Wood is composed mainly of lignin, hemicellulose and cellulose fractions. Xylans constitute the main polymeric component of the hemicellulose fraction of plant cell walls. Endo-xylanases (E.C. 3.2.1.8) are glycosyl-hydrolases, which hydrolyse internal b-1,4 bonds in the main chain of xylan. In the transformation of wood to paper, the lignin fraction must be eliminated and this occurs in two stages: the Kraft process and pulp bleaching. In this process, a brown pulp is obtained with a residual lignin content, which is eliminated in the subsequent bleaching process. Most paper industries have used a bleaching sequence based on chlorine dioxide and extraction by alkaline solutions.  In this, the percentage of chlorine used in the bleaching process is calculated from the brown pulp Kappa number. Xylanases are the focus of research to harness this catalytic function since they have potential as an environmentally acceptable alternative to chemical bleaching of wood pulp in the Kraft process. We have previously cloned the b-1,4 endoxylanase from Bacillus subtilis into the pT7T3 vector, and after transformation of Escherichia coli DH5a with this construct, recombinant XynA was secreted into the culture supernatant and could be purified by ethanol precipitation and cation exchange chromatography. Hydrolytic activity of the purified recombinant protein was determined by the DNS method, and the Kappa number was determined for the crude supernatant of culture and purified protein by monitoring the reaction of pulp samples with potassium permanganate. The optimum conditions for the experimental kappa number determination was 36 U of recombinant xylanase at 8g dry pulp mass per 100g of water, at 55 ⁰C for 1h. After the enzymatic treatment, the amount of ClO2 necessary to obtain the same Kappa number was reduced to 6,45 and brightness of the pulp increased by 24,92% in comparison to the untreated samples.

Supported by FAPESP, CNPq and PRP-USP