Structures and Substrate Recognitions of Animal, Botanical and Microbial α-Glucosidases and Their Molecular Evolution

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α-Glucosidase (EC 3.2.1.20) is a typical exo-type glycosidase that releases the α-glucose from non-reducing side of substrate. We are interested in the relationship between catalytic action and the structure, since the substrate specificity differs greatly with the source of enzyme. There are at least two types of α-glucosidases which show different substrate recognitions, suggesting that enzymes can be classified into two groups (family I and family II). However, the structural information is not enough to learn that structures of enzymes belonging to each group are homologous or not. In this project, we analyzed the primary structures of α-glucosidases from animal, botanical and microbial origins, and found that the enzymes of family I and II had different primary amino acid sequences.α-Glucosidases from insect and bacteria belonged to family I, of which molecular weight was about 70 kDa. Four catalytic regions found were similar to those of α-amylase. The activity toward heteroside (sucrose or p-nitrophenyl α-glucoside) was higher than holoside (maltooligosaccharides). Members of family II were from animal, botanical and mold origins, and showed the opposite substrate specificity, high activity to holoside and low to heteroside. The molecular sizes were about 100 kDa. We analyzed the primary structures of six kinds of α-glucosidases belonging to this group. The sequences obtained were homologous each other, but no similarity was observed with family I enzymes. The findings suggest that the α-glucosidase was evolved from two different ancestral proteins.