ADSORPTION OF CLOSTRIDIUM-STERCORARIUM XYLANASE-A TO INSOLUBLE XYLAN AND THE IMPORTANCE OF THE CBD TO XYLAN HYDROLYSIS

Xylanase A (XynA) from Clostridium stercorarium, which consists of a c atalytic domain and two family VI cellulose-binding domains (CBDs) eac h connected by a Linker sequence, was found to bind to insoluble oat-s pelt xylan as well as acid-swollen cellulose (ASC). Its adsorption to xylan was greatly dependent on the concentration of the phosphate buff er in the binding assay mixtures. The adsorption of XynA to insoluble xylan proceeded in accordance with a Langmuir-type isotherm. The Ka an d [PX](max) values of XynA for oat-spelt were 0.17 l/mu mol and 5.0 mu mol/g, respectively. Removal of the CBDs from XynA abolished the cell ulose-and xylan-binding abilities of this enzyme and reduced the enzym e activity toward insoluble xylan but not soluble xylan. These results clearly indicated that the CBDs of XynA play an important role in the hydrolysis of insoluble xylan. Desorption of XynA from the ASC-XynA c omplex was effected by soluble saccharide solutions such as barley bet a-glucan, birchwood xylan, and glucose solutions as well as a cellobio se solution, indicating that the CBDs of XynA have an affinity for sol uble saccharides in addition to insoluble polysaccharides.