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The modular carbohydrate-active enzyme belonging to glycoside hydrolase family 30 (GH30) from Clostridium thermocellum (CtXynGH30) is a cellulosomal protein which plays an important role in plant cell-wall degradation. The full-length CtXynGH30 contains an N-terminal catalytic module (Xyn30A) followed by a family 6 carbohydrate-binding module (CBM6) and a dockerin at the C-terminus. The recombinant protein has a molecular mass of 45 kDa. Preliminary structural characterization was carried out on Xyn30A crystallized in different conditions. All tested crystals belonged to space group P1 with one molecule in the asymmetric unit. Molecular replacement has been used to solve the Xyn30A structure.

The cellulosome, a highly elaborate extracellular multi-enzyme complex of cellulases and hemicellulases, is responsible for the degradation of plant cell walls. The xylanase CtXyl5A (Cthe_2193) is a multimodular arabinoxylanase which is one of the largest components of the Clostridium thermocellum cellulosome. The N-terminal catalytic domain of CtXyl5A, which is a member of glycoside hydrolase family 5 (GH5), is responsible for the hydrolysis of arabinoxylans. Appended after it are three noncatalytic carbohydrate-binding modules (CBMs), which belong to families 6 (CBM6), 13 (CBM13) and 62 (CBM62). In addition, CtXyl5A has a fibronectin type III-like (Fn3) module preceding the CBM62 and a type I dockerin (DOK) module following it which allows the enzyme to be integrated into the cellulosome through binding to a cohesin module of the protein scaffold CipA. Crystals of the pentamodular enzyme without the DOK module at the C-terminus, with the domain architecture CtGH5-CBM6-CBM13-Fn3-CBM62, have been obtained. The structure of this pentamodular xylanase has been determined by molecular replacement to a resolution of 2.64 Å using coordinates of CtGH5-CBM6, Fn3 and CBM62 from the PDB as search models.