Carbohydrate binding module family 17/28 (IPR005086)
Short name: CBM_fam_17/28
Overlapping homologous superfamilies
- Galactose-binding-like domain superfamily (IPR008979)
A carbohydrate-binding module (CBM) is defined as a contiguous amino acid sequence within a carbohydrate-active enzyme with a discreet fold having carbohydrate-binding activity. A few exceptions are CBMs in cellulosomal scaffolding proteins and rare instances of independent putative CBMs. The requirement of CBMs existing as modules within larger enzymes sets this class of carbohydrate-binding protein apart from other non-catalytic sugar binding proteins such as lectins and sugar transport proteins.
CBMs were previously classified as cellulose-binding domains (CBDs) based on the initial discovery of several modules that bound cellulose [PMID: 3338453, PMID: 3134347]. However, additional modules in carbohydrate-active enzymes are continually being found that bind carbohydrates other than cellulose yet otherwise meet the CBM criteria, hence the need to reclassify these polypeptides using more inclusive terminology.
Previous classification of cellulose-binding domains were based on amino acid similarity. Groupings of CBDs were called "Types" and numbered with roman numerals (e.g. Type I or Type II CBDs). In keeping with the glycoside hydrolase classification, these groupings are now called families and numbered with Arabic numerals. Families 1 to 13 are the same as Types I to XIII. For a detailed review on the structure and binding modes of CBMs see [PMID: 15214846].
CBM17 binds to amorphous cellulose and soluble beta-1,4-glucans, with a minimal binding requirement of cellotriose and optimal affinity for cellohexaose. Family 17 CBMs appear to have a very shallow binding cleft that may be more accessible to cellulose chains in non-crystalline cellulose than the deeper binding clefts of family 4 CBMs [PMID: 11733998]. CBM28 does not compete with CBM17 modules when binding to non-crystalline cellulose but does have a "beta-jelly roll" topology, which is similar in structure to the CBM17 domains. Sequence and structural conservation in families 17 and 28 suggests that they have evolved through gene duplication and subsequent divergence [PMID: 15136030].
This entry includes family 17 and 28 which show structural homology. The domain is found in a number of alkaline cellulases.
- PF03424 (CBM_17_28)