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Role of the thiol group in the activity and stability of xylanase from Thermomyces lanuginosus

Microbial xylanases belong to most important enzymes with industrial application. Recently, a thermophilic fungus Thermomyces lanuginosus was described as one of the best producers of this enzyme. Studies of the Termomyces lanuginosus xylanase structure and properties have revealed that the enzyme contains only one cysteine, and that the enzyme activity is strongly affected by thiol reacting agents p-hydroxymercuribenzoic acid and 5,5"-dithiobis(2-nitro)benzoic acid. Therefore, the role of the unique cysteine residue in the activity and stability of the Thermomyces lanuginosus xylanase was examined. To get an insight into the function of the cysteine residue in the activity and stability of the T. lanuginosus xylanase, the enzyme activity was determined in the presence of different concentrations of thiol reagents, beta-mercaptoethanol and dithiothreitol. Besides, the influence of beta-mercaptoethanol on the inactivation of the enzyme at acidic pH was studied. It was found that thiol reagents, beta-mercaptoethanol and dithiothreitol enhanced xylanase activity. Furthermore, the addition of beta-mercaptoethanol markedly increased the enzyme stability at acidic pH. A direct involvement of the cysteine residue in the active site of T. lanuginosus xylanase is proposed.

xylanase; Thermomyces lanuginosus

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