Thermal inactivation of alkali stable xylanases (XylA & XylB) from alkali tolerant fungus Aspergillus fischeri Fxn 1: effect of trehalose on their thermostability

Alkali tolerant fungus Aspergillus fischeri Fxn 1 secretes two xylanases, an exoxylanase (XylA) and an endoxylanase (XylB) in solid state fermentation. Kinetic studies showed that thermal inactivation of purified XylA and XylB at pH 9, in the temperature ranging from 303K to 333K, followed first-order kinetics, with denaturation rate constants as 9.6 10-3 and 8.2 10-3 at 323K for XylA & XylB respectively. Heat-inactivation plots for purified enzymes were linear from which thermodynamic activation parameters, ΔH#, ΔS# and ΔG# have been estimated. The enzymatic activity was relatively stable with a respective half-life (t1/2) at 323K of 72 min for XylA and 84.5 min for XylB. The half life values of XylA & XylB increased to 169 and 136 min respectively, in the presence of 1 M trehalose. Separate tests at 60 C in the presence of additives (polyols, carbohydrates) showed that trehalose was the most effective stabiliser, which increased the stability of XylA & XylB to 64.7 & 17.63 % respectively