Escherichia coil L-asparaginases have great significance in the treatment of leukemia. Consequently, there is considerable interest in engineering this enzyme for improving its stability. In this work, the effect of surface charge on the stability of the enzyme L-asparaginase 11 was studied by site-directed mutagenesis of the cloned ansB gene from Escherichia sp. Replacement of two positively charged residues (K139 and K207) on the surface loops with neutral and reverse charges resulted in altered thermo stability in designed variants. Neutral charge substitutions (K139A and K207A) retained greater tolerance and stability followed by negative charge substitutions (K139D and K207D) compared to control mutant K139R and wild enzyme. From the results, it was concluded that the optimization of surface charge contributed much to the thermal properties of proteins without affecting the structure. (c) 2013 Elsevier Inc. All rights reserved.