A mesophilic Aspergillus oryzae xylanase (AoXyn11A) belongs to glycoside hydrolase family 11. Hydrogen bonds and a disulfide bridge were introduced between the N-terminus extension and the beta-sheet A2 of AoXyn11A, which were located in the corresponding region of a hyperthermostable xylanase. The mutants were designated as AoXyn11A(C5) and AoXyn11A(C5-C32), respectively. The thermostabilities of AoXyn11A and the mutants were assessed by the molecular dynamics simulations. After being incubated at 55 A degrees C for 30 min, AoXyn11A(C5-C32) retained 49 % of its original activity, AoXyn11A(C5) retained 12 % and AoXyn11A retained 3 %. The interactions between the N-terminus extension and the beta-sheet A2 were analyzed in depth: there was enhancement of the interactions between the N-terminus extension and the beta-sheet A2 of AoXyn11A that improved its thermostability.