InterPro: IPR008960 Carbohydrate-binding domain family 9-like

This entry represents an immunoglobulin-like beta-sandwich domain found in a variety of protein families, including family 9 carbohydrate-binding (CDB9) enzymes like endo-1,4-beta-xylanase (EC:3.2.1.8), as the C-terminal domain of glucodextranase enzymes (EC:3.2.1.70), and as the cytochrome domain of cellobiose dehydrogenase enzymes (EC:1.1.99.18). In general, these domains consist of 7 beta-strands in 2 sheets with a Greek key topology, but with an additional beta-strand at the N terminus [1, 2].

Bacterial extracellular cellulases and hemicellulases are involved in the hydrolysis of the major structural polysaccharides of plant cell walls. These are usually modular enzymes that contain catalytic and non-catalytic domains. The CBD9 domain binds to cellulose, xylan, as well as to a range of soluble di- and mono-saccharides, and is found in cellulose- and xylan-degrading enzymes [3].

Cellobiose dehydrogenases (CDHs) are extracellular hemoflavoenzymes produced by various wood-degrading fungi that are involved in the degradation of cellulose and lignin [4]. These enzymes consist of a cytochrome domain and a flavin domain, which are believed to have evolved in parallel as fused genes, because the two domains can be cleaved proteolytically to make a functional cytochrome and flavodehydrogenase [5]. The b-type cytochrome domain contains a 6-coordinate low spin b-type haem with unusual iron ligands and coordination geometry.

Glucodextranase hydrolyzes alpha-1,6-glucosidic linkages of dextran from the non-reducing end to produce beta-D-glucose via an inverting reaction mechanism and classified into the glycoside hydrolase family 15 (GH15) [6].