful industrially. The compound is formed by organic oxidation of glyoxal with hot nitric acid, the main side product being oxalic acid. Ozonolysis of maleic acid is also effective. The glyoxylate cycle is an anabolic metabolic pathway occurring in plants, and several microorganisms, such as E. coli and yeast. It has long been thought to be absent in most animals, and the glyoxylate cycle allows these organisms to use fats for the synthesis of carbohydrates.
In these organisms, in the absence of available carbohydrates, the glyoxylate cycle permits the synthesis of glucose from lipids via acetate generated in fatty acid β-oxidation. The glyoxylate cycle bypasses the steps in the citric acid cycle where carbon is lost in the form of CO₂. In the next step, catalyzed by the first glyoxylate cycle enzyme, isocitrate lyase, isocitrate undergoes cleavage into succinate and glyoxylate. Glyoxylate condenses with acetyl-CoA, yielding malate. Both malate and oxaloacetate can be converted into phosphoenolpyruvate, which is the substrate of phosphoenolpyruvate carboxykinase, the first enzyme in gluconeogenesis. The net result of the glyoxylate cycle is therefore the production of