Structure and Function of Domains in Amylolytic Enzymes

Primary, secondary and tertiary structures of amylolytic enzymes from a wide variety of sources and functions (the a-amylases, bacterial cyclomaltodextrin glucanosyltrans-ferases, isoamylases and starch-branching enzymes) have been found to be closely related, and have been placed into the so-called structural a-amylase family.176,177 These enzymes have been studied with regard to the number, structure, organization and function of domains.178 

As discussed above, glucoamylases consist of three domains a catalytic domain a linker domain and a starch-binding domain. These domains can be in different orders in which the catalytic domain is located at the N-terminus or the C-terminus and conversely the starch-binding domain is at the C-terminus or the N-terminus, with the linker domain between the two. 

oryzae a-amylase consists of three domains (A, B and C).179,180 Domain A has an amino-terminal ((3/a)8-barrel structure, followed by domain C, consisting of (3-strands that are folded into a Greek motif. Domain B is inserted between the third (3-strand and the third a-helix of the ((3/a)8-barrel. This is a highly variable domain in both its length and amino acid sequence, depending on the source and type of the enzyme.181,182 Cyclomaltodextrin glucanosyltransferases generally consist of five domains (A, B, C, D and E). Domains A, B and C consist of the same catalytic domains found in 

oryzae a-amylase.178 182 185 The structure of domain E is typical of enzymes that bind and react with raw starch granules.186 Domain D contains antiparallel [3-barrels,183-185 but nothing is known about its function. 

A few chimeric enzymes have been constructed by adding one or more domains from one amylolytic enzyme to another. Some of these chimeric enzymes have been studied in regard to the secretion of the enzyme,187 substrate specificity188 and product specificity.189 A starch binding domain from a Bacillus sp. cyclomaltodextrin glucanosyltransferase was fused with B. subtilis a-amylase, and an a-amylase was 

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