Oxaloacetic acid

Oxaloacetic acid is not only important as the last step in the citric acid cycle and thus the substance required of acetate (as acetyl CoA) to enter the cycle. It too, is a major jump off material used to form many others. It forms aspartate  which in turn forms asparagine, lysine, urea, nucleotides, homoserine, methionine, threonine, and isoleucine. That a big bite out of the list of amino acids and key elements in important synthesis cycles important to metabolism.