Bacillus thermoglucosidius alpha-glucosidase. Temperature dependence of activity and stability

A p-nitrophenyl-alpha-D-glucopyranoside-hydrolyzing alpha-glucosidase from an obligate thermophile, Bacillus thermoglucosidius KP 1006, gave a triphasic relationship at pH 6.8 in the van't Hoff plot of Km, in the Arrhenius plot of the first order rate constant of inactivation with 0.1% sodium dodecyl sulfate, and in the logarithmic plot of the maximal fluorescence intensity at 346 nm versus reciprocal of temperature. The respective plots exhibited two breaks at 40 and 61 degrees C, 43 and 62 degrees C, and 40 and 61 degrees C. However, the Arrhenius plot of the molecular activity at pH 6.8 had a single discontinuity at 64 degrees C. These findings, together with thermodynamic quantities for the enzyme, suggest that the thermal conformation changes in the enzyme protein occur around 40--43 degrees C and 61--64 degrees C. The Arrhenius plot of the rate constant of heat inactivation at pH 6.8 was bent at 73 degrees C. Thermodynamic data indicate that the enzyme is transformed from a heat stable form into a heat unstable form at 73 degrees C with temperature elevation. The critical points localized near the minimum, optimum, and maximum temperatures (40, 60, and 72 degrees C) of the cell growth, respectively.