α-Glucosidases hydrolyze the terminal nonreducing α-d
-glucosidic linkage of disaccharides, oligosaccharides and polysaccharides, with the release of α-glucose. The substrate-specificity of α-glucosidases differs greatly depending on the source of the enzyme. Most α-glucosidases (EC 22.214.171.124; α-d
-glucoside glucohydrolases) preferentially hydrolyze maltose (Needleman et al.
), whereas another class of α-glucosidases, oligo-1,6-glucosidases (EC 126.96.36.199; oligosaccharide oligo-1,6-glucohydrolases), act on the α-1,6-glucosidic linkage of isomaltooligosaccharides and dextran (Linder & Sund, 1981
; Suzuki et al.
α-Glucosidase is classified into glucoside hydrolase (GH) family 13 on the basis of its amino-acid sequence relationship to other glycosyl hydrolases. Many primary structures of members of GH family 13 from various origins are now available and have been compared with each other. The existence of four highly conserved regions (regions I–IV) and three acidic residues located in the conserved regions as catalytic residues have been reported (Matsuura et al.
; Nakajima et al.
; Svensson, 1988
). Furthermore, all GH family 13 enzymes for which three-dimensional structures have been solved have a common multidomain structure composed of three domains: A, B and C (MacGregor et al.
). Domain A is a catalytic domain containing a (β/α)8
-barrel. Domain B is found inserted between the third β-sheet and the helix of the (β/α)8
-barrel and may play a role in both enzyme stability and substrate binding. Domain C has a β-barrel structure of eight antiparallel β-strands in a double Greek-key motif. The catalytic nucleophile in the conserved region II is located in the loop extending from β-strand 4 and the general acid–base catalyst in the conserved region III is located at the C-terminus of β-strand 5.
contains two α-glucosidases, namely α-1,4-glucosidase (maltase) and oligo-1,6-glucosidase (isomaltase), which preferentially hydrolyze maltose or isomaltose and methyl α-d
-glucopyroside (α-mg), respectively. The expression of these enzymes is independently controlled by different polymeric genes, namely MAL
; Vanoni et al.
; Johnson & Carlson, 1992
). Maltase (the MAL
6 product of S. cerevisiae
) preferentially hydrolyzes maltose but not isomaltose or α-mg, whereas isomaltase hydrolyzes isomaltose and α-mg but not maltose (Khan & Eaton, 1967
; Needleman et al.
Isomaltase from S. cerevisiae
shows a notable substrate-specificity. Generally, oligo-1,6-glucosidases preferentially hydrolyze isomaltotriose and show high activity toward isomaltooligosaccharides or dextran (Linder & Sund, 1981
; Suzuki et al.
; Russell & Ferretti, 1990
; Saburi et al.
). However, isomaltase from S. cerevisiae
shows highest activity toward isomaltose and little activity toward isomaltotriose and isomaltotetraose.
In 1997, the crystal structure of oligo-1,6-glucosidase from Bacillus cereus
was solved at 2.0 Å resolution (Watanabe et al.
). Recently, the structures of dextran glucosidase in an uncomplexed form and of its mutant in complex with isomaltotriose at 2.2 Å resolution have been determined (Hondoh et al.
). However, in order to obtain a better understanding of the structure–function relationship of oligo-1,6-glucosidases, more precise structures are required. In the present work, we report the crystallization of S. cerevisiae
isomaltase and present a preliminary crystallographic analysis of the crystals obtained.