This family includes the following closely related glycosyl hydrolase family 31 (GH31) enzymes: maltase-glucoamylase (MGAM), sucrase-isomaltase (SI), lysosomal acid alpha-glucosidase (GAA), neutral alpha-glucosidase C (GANC), the alpha subunit of neutral alpha-glucosidase AB (GANAB), and alpha-glucosidase II. MGAM is one of the two enzymes responsible for catalyzing the last glucose-releasing step in starch digestion. SI is implicated in the digestion of dietary starch and major disaccharides such as sucrose and isomaltose, while GAA degrades glycogen in the lysosome, cleaving both alpha-1,4 and alpha-1,6 glucosidic linkages. MGAM and SI are anchored to small-intestinal brush-border epithelial cells. The absence of SI from the brush border membrane or its malfunction is associated with malabsorption disorders such as congenital sucrase-isomaltase deficiency (CSID). The domain architectures of MGAM and SI include two tandem GH31 catalytic domains, an N-terminal domain found near the membrane-bound end and a C-terminal luminal domain. Both of the tandem GH31 domains of MGAM and SI are included in this family. The domain architecture of GAA includes an N-terminal TFF (trefoil factor family) domain in addition to the GH31 catalytic domain. Deficient GAA expression causes Pompe disease, an autosomal recessive genetic disorder also known as glycogen storage disease type II (GSDII). GANC and GANAB are key enzymes in glycogen metabolism that hydrolyze terminal, non-reducing 1,4-linked alpha-D-glucose residues from glycogen in the endoplasmic reticulum. Alpha-glucosidase II is a GH31 enzyme, found in bacteria and plants, which has exo-alpha-1,4-glucosidase and oligo-1,6-glucosidase activities. Alpha-glucosidase II has been characterized in Bacillus thermoamyloliquefaciens where it forms a homohexamer. This family also includes the MalA alpha-glucosidase from Sulfolobus solfataricus and the AglA alpha-glucosidase from Picrophilus torridus. MalA is part of the carbohydrate-metabolizing machinery that allows this organism to utilize carbohydrates, such as maltose, as the sole carbon and energy source. The MGAM-like family corresponds to subgroup 1 in the Ernst et al classification of GH31 enzymes.
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