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Glycoside hydrolases of family 141 display α-L-fucosidase (EC 3.2.1.51) [1] or xylanase (EC 3.2.1.8) [2] activities. The Bacteroides thetaiotaomicron enzyme BT1002 is the founding member of this family. This enzyme cleaves the fucosodic linkage in 2-O-methyl-D-xylosyl-α-1,3-L-fucosyl-α1,4-L-Rhap, a component of chain B of rhamnogalacturonan II, a complex pectin conserved in the primary cell walls of plants [1]. Recently, an endo-xylanase from Clostridium thermocellum (Xyn141E) was also described. Xyn141E is active against a range of xylans, displaying low level activity against other β1,4-glycans such as carboxymethyl cellulose, barley beta glucan and ivory nut mannan [2].
Kinetics and Mechanism
Very little is known about the kinetics or mechanism of family 141 enzymes. However, in the BT1002 crystal structure, the distance of 6.1 Å between the catalytic residues suggests that members of this family may be retaining enzymes and follow a double displacement mechanism [1].
Catalytic Residues
In the X-ray crystal structure of B. thetaiotaomicron BT1002, two aspartates (Asp523 and Asp564), which are highly conserved and located within the active site pocket, are the proposed catalytic residues. Site directed mutagenesis of these amino acids abolished the catalytic activity of BT1002 indicating the essential role in catalysis. Additionally, the structural location of the catalytic residues suggests that Asp523 (at the base of the pocket) acts as catalytic nucleophile and Asp564 (at the lip of the active site) is the general acid-base residue [1].
Three-dimensional structures
Figure 1.BT1002 strucutre. (PDB ID 5PDB) The figure shows the β-parallel catalytic domain (purple) and the additional β-sandwich domain (yellow)
The three-dimensional structure has been determined for B. thetaiotaomicron BT1002 at 2 Å (PDB ID 5PDB). The protein has two domains: a N-terminal β-sandwich and a C-terminal catalytic domain that folds into a right-handed parallel β-helix . The overall fold displays similarity to members of Polysaccharide Lyase Family 1 and Glycoside Hydrolase Family 28 (Figure 1) [1]. The active site pocket is in a similar location to the catalytic centre of GH120 β-xylosidases [3], which also displays the same fold as the GH141 enzyme BT1002. The catalytic resisdues identified in the GH120 β-xylosidase [4], however, are not conserved in the GH141 α-L-fucosidase.
