Cloning and expression of a gene for an alpha-glucosidase from Saccharomycopsis fibuligera homologous to family GH31 of yeast glucoamylases

Development of a strategy for the screening of α-glucosidase-producing microorganisms

α-Glucosidase is a crucial enzyme for the production of isomaltooligosaccharide. In this study, a novel method comprising eosin Y (EY) and α-D-methylglucoside (AMG) in glass plates was tested for the primary screening of α-glucosidaseproducing strains. First, α-glucosidase-producing Aspergillus niger strains were selected on plates containing EY and AMG based on transparent zone formation resulting from the solubilization of EY by the hydrolyzed product. Conventional methods that use trypan blue (TB) and p-nitrophenyl-α-D-glucopyranoside (pPNP) as indicators were then compared with the new strategy. The results showed that EY-containing plates provide the advantages of low price and higher specificity for the screening of α-glucosidase-producing strains. We then evaluated the correlation between the hydrolytic activity of α-glucosidase and diffusion distance, and found that good linearity could be established within a 6-75 U/ml enzyme concentration range. Finally, the hydrolytic and transglycosylation activities of α-glucosidase obtained from the target isolates were determined by EY plate assay and 3,5-dinitrosalicylic acid-Saccharomyces cerevisiae assay, respectively. The results showed that the diameter of the transparent zone varied among isolates was positively correlated with α-glucosidase hydrolytic activity, while good linearity could also be established between α-glucosidase transglycosylation activity and non-fermentable reducing sugars content. With this strategy, 7 Aspergillus niger mutants with high yield of α-glucosidase from 200 obvious single colonies on the primary screen plate were obtained.

Aromatic residue on β→α loop 1 in the catalytic domain is important to the transglycosylation specificity of glycoside hydrolase family 31 α-glucosidase

The specificity for the α-1,4- and α-1,6-glucosidic linkages varies among glycoside hydrolase family 31 α-glucosidases. This difference in substrate specificity has been considered to be due to the difference in an aromatic residue on β→α loop 1 in the catalytic domain with a (β/α)8 barrel fold; i.e., the enzymes having Tyr and Trp on β→α loop 1 were respectively described as α-1,4-specific and α-1,6-specific α-glucosidases. Schwanniomyces occidentalis α-glucosidase, however, prefers the α-1,4-glucosidic linkage, although the enzyme possesses Trp324 at the corresponding position. The mutation of Trp324 to Tyr decreased the ability for hydrolysis of the α-1,6-glucosidic linkage and formation of the α-1,6-glucosidic linkage in transglycosylation, indicating Trp324 to be closely associated with α-1,6 specificity, even if the enzyme preferred the α-1,4-glucosidic linkage. The mutant enzyme was found to catalyze the production of the branched oligosaccharide, 2,4-di-O-(α-D-glucopyranosyl)-D-glucopyranose, more efficiently than the wild-type enzyme.

Purification and biochemical characterization of an alpha-glucosidase from Xanthophyllomyces dendrorhous

Xanthophyllomyces dendrorhous grown in different media shows amylolytic activity, consisting in an extracellular exo-acting enzyme able to hydrolysed alpha,1-4 glycosidic bonds from soluble starch, which also cleaves maltose and malto-oligosaccharides. The enzyme was purified, using basically a couple of chromatography process on DEAE-Sephacel. It is a glycoprotein with a molecular weight estimated to be 60.2 kDa based on its mobility in SDS-PAGE and 115 kDa based on gel filtration. N-linked carbohydrate accounts for 12% of the total mass. It exhibited optimum activity at pH 5.5 and 45 degrees C. Thermostability analysis indicated that it was stable to thermal treatment up to 50 degrees C; 50% of the activity was maintained after 3 h. The rate parameters measured for the hydrolysis of starch and various chain length malto-oligosaccharides shows high catalytic efficiency, calculated by the relationship V(cat)/K(m), for malto-oligosaccharides, such as maltotriose (873 mM(-1) min(-1)), or maltoheptose (698 mM(-1) min(-1)). The new enzyme hydrolysed soluble starch with nearly 3.5- and 1.4-fold lower efficiency than that for maltotriose and maltose, respectively. No activity was found on heterogeneous substrates, such as sucrose and aryl alpha-glucoside, or on isomalto-oligosaccharides. In accordance to substrate specificity profile, the new enzyme was classified as an alpha-glucosidase.