Homologous xylanases from Clostridium thermocellum: evidence for bi-functional activity, synergism between xylanase catalytic modules and the presence of xylan-binding domains in enzyme complexes

Clostridium thermocellum produces a consortium of plant-cell-wall hydrolase s that form a cell-bound multi-enzyme complex called the cellulosome, In th e present study two similar xylanase genes, xynU and xynV, were cloned from C. thermocellum strain YS and sequenced. The deduced primary structures of both xylanases, xylanase U (XylU) and xylanase V (XylV), were homologous w ith the previously characterized xylanases from C. thermocellum strain F1. Truncated derivatives of XylV were produced and their biochemical propertie s were characterized. The xylanases were shown to be remarkably thermostabl e and resistant to proteolytic inactivation. The catalytic domains hydrolys ed xylan by a typical endo-mode of action. The type VT cellulose-binding do main (CBD) homologue of XylV bound xylan and, to a smaller extent, Avicel a nd acid-swollen cellulose. Deletion of the CBD from XylV abolished the capa city of the enzymes to bind polysaccharides. The polysaccharide-binding dom ain was shown to have a key role in the hydrolysis of insoluble substrates by XylV. The C-terminal domain of XylV, which is absent from XylU, removed acetyl groups from acetylated xylan and acted in synergy with the glycosyl hydrolase catalytic domain of the enzyme to elicit the hydrolysis of acetyl ated xylan.