Glucan 1,4-alpha-maltotetraohydrolase (alpha-MTH), C-terminal CBM20 (carbohydrate-binding module, family 20) domain. Alpha-MTH, also known as maltotetraose-forming exo-amylase or G4-amylase, is an exo-amylase found in bacteria that degrades starch from its non-reducing end. Most alpha-MTHs have, in addition to the C-terminal CBM20 domain, an N-terminal glycosyl hydrolase family 13 catalytic domain. 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.