The amino group of aspartate attacks carbamoyl phosphate, yielding N-carbamoyl aspartate and Pi in a reaction catalyzed by aspartate transcarbamoylase (Figure 14-4). Carbamoyl phosphate possesses an energy-rich bond; there are no high energy bonds in the products. The reaction is exergonic and unidirectional. Next, dihydroorotase mediates a cyclization with the elimination of water, yielding dihydroorotate. This reaction is isoergonic.