Since it is the rate-limiting step, CPS I is essentially the urea cycle's pacemaker enzyme. CPS catalyses the synthesis of carbamoyl phosphate (CP) from ATP and bicarbonate, depending on the type of enzyme, using glutamine or ammonia as a nitrogen source. Thus, eukaryotes have three CPS isoforms that are used to generate CP for use in various metabolic pathways. Prokaryotes, on the other hand, have a single CPS, classified as Type II, for which glutamine is the sole nitrogen source.