1B7B

Carbamoyl phosphate (CP) can be synthesised from mixtures of ATP, bicarbonate and ammonia by two types of enzymes: carbamoyl phosphate synthases (CPS) and carbamate kinases (CK). CPS uses a three step mechanism and consumes two molecule of ATP per molecule of CP synthesised; this reaction is essentially irreversible and is the first committed step in the biosynthesis of pyrimidines, arginine and urea. By contrast, carbamate kinases (CK) make CP reversibly in a one-step reaction using one molecule of ATP per CP molecule synthesised. The actual substrate for these enzymes is carbamate which is in equilibrium with bicarbonate and ammonia. The in vivo role of CK from many organisms in which it has been studied (such as Enterococcus faecalis) seems to be to generate ATP from ADP using carbamoyl phosphate derived from the catabolism of arginine. However in some species, such as the hypothermophilic archaea Pyrococcus furiosus and Pyrococcus abyssi the enzyme appears to have an anabolic role. The enzyme from these species has in fact been called a \"carbamate kinase-like carbamoyl phosphate synthetase\", although studies have shown that is structurally and enzymologically a carbamate kinase.