Abstract
This paper describes the study on the structure and function of enzymes hydrolyzing a polysaccharide, pullulan and the related enzymes. The paper is composed of the following three major sections. (1) A thermophilic actinomycete, Thermoactinomyces vulgaris R-47, produces two pullulan hydrolyzing-enzymes, TVA I and TVA II. TVA II forms a dimer, and this is the first observation of a dimeric enzyme in the α-amylase family. In contrast, TVA I functions as a monomer, and domain N of TVA I is identified as a novel carbohydrate-binding module, CBM34. (2) The crystal structure and function of the isopullulanase were determined. The isopullulanase forms a unique β-helix fold, which is not found in other pullulan-hydrolyzing enzymes. (3) A glucoamylase gene was found downstream of the TVA II gene, and the primary structure of this glucoamylase resembled that of glucodextranase. The glucoamylase and related enzymes were further studied, and the crystal structures and functions of Arthrobacter globiformis I42 glucodextranase (GDase) and Escherichia coli glucosidase, YgjK, have been determined. Although GDase and YgjK scarcely hydrolyzed starch and their substrate specificities were completely different from that of glucoamylase, these enzymes have a common (α/α)6-barrel domain like glucoamylase.
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