21] Carbamoyl phosphate synthesis: Carbamate kinase from Pyrococcus furiosus

Abstract

Carbamoyl phosphate (CP) is a precursor of both arginine and the pyrimidines in their de novo biosynthetic pathways. This molecule is highly thermolabile and its thermal decomposition at neutral pH yields cyanate, a nondiscriminate carbamoylating agent. The metabolism of carbamoyl phosphate in hyperthermophilic organisms is therefore of interest. Both carbamate kinase (CK) and carbamoyl-phosphate synthase (CPS)can synthesize CP from mixtures of ATP, bicarbonate, and ammonia. The reaction catalyzed by CK is reversible, although the equilibrium favors ATP synthesis. The carbamate kinase described in this article is remarkable by a number of intrinsic properties that are in keeping with the anabolic role it appears to play in vivo in a hyperthermophilic organism: a relatively high affinity for carbamate, a high efficiency in the synthesis of CP as compared to the enteroccocal homolog, and a high thermostability. Moreover, it appears to be engaged in CP channeling with ornithine and aspartate carbamoyltransferases. The CP-synthesizing activity reported in P. abyssi is probably also because of a true carbamate kinase adapted to anabolism. However, channeling of CP between classical CPS and carbamoyltransferases specific for ornithine and aspartate has been reported in the extreme thermophilic bacterium Thermus ZO5.