Pcal_0970: an extremely thermostable L-asparaginase from Pyrobaculum calidifontis with no detectable glutaminase activity.

The genome sequence of Pyrobaculum calidifontis contains two open reading frames, Pcal_0144 and Pcal_0970, exhibiting homology with L-asparaginases. In search of a thermostable L-asparaginase with no glutaminase activity, we have cloned and expressed the gene encoding Pcal_0970 in Escherichia coli. Recombinant Pcal_0970 was produced in insoluble and inactive form which was solubilized and refolded into enzymatically active form. The refolded Pcal_0970 showed the highest activity at or above 100 °C. Optimum pH for the enzyme activity was 6.5. Addition of divalent metal cations or EDTA had no significant effect on the activity. The enzyme was capable of hydrolyzing D-asparagine with a 20% activity as compared to 100% with L-asparagine. Pcal_0970 did not show any detectable activity when L-glutamine or D-glutamine was used as substrate. Pcal_0970 exhibited a Km value of 4.5 ± 0.4 mmol/L and Vmax of 355 ± 13 μmol min-1 mg-1 towards L-asparagine. The activation energy, from the linear Arrhenius plot, was determined as 39.9 ± 0.6 kJ mol-1. To the best of our knowledge, Pcal_0970 is the most thermostable L-asparaginase with a half-life of more than 150 min at 100 °C and this is the first report on characterization of an L-asparaginase from phylum Crenarchaeota.