Biochemical characterization of a novel thermostable GH11 xylanase with CBM6 domain from Caldicellulosiruptor kronotskyensis

The extreme thermophilic bacterium Caldicellulosiruptor kronotskyensis can use hemicelluloses and cellulose as carbohydrate source. The gene Calkro_0081 encoded a novel GH11 xylanase (Xyn11A) with a catalytic domain (GH-CD) and a carbohydrate binding module (CBM6). The native Xyn11A and two corresponded truncations Xyn11A-CD (catalytic domain of Xyn11A) and Xyn11A-CBM (carbohydrate binding module of Xyn11A) were respectively cloned, heterologously expressed, and biochemically characterized. The recombinant Xyn11A is active in a wide temperature range from 40 to 95 °C with the highest activity at 75 °C. Xyn11A was completely stable at 70 °C for 6 h and more than 50% activity was retained after incubation for 6 h at 75 °C. The optimum pH of Xyn11A was 6.0, and it retained 100% activity after 15 h incubation in pH 5.5–7.5 at 70 °C. As for Xyn11A-CD, the optimal pH value and temperature were 6.0 and 65 °C, the residual activity retained 30% after 6 h at 60 °C. The respective specific activities of Xyn11A, Xyn11A-CD, and Xyn11A-CBM were 1752.0, 986.8, and 0 IU/mg on beechwood xylan (BWX) at optimum conditions. The activity of Xyn11A is the highest among the reported thermostable xylanases at 75 °C. Using BWX as substrate, the final products were xylose and xylobiose after hydrolysis with Xyn11A or Xyn11A-CD. No hydrolytic activity of CBM6 was found, while which shows crucial functions on thermostability and activity for Xyn11A. The characteristics of thermostability and high activity make Xyn11A a potential enzyme for industry application.