urea cycle

Biochemical process discovered by German-born British biochemist Hans Krebs and German physician Kurt Henseleit in 1932, by which nitrogenous waste is converted into urea, which is easily excreted. When proteins and amino acids break down, ammonia, which is highly toxic, is formed. Most of the ammonia is converted into glutamate, and becomes usable for the synthesis of more amino acids and proteins. Any excess is converted into the water-soluble compound urea, which can be excreted as urine. Free ammonia, carbon dioxide, and ATP react to form carbamyl phosphate. This compound then forms citrulline by reacting with the amino acid ornithine, which can then accept another amino group giving arginine. Arginine can then break down, giving urea, for excretion, and ornithine, which can take part in the cycle again.