Highly thermostable purified xylanase from Rhizomucor miehei NRRL 3169

A thermostable xylanase was purified and characterized from the thermophilic fungus Rhizomucor miehei (Cooney & R.Emers.) Schipper. The enzyme was purified 29.1-fold to homogeneity by ammonium sulfate precipitation, sephadex G-100 gel filtration and diethylaminoethyl cellulose anion exchange chromatography. The enzyme was highly active with a range of pH from 5.0 to 6.5. The optimum temperature of the purified enzyme was 75 degrees C. The enzyme showed high thermal stability at 70 and 75 degrees C, and the half-life of the xylanase at 90 degrees C was 30 min. Km and Vmax values at 50 degrees C of the purified enzyme were 0.055 mg/ml and 113.5 mu mol min(-1) mg(-1), respectively. The enzyme was activated by Ca2+, Cu2+, K+ and Na+. On the other hand, Ag2+, Hg2+, Ba2+, and Zn2+ inhibited the enzyme. The molecular weight of the xylanase was estimated to be 27 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The present study is among the first work to examine and describe a secreted highly thermostable xylanase from the Rhizomucor miehei fungus. This enzyme displays a number of biochemical properties that make it a potentially strong candidate for industrial and commercial application in pulp bleaching.