Oxaloacetate

Oxaloacetate is a key component in the citric acid cycle, gluconeogenesis and in the biosynthesis of other biological molecules, such as asparate and other amino acids, purines and pyrimidines.  

In the citric acid cycle, oxaloacetate (a 4 carbon molecule) is combined with acetyl CoA (a 2 carbon molecule) to generate the 6 carbon molecule citric acid.  Acetyl CoA is generated from the oxidation of pyruvate (which also generates C0₂₎, and from the oxidation of fatty acids.  In the eigth and last step of the citric acid cycle, oxaloacetate is regenerated from malate by malate dehydrogenase so it can once again combine with acetyl CoA and keep the cycle going.