The enzyme ornithine carbamoyltransferase (OTCase) of Moritella abyssi, a new, strictly psychrophilic and piezophilic bacterial species, was purified. The enzyme displays maximal activity at rather low temperatures (23 to 25°C) compared to other cold-active enzymes and is much less thermoresistant than its homologue from Escherichia coli or thermophilic prokaryotes. 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 thereoff. The OTCase from M. abyssi displays higher Km values for ornithine and carbamoyl pphosphate than mesophilic and thermophilic OTCases and is only weakly inhibited by the bisubstrate analogue PALO. The M. abyssi OTCase differs from other, nonpsychrophilic OTCases by substitutions in the most conserved motifs, which probably contribute to the comparatively high Km values and the lower sensitivity to PALO. The Km for ornithine, however, is substatially lower at low temperatures. A survey of the catalytic efficiencies (kcat/Km) of OTCases adapted to different temperatures showed that the activity of the M. abyssi OTCase remains suboptimal at low temperature despite the 4.5-fold devrease in Km value for ornthine observed when the temperature is brought from 20 to 5°C. The 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.
|Journal of Bacteriology
|Published - 1 apr 2003