Thermostability of beta-xylosidase from Aspergillus sydowii MG49.

Heating of Aspergillus beta-xylosidase at 85 degrees C +/- 1 degree C and pH 5.5-6.0 (optimum for activity), causes irreversible, covalent thermoinactivation of the enzyme, involving oxidation of the thiol groups that are required for catalysis. Exogenous addition of cysteine, DTT, GSH and mercaptoethanol stabilizes the enzyme by extending its half-life. A similar effect is also exhibited by bivalent cations like Mg2+, Mn2+, Co2+, Ca2+ and Zn2+ while, on the other hand Cu2+ accelerates thermoinactivation. Chemical modification of crude beta-xylosidase with cross-linking agents like glutaraldehyde or covalent immobilization to a nonspecific protein like gelatin and BSA also enhances enzyme thermostability. These results suggest that addition of thiols and bivalent metal ions to a crude beta-xylosidase preparation or immobilization/chemical modification enhances its thermal stability, thus preventing loss of catalytic activity at elevated temperatures.