EVIDENCE THAT LINKER SEQUENCES AND CELLULOSE-BINDING DOMAINS ENHANCE THE ACTIVITY OF HEMICELLULASES AGAINST COMPLEX SUBSTRATES

Xylanase A (XYLA) and arabinofuranosidase C (XYLC) from Pseudomonas fl uorescens subsp. cellulosa are modular enzymes consisting of discrete cellulose-binding domains (CBDs) and catalytic domains joined by serin e-rich linker sequences. To evaluate the role of the CBDs and interdom ain regions, the capacity of full-length and truncated derivatives of the two enzymes, lacking either the linker sequences or CBDs, to hydro lyse a range of substrates, and bind to cellulose, was determined. Rem oval of the CBDs did not affect either the activity of XYLA or XYLC ag ainst soluble arabinoxylan. Similarly, deletion of the linker sequence s did not alter the affinity of the enzymes for cellulose or their act ivity against soluble substrates, even when bound to cellulose via the CBDs. Truncated derivatives of XYLA lacking either the linker sequenc es or the CBD were less active against xylan contained in cellulose-he micellulose complexes, compared with the full-length xylanase. Similar ly, removal of the CBD from XYLC diminished the activity of the enzyme (XYLC''') against plant-cell-wall material containing highly substitu ted arabinoxylan. The role of CBDs and linker sequences in the catalyt ic activity of hemicellulases against the plant cell wall is discussed .