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Concept
Oxaloacetic acid
Summary
Oxaloacetic acid (also known as oxalacetic acid or OAA) is a crystalline organic compound with the chemical formula HO2CC(O)CH2CO2H. Oxaloacetic acid, in the form of its conjugate base oxaloacetate, is a metabolic intermediate in many processes that occur in animals. It takes part in gluconeogenesis, the urea cycle, the glyoxylate cycle, amino acid synthesis, fatty acid synthesis and the citric acid cycle.
Oxaloacetic acid undergoes successive deprotonations to give the dianion:
HO2CC(O)CH2CO2H −O2CC(O)CH2CO2H + H+, pKa = 2.22
−O2CC(O)CH2CO2H −O2CC(O)CH2CO2− + H+, pKa = 3.89
At high pH, the enolizable proton is ionized:
−O2CC(O)CH2CO2− −O2CC(O−)CHCO2− + H+, pKa = 13.03
The enol forms of oxaloacetic acid are particularly stable. Keto-enol tautomerization is catalyzed by the enzyme oxaloacetate tautomerase. trans-Enol-oxaloacetate also appears when tartrate is the substrate for fumarase.
Oxaloacetate forms in several ways in nature. A principal route is upon oxidation of L-malate, catalyzed by malate dehydrogenase, in the citric acid cycle. Malate is also oxidized by succinate dehydrogenase in a slow reaction with the initial product being enol-oxaloacetate.
It also arises from the condensation of pyruvate with carbonic acid, driven by the hydrolysis of ATP:
CH3C(O)CO2− + HCO3− + ATP → −O2CCH2C(O)CO2− + ADP + Pi
Occurring in the mesophyll of plants, this process proceeds via phosphoenolpyruvate, catalysed by phosphoenolpyruvate carboxylase. Oxaloacetate can also arise from trans- or de- amination of aspartic acid.
Oxaloacetate is an intermediate of the citric acid cycle, where it reacts with acetyl-CoA to form citrate, catalyzed by citrate synthase. It is also involved in gluconeogenesis, the urea cycle, the glyoxylate cycle, amino acid synthesis, and fatty acid synthesis. Oxaloacetate is also a potent inhibitor of complex II.
Gluconeogenesis is a metabolic pathway consisting of a series of eleven enzyme-catalyzed reactions, resulting in the generation of glucose from non-carbohydrates substrates.
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