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

摘要

The enzyme ornithine carbamoyltransferase (OTCase) of Moritella abyssi (OTCaseMab), a new, strictly psychrophilic and piezophilic bacterial species, was purified. OTCaseMab 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 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. OTCaseMab displays higher Km 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). OTCaseMab 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 substantially lower at low temperatures. A survey of the catalytic efficiencies (kcat/Km) of OTCases adapted to different temperatures showed that OTCaseMab activity remains suboptimal at low temperature despite the 4.5-fold decrease in the Km value for ornithine observed when the temperature is brought from 20 to 5°C. OTCaseMab 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.