Molecular physiology of carbamoylation under extreme conditions: what can we learn from extreme thermophilic microorganisms?

The importance of protein-protein interactions in the physiology of extreme thermophiles was investigated by analyzing the enzymes involved in biosynthetic carbamoylation in Thermus ZO5 and by comparing the results obtained with already available or as yet unpublished information concerning other thermophilic eu- and archaebacteria such as Thermotoga, Sulfolobus, and Pyrococcus. Salient observations were that (i) the highly thermolabile and reactive carbamoylphosphate molecule appears to be protected from thermodegradation by channelling towards the synthesis of citrulline and carbamoylaspartate, respectively precursors of arginine and the pyrimidines; (ii) Thermus ornithine carbamoyltransferase is clearly a thermophilic enzyme, intrinsically thermostable and showing a biphasic Arrhenius plot, whereas aspartate carbamoyltransferase is inherently unstable and is stabilized by its association with dihydroorotase, another enzyme encoded by the Thermus pyrimidine operon. Possible implications of these results are discussed.