L-Asparaginase II of E. coli is a kind of effective drug in the treatment of acute lymphoblastic leukaemia. However, during asparaginase therapy, repeated using of the drug is commonly needed because of the enzyme's relatively short half-life and instability in the processes of production and treatment. This leads to more serious toxic effects on patients. In order to stabilize the enzyme, a higher thermostable mutant L-asparaginase II was created in the present study by replacing Asp178 with proline in a hydrogen-bonded turn (178-18ODGR) which is contribute to the thermostability of the enzyme. The results displayed that values of K-m and K-cat for the mutant enzyme are not affected although the energy of activation is increased comparing to the wild-type enzyme. These data suggest that such alteration for L-asparaginase II enhances the thermostability of the enzyme without changing the enzyme's activity and thus the therapeutical use Of L-asparaginase II might be benefit from these results. (c) 2007 Elsevier Inc. All rights reserved.