Glucoamylase (glucan1,4-alpha-glucosidase), C-terminal CBM20 (carbohydrate-binding module, family 20) domain. Glucoamylases are inverting, exo-acting starch hydrolases that hydrolyze starch and related polysaccharides by releasing the nonreducing end glucose. They are mainly active on alpha-1,4-glycosidic bonds but also have some activity towards 1,6-glycosidic bonds occurring in natural oligosaccharides. The ability of glucoamylases to cleave 1-6-glycosidic binds is called "debranching activity" and is of importance in industrial applications, where complete degradation of starch to glucose is needed. Most glucoamylases are multidomain proteins containing an N-terminal catalytic domain, a C-terminal CBM20 domain, and a highly O-glycosylated linker region that connects the two. The CBM20 domain is found in a large number of starch degrading enzymes including alpha-amylase, beta-amylase, glucoamylase, and CGTase (cyclodextrin glucanotransferase). CBM20 is also present in proteins that have a regulatory role in starch metabolism in plants (e.g. alpha-amylase) or glycogen metabolism in mammals (e.g. laforin). CBM20 folds as an antiparallel beta-barrel structure with two starch binding sites. These two sites are thought to differ functionally with site 1 acting as the initial starch recognition site and site 2 involved in the specific recognition of appropriate regions of starch.
Lu S et al.(2020). "CDD/SPARCLE: the conserved domain database in 2020.",