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

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To increase the thermostability of β-amylase, seven kinds of single-mutant plasmids were constructed with an expression vector of barley β-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°C. To express the sevenfoldmutant β-amylase in Escherichia coli, plasmid pB927 was constructed. E. coli harboring plasmid pB927 produced sevenfold-mutant β-amylase. The T50 value of purified sevenfoldmutant β-amylase (69.0°C) was higher than that of not only the original recombinant β-amylase (57.4°C) by 11.6°C but also soybean β-amylase (63.2°C) by 5.8°C. The intragenic amino acid replacements were found to have simple additive effects on the thermostability of β-amylase. The sevenfold-mutant β-amylase was found to be stable at pHs up to 12.5, while the original recombinant β-amylase was unstable at pHs above 9.5. The data obtained from kinetics studies suggested that the sevenfold-mutant β-amylase acquired enhanced thermostability, but its function as a β-amylase remained unchanged.

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