Carbamoyl phosphate.

When amino acids are used for energy, their carbon skeletons are transformed by a variety of pathways into acetyl CoA or intermediates of glycolysis or the citric acid cycle. While the degradative pathways of some amino acids are complex and beyond the scope of the MCAT, the simpler transaminase pathways are within the scope of study. It is very good to have a general sense of this subject even though you don't need to memorize all six steps in the breakdown of leucine.

One very important topic is the fate of α-amine group when the breakdown of an amino acid occurs. As a general rule these amine groups will be converted into urea and then excreted. The pathway by which this occurs is called the urea cycle. The urea cycle takes place primarily in the liver, and to a lesser extent in the kidney. It was the first metabolic cycle discovered (Hans Krebs and Kurt Henseleit, 1932), five years before the discovery of the citric acid cycle.