Structural elements in dextran glucosidase responsible for high specificity to long chain substrate.
Dextran glucosidase from Streptococcus mutans (SMDG) and Bacillus oligo-1,6-glucosidases, members of glycoside hydrolase family 13 enzymes, have the high sequence similarity. Each of them is specific to alpha-1,6-glucosidic linkage at the non-reducing end of substrate to liberate glucose. The activities toward long isomaltooligosaccharides were different in both enzymes, in which SMDG and oligo-1,6-glucosidase showed high and low activities, respectively. We determined the structural elements essential for high activity toward long-chain substrate. From conformational comparison between SMDG and B. cereus oligo-1,6-glucosidase (three-dimensional structure has been solved), Trp238 and short beta-->alpha loop 4 of SMDG were considered to contribute to the high activity to long-chain substrate. W238A had similar kcat/Km value for isomaltotriose to that for isomaltose, suggesting that the affinity of subsite +2 was decreased by Trp238 replacement. Trp238 mutants as well as the chimeric enzyme having longer beta-->alpha loop 4 of B. subtilis oligo-1,6-glucosidase showed lower preference for long-chain substrates, indicating that both Trp238 and short beta-->alpha loop 4 were important for high activity to long-chain substrates.
Saburi
Wataru
W
Graduate School of Agriculture, Hokkaido University, Sapporo 060-8589, Japan.
Mori
Haruhide
H
Saito
Saori
S
Okuyama
Masayuki
M
Kimura
Atsuo
A
D000595:Amino Acid Sequence; D020134:Catalytic Domain; D003911:Dextrans / Q000378:metabolism; D005959:Glucosidases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D008969:Molecular Sequence Data; D013295:Streptococcus mutans / Q000201:enzymology; D013379:Substrate Specificity
D003911:Dextrans; D005959:Glucosidases; C023149:exo-1,6-alpha-glucosidase