Frederic L. Holmes, “Hans Krebs”, 2 volumes, 1991 has in the first volume the Ornithine-Urea cycle which Hans Krebs discovered. Hans Krebs was an apprentice to Otto Warburg, in studying cellular respiration. CO2, carbon dioxide combined with NH3 ammonia gives urea NH2-CO-NH2. Hans Krebs discovered the Urea Cycle in 1933. On the Roche Biochemical webpage the Urea Cycle is location “G” and coordinate “7”, and is shown below.