Effect of Cys168 substitutions on the Thermostability and the Thermal Aggregation of Thermus thermophilus Inorganic Pyrophosphatase.

Thermus thermophilus Inorganic pyrophosphatase (Tth PPase) is comprised of homohexamer，and exhibits
high thermostability. However，the thermal aggregation containing the cross-linked dimer was observed after
heating above 85℃. Therefore，we focused on the sole cysteine (Cys168) in C-terminalregion，and evaluated the
effects of substitutions at this position on thermostability and thermal aggregation of Tth PPase.
Firstly，we prepared the four Cys168-substituted variants (C168A，L ，1，and F) by site-directed mutagenesis.
Although all variants formed hexamer in native state，C168A variant exhibited the highest thermostabilities for
the enzyme activity and quatemary structure in wild type and all variants，while the other variants decreased
them drastically as the side chain at the 168 position was much more bulky and hydrophobic in Tth PPase.
Moreover， suppression of thermal aggregation for C168A variant was observed in the ANS fluorescence
experiments. Therefore，we suggest that the small volume and less hydrophobicity of side chain at 168 position
may contribute to the conformational thermostability, and substitution with Ala is the most suitable for
thermostabilization and suppression ofthermal aggregation of Tth PPase.