What are essential features of the urea cycle?
A27. The essential features of the urea cycle reactions and their metabolic regulation are as follows: 1. 1. Arginine from the diet or from protein breakdown is cleaved by the cytosolic enzyme arginase, generating urea and ornithine. 2. 2. Ornithine arising in the cytosol is transported to the mitochondrial matrix, where ornithine transcabamoylase catalyzes the condensation of ornithine with carbamoyl phosphate, producing citrulline. The energy for the reaction is provided by the high-energy anhydride of carbamoyl phosphate. 3. 3. The product, citrulline, is then transported to the cytosol, where the remaining reactions of the cycle take place. 4. 4. In a 2-step reaction, catalyzed by cytosolic argininosuccinate synthetase, citrulline is converted to argininosuccinate. The reaction involves the addition of AMP (from ATP) to the amido carbonyl of citrulline, forming an activated intermediate on the enzyme surface (AMP-citrulline), and the subsequent addition of aspartate to form arginin
