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Oxaloacetic acid (also known as oxalacetic acid or OAA) is a crystalline with the HO2CC(O)CH2CO2H. Oxaloacetic acid, in the form of its oxaloacetate, is a metabolic intermediate in many processes that occur in animals. It takes part in , the , the , amino acid synthesis, fatty acid synthesis and the citric acid cycle.


Properties
Oxaloacetic acid undergoes successive to give the :
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 forms of oxaloacetic acid are particularly stable. Keto-enol tautomerization is catalyzed by the enzyme oxaloacetate tautomerase. trans-Enol-oxaloacetate also appears when is the substrate for .


Biosynthesis
Oxaloacetate forms in several ways in nature. A principal route is upon of , 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 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 or de- amination of .


Biochemical functions
Oxaloacetate is an intermediate of the citric acid cycle, where it reacts with to form , catalyzed by . It is also involved in , the , the , amino acid synthesis, and fatty acid synthesis. Oxaloacetate is also a potent inhibitor of .


Gluconeogenesis
is a metabolic pathway consisting of a series of eleven enzyme-catalyzed reactions, resulting in the generation of from non-carbohydrate substrates. The beginning of this process takes place in the mitochondrial matrix, where molecules are found. A pyruvate molecule is carboxylated by a pyruvate carboxylase enzyme, activated by a molecule each of ATP and water. This reaction results in the formation of oxaloacetate. reduces oxaloacetate to . This transformation is needed to transport the molecule out of the . Once in the , malate is oxidized to oxaloacetate again using NAD+. Then oxaloacetate remains in the cytosol, where the rest of reactions will take place. Oxaloacetate is later and by phosphoenolpyruvate carboxykinase and becomes 2-phosphoenolpyruvate using guanosine triphosphate (GTP) as phosphate source. Glucose is obtained after further downstream processing.


Urea cycle
The is a metabolic pathway that results in the formation of using one ammonium molecule from degraded amino acids, another ammonium group from aspartate and one bicarbonate molecule. This route commonly occurs in . The reactions related to the urea cycle produce , and NADH can be produced in two different ways. One of these uses oxaloacetate. In the cytosol there are molecules. Fumarate can be transformed into by the actions of the enzyme . Malate is acted on by malate dehydrogenase to become oxaloacetate, producing a molecule of NADH. After that, oxaloacetate will be recycled to , as prefer these over the others. This recycling maintains the flow of into the cell.


Glyoxylate cycle
The is a variant of the citric acid cycle. It is an pathway occurring in and utilizing the enzymes and . Some intermediate steps of the cycle are slightly different from the citric acid cycle; nevertheless oxaloacetate has the same function in both processes. This means that oxaloacetate in this cycle also acts as the primary reactant and final product. In fact the oxaloacetate is a net product of the because its loop of the cycle incorporates two molecules of acetyl-CoA.


Fatty acid synthesis
In previous stages acetyl-CoA is transferred from the mitochondria to the cytoplasm where fatty acid synthase resides. The acetyl-CoA is transported as a citrate, which has been previously formed in the mitochondrial matrix from acetyl-CoA and oxaloacetate. This reaction usually initiates the citric acid cycle, but when there is no need of energy it is transported to the cytoplasm where it is broken down to cytoplasmic acetyl-CoA and oxaloacetate.

Another part of the cycle requires NADPH for the synthesis of fatty acids. Part of this reducing power is generated when the cytosolic oxaloacetate is returned to the mitochondria as long as the internal mitochondrial layer is non-permeable for oxaloacetate. Firstly the oxaloacetate is reduced to malate using NADH. Then the malate is decarboxylated to pyruvate. Now this pyruvate can easily enter the mitochondria, where it is carboxylated again to oxaloacetate by pyruvate carboxylase. In this way, the transfer of acetyl-CoA that is from the mitochondria into the cytoplasm produces a molecule of NADH. The overall reaction, which is spontaneous, may be summarized as:

HCO3 + ATP + acetyl-CoA → ADP + Pi + malonyl-CoA


Amino acid synthesis
Six essential amino acids and three nonessential are synthesized from oxaloacetate and pyruvate. Aspartate and alanine are formed from oxaloacetate and pyruvate, respectively, by transamination from . Asparagine is synthesized by amidation of aspartate, with glutamine donating the NH4. These are nonessential amino acids, and their simple biosynthetic pathways occur in all organisms. Methionine, threonine, lysine, isoleucine, valine, and leucine are essential amino acids in humans and most vertebrates. Their biosynthetic pathways in bacteria are complex and interconnected.


Oxalate biosynthesis
Oxaloacetate produces oxalate by hydrolysis.Gadd, Geoffrey M. "Fungal production of citric and oxalic acid: importance in metal speciation, physiology and biogeochemical processes" Advances in Microbial Physiology (1999), 41, 47-92.
oxaloacetate + H2O oxalate + acetate
This process is catalyzed by the . This enzyme is seen in plants, but is not known in the animal kingdom.Xu, Hua-Wei. "Oxalate accumulation and regulations is independent of glycolate oxidase in rice leaves" Journal of Experimental Botany, Vol 57, No. 9 pp. 1899-1908, 2006


Interactive pathway map


See also
  • Dioxosuccinic acid
  • Oxidative phosphorylation
  • Citric acid cycle

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