Cumulative effect of amino acid replacements results in enhanced thermostability of potato type L alpha-glucan phosphorylase.

The thermostability of potato type L alpha-glucan phosphorylase (EC 2.4.1.1) was enhanced by random and site-directed mutagenesis. We obtained three single-residue mutations-Phe39-->Leu (F39L), Asn135-->Ser (N135S), and Thr706-->Ile (T706I)-by random mutagenesis. Although the wild-type enzyme was completely inactivated, these mutant enzymes retained their activity even after heat treatment at 60 degrees C for 2 h. Combinations of these mutations were introduced by site-directed mutagenesis. The simultaneous mutation of two (F39L/N135S, F39L/T706I, and N135S/T706I) or three (F39L/N135S/T706I) residues further increased the thermostability of the enzyme, indicating that the effect of the replacement of the residues was cumulative. The triple-mutant enzyme, F39L/N135S/T706I, retained 50% of its original activity after heat treatment at 65 degrees C for 20 min. Further analysis indicated that enzymes with a F39L or T706I mutation were resistant to possible proteolytic degradation.