Enzymes of the Glycoside hydrolase family 13 typically show three domains A, B and C. In addition to these, Alpha-amylase SusG from Bacteroides thetaiotaomicron (Swiss:Q8A1G3) includes the insertion of a unique carbohydrate binding module (CBM) family 58 (this entry), which gives the protein an elongated shape. SusG, located on the outer membrane of the cell, cleaves starch into oligosaccharides before internalisation for degradation. CBM58 is approximately 120 residues in length. This domain folds into a beta sandwich and adopts an immunoglobulin-like topology with a flat five-stranded antiparallel beta sheet opposing a four-stranded antiparallel beta sheet which features three protruding loops. Y260, W287, and W299 form the starch-binding site [1, 2, 3].

1: SusG: a unique cell-membrane-associated alpha-amylase from a prominent human gut symbiont targets complex starch molecules. Koropatkin NM, Smith TJ; Structure. 2010;18:200-215. PMID:20159465

2: Structural basis for the flexible recognition of alpha-glucan substrates by Bacteroides thetaiotaomicron SusG. Arnal G, Cockburn DW, Brumer H, Koropatkin NM; Protein Sci. 2018;27:1093-1101. PMID:29603462

3: The Sus operon: a model system for starch uptake by the human gut Bacteroidetes. Foley MH, Cockburn DW, Koropatkin NM; Cell Mol Life Sci. 2016;73:2603-2617. PMID:27137179