The microbial degradation of cellulose and xylans requires several types of
enzyme such as endoglucanases (EC 22.214.171.124), cellobiohydrolases (EC 126.96.36.199)
(exoglucanases), or xylanases (EC 188.8.131.52) . Structurally, cellulases and
xylanases generally consist of a catalytic domain and a conserved region of
~100 amino acid residues, the carbohydrate-binding module 2 (CBM2) [2,E1]. It
is found either at the N-terminal or at the C-terminal extremity of these
CBM2 can be classified in 2 subfamilies according to substrate specificities:
CBM2a which binds cellulose and CBM2b which interacts specifically with xylan.
CBM2a and CBM2b also display significant differences at the residue level as
shown in the following schematic representation:
'C': conserved cysteine involved in a disulfide bond.
'*': position of the pattern.
'W': surface-exposed tryptophan
Like other CBM domains CBM2 is a β-sheet domain containing a planar face
which interacts with its ligand via a hydrophobic strip of aromatic residues
(see <PDB:1EXG>) . In family 2a this hydrophobic surface consists of three
tryptophan residues, which are all required for binding soluble and insoluble
forms of cellulose . In family 2b only 2 surface-exposed tryptophans are
conserved and the first one is oriented differently. It is therefore not well
oriented to interact with cellulose but is ideal for binding xylan .
Enzymes known to contain such a domain are:
- Endoglucanase (gene end1) from Butyrivibrio fibrisolvens.
- Endoglucanases A (gene cenA) and B (cenB) from Cellulomonas fimi.
- Exoglucanases A (gene cbhA) and B (cbhB) from Cellulomonas fimi.
- Endoglucanase E-2 (gene celB) from Thermomonospora fusca.
- Endoglucanase A (gene celA) from Microbispora bispora.
- Endoglucanases A (gene celA), B (celB) and C (celC) from Pseudomonas
- Endoglucanase A (gene celA) from Streptomyces lividans.
- Exocellobiohydrolase (gene cex) from Cellulomonas fimi.
- Xylanases A (gene xynA) and B (xynB) from Pseudomonas fluorescens.
- Arabinofuranosidase C (EC 184.108.40.206) (xylanase C) (gene xynC) from
- Chitinase 63 (EC 220.127.116.11) from Streptomyces plicatus.
- Chitinase C from Streptomyces lividans.
To recognize the CBM2a domain we developed a pattern which is located in a
conserved region specific to CBM2a. We also developed a profile which covers
the whole CBM2 domain and recognizes both CBM2 subfamilies.
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