Metabolic Enzymes from Psychrophilic Bacteria: Challenge of Adaptation to Low Temperatures in Ornithine Carbamoyltransferase from Moritellaabyssi

ABSTRACT

The enzyme ornithine carbamoyltransferase (OTCase) of Moritellaabyssi (OTCase_Mab), a new, strictly psychrophilic and piezophilic bacterial species, was purified. OTCase_Mab displays maximal activity at rather low temperatures (23 to 25°C) compared to other cold-active enzymes and is much less thermoresistant than its homologues from Escherichiacoli 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 Pyrococcusfuriosus 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 δ-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_m 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 5°C. 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.