Two structurally related starch-binding domain families CBM25 and CBM26

Starch binding domains (SBDs) are able to bind to and facilitate the degradation of raw starch and starchy substrates. In general, in the CAZy database they have been classified among the carbohydrate-binding module (CBM) families. The two families CBM25 and CBM26 together with families CBM20, 21, 34, 41, 45, 48, 53, 58, 68 and 69 belong to twelve SBD CAZy families. They represent a group of closely related modules exhibiting some sequence similarity, although each of the two families possesses its own features. Both CBM25 and CBM26 adopt a typical SBD fold of distorted β-barrel as recognized in the modules present in the maltohexaose-producing amylase from Bacillus halodurans. With regard to catalytic domains, most members are α-amylases and maltooligosaccharide-producing amylases from the α-amylase glycoside hydrolase (GH) family GH13, but also some β-amylases (GH14) and hypothetical proteins (e.g. from the family GH31) are known. The main goal of this review was to compare the available amino acid sequences of SBDs from both families CBM25 and CBM26 and to reveal, if possible, SBD(s) with the character “intermediary” between the CBM25 and CBM26. Emphasis was also given on a structural comparison of the identified intermediary SBD with the CBM25 and CBM26 representatives and a detailed evolutionary division of both CBM families that can be utilized for defining the future subfamilies.