[en] The enzyme ornithine carbamoyltransferase (OTCase) of Motitella abyssi (OTCase(Mab)), a new, strictly psychrophilic and piezophilic bacterial species, was purified. OTCase(Mab) displays maximal activity at rather low temperatures (23 to 25degreesC) compared to other cold-active enzymes and is much less thermoresistant than its homologues from Escherichia coli or thermophilic procaryotes. In vitro the enzyme is in equilibrium between a trimeric state and a dodecameric, more stable state. The melting point and denaturation enthalpy changes for the two forms are considerably lower than the corresponding values for the dodecameric Pyrococcus furiosus OTCase and for a thermolabile trimeric mutant thereof. OTCase(Mab) displays higher K-m values for ornithine and carbamoyl phosphate than mesophilic and thermophilic OTCases and is only weakly inhibited by the bisubstrate analogue delta-N-phosphonoacetyl-L-ornithine (PALO). OTCase(Mab) differs from other, nonpsychrophilic OTCases by substitutions in the most conserved motifs, which probably contribute to the comparatively high K-m values and the lower sensitivity to PALO. The K. for ornithine, however, is substantially lower at low temperatures. A survey of the catalytic efficiencies (k(cat)/K-m) of OTCases adapted to different temperatures showed that OTCase(Mab) activity remains suboptimal at low temperature despite the 4.5-fold decrease in the K-m value for ornithine observed when the temperature is brought from 20 to 5degreesC. OTCase(Mab) adaptation to cold indicates a trade-off between affinity and catalytic velocity, suggesting that optimization of key metabolic enzymes at low temperatures may be constrained by natural limits.