DO THE NONCATALYTIC POLYSACCHARIDE-BINDING DOMAINS AND LINKER REGIONSENHANCE THE BIOBLEACHING PROPERTIES OF MODULAR XYLANASES

Xylanase A (XylA) from Pseudomonas fluorescens subsp. cellulosa consis ts of an N-terminal non-catalytic cellulose-binding domain joined to a functionally independent C-terminal catalytic domain by a sequence ri ch in serine residues. Xylanase D (XylD) from Cellulomonas fumi also e xhibits a modular structure comprising an N-terminal catalytic domain linked to an internal non-catalytic xylan-binding domain and a C-termi nal cellulose-binding domain. To determine the importance of the non-c atalytic polysaccharide-binding domains and linker sequences of XylA a nd XylD in relation to their capacity to hydrolyse pulp xylan and enha nce bleachability, purified foil-length and modified derivatives of bo th enzymes were incubated with a hardwood kraft pulp. Deletion of the cellulose-binding domain or linker region from XylA decreased the acti vity of the enzyme against pulp xylan, but had no significant effect o n the capacity of the enzyme to facilitate delignification and reduce pulp kappa number. While full-length and truncated forms of XylD, lack ing either the cellulose-binding or the cellulose- and xylan-binding d omains, were equally effective in hydrolysing pulp xylan, enzyme deriv atives containing a polysaccharide-binding domain were marginally more efficient in reducing pulp kappa number. The reduction in kappa numbe r elicited by full-length and isolated catalytic domains of XylA and X ylD was reflected in an increase in the brightness of paper handsheets derived from pretreated pulps. Thus, the polysaccharide-binding domai ns of XylA and XylD did not appear to confer any advantage in terms of the ability of the enzymes to improve pulp bleachability. However, Xy lA and XylD, which belong to different glycosyl hydrolase families, di ffered in their ability to hydrolyse pulp xylan and facilitate the del ignification of kraft pulp.