Thermal unfolding of selected enzyme variants was carried out using a Chirascan spectropolarimeter (Applied Photophysics, UK). Each protein sample was diluted in 50mM Phosphate buffer to the concentration of 0.18 mg•mL -1 and measured in a 0.1 cm quartz cuvette. Changes of ellipticity were monitored at three wavelengths, 195 nm, 210 nm, and 227 nm from 15°C to 80°C with a 0.1°C resolution and 1°C•min -1 heating rate. Recorded data were fitted using the model "Sigmoid curve + slope" in the Pro Data Viewer software (Applied Photophysics, UK). The apparent melting temperature (Tm) was evaluated as a midpoint of the normalized thermal transition.