PROSITE documentation PDOC00486

CBM1 (Carbohydrate-binding type-1) domain signature and profile

The microbial degradation of cellulose and xylans requires several types of enzymes such as endoglucanases (EC 3.2.1.4), cellobiohydrolases (EC 3.2.1.91) (exoglucanases), or xylanases (EC 3.2.1.8) [1]. Cellulases and xylanases generally consist of a catalytic domain and a cellulose-binding domain (CBD) also called carbohydrate-binding module (CBM).

The carbohydrate-binding module of a number of fungal cellulases (CBM1) has been shown to consist of 38 amino acid residues. Enzymes known to contain such a domain are:

• Endoglucanase I (gene egl1) from Trichoderma reesei.
• Endoglucanase II (gene egl2) from Trichoderma reesei.
• Endoglucanase V (gene egl5) from Trichoderma reesei.
• Exocellobiohydrolase I (gene CBHI) from Humicola grisea, Neurospora crassa, Phanerochaete chrysosporium, Trichoderma reesei, and Trichoderma viride.
• Exocellobiohydrolase II (gene CBHII) from Trichoderma reesei.
• Exocellobiohydrolase 3 (gene cel3) from Agaricus bisporus
• Endoglucanases B, C2, F and K from Fusarium oxysporum.

CBM1 is found either at the N-terminal (Cbh-II or egl2) or at the C-terminal extremity (Cbh-I, egl1 or egl5) of these enzymes. As it is shown in the following schematic representation, there are four conserved cysteines in this type of CBD domain, all involved in disulfide bonds. The CBM1 fold consists of a three-stranded antiparallel β-sheet (see <PDB:1AZJ>) [2]. The domain folds into a wedge-shaped structure with a flat face containing three conserved aromatic residues separated by about 10.4 Angstroms which corresponds to the periodicity of the glucose units in the cellulose. These residues are thus probably involved in cellulose binding.

                         +----------------+
                         |          +-----|---------+
                         |          |     |         |
                  xxxxxxxCxxxxxxxxxxCxxxxxCxxxxxxxxxCx
                         ****************************

'C': conserved cysteine involved in a disulfide bond.
'*': position of the pattern.

Such a domain has also been found in a putative polysaccharide binding protein from the red alga, Porphyra purpurea. Structurally, this protein consists of four tandem repeats of the CBD domain.