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We here report the first molecular characterization of an α-xylosidase (XylS) from an Archaeon. Sulfolobus solfataricus is able to grow at temperatures higher than 80 °C on several carbohydrates at acidic pH. The isolatedxylS gene encodes a monomeric enzyme homologous to α-glucosidases, α-xylosidases, glucoamylases and sucrase-isomaltases of the glycosyl hydrolase family 31.xylS belongs to a cluster of four genes in the S. solfataricus genome, including a β-glycosidase, an hypothetical membrane protein homologous to the major facilitator superfamily of transporters, and an open reading frame of unknown function. The α-xylosidase was overexpressed in Escherichia colishowing optimal activity at 90 °C and a half-life at this temperature of 38 h. The purified enzyme follows aretaining mechanism of substrate hydrolysis, showing high hydrolytic activity on the disaccharide isoprimeverose and catalyzing the …