Construction of a Plasmid Used for the Expression of a Sevenfold-Mutant Barley β-Amylase with Increased Thermostability in Escherichia coli and Properties of the Sevenfold-Mutant β-Amylase

To increase the thermostability of beta-amylase, seven kinds of single-mutant plasmids were constructed with an expression vector of barley beta-amylase by mutagenesis. The remaining activity versus temperature curves were used to determine the temperatures (T50) at which 50% of the initial activity was lost during a 30-min heating period. These mutations increased the T50 values by amounts ranging from 0.8 to 3.2 degrees C. To express the sevenfold-mutant beta-amylase in Escherichia coli, plasmid pB927 was constructed. E. coli harboring plasmid pB927 produced sevenfold-mutant beta- amylase. The T50 value of purified sevenfold-mutant beta-amylase (69.0 degrees C) was higher than that of not only the original recombinant beta-amylase (57.4 degrees C) by 11.6 degrees C but also soybean beta-amylase (63.2 degrees C) by 5.8 degrees C. The intragenic amino acid replacements were found to have simple additive effects on the thermostability of beta-amylase. The sevenfold-mutant beta-amylase was found to be stable at pHs up to 12.5, while the original recombinant beta-amylase was unstable at pHs above 9.5. The data obtained from kinetics studies suggested that the sevenfold-mutant beta-amylase acquired enhanced thermostability, but its function as a beta-amylase remained unchanged.