Stability characteristics of a calcium-independent alkaline protease from Nesterenkonia sp.

Thermodynamic stability of an alkaline protease from a new alkaliphilic Nesterenkonia sp. AL-20, was investigated and compared with that of Subtilisin Carlsberg. The amount of calcium bound to the AL-20 protease was determined to be only about 0.14 mol/mol of protease. Differential scanning calorimetry scan of the enzyme at increasing temperature showed the denaturation of the enzyme to be a two-state process with melting temperature, Tm of about 74 °C at pH 10.0, which was unaltered upon addition of calcium as well as after treatment with chelating agents. The thermodynamic parameters were nearly the same over a pH range of 7.0–10.0. Tm was reduced to 69.7 °C at pH 6.0 and 72 °C at pH 11.0. The secondary structure of the protease was unaffected during storage at 50 °C, even in the presence of 1% SDS as observed by circular dichroism. The protease activity was extremely stable in the presence of hydrogen peroxide and various sequestering agents used in detergents.