Thermus thermophilus無機ピロリン酸加水分解酵素のCys168置換による耐熱性と熱凝集への影響 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 exhibitshigh thermostability. However，the thermal aggregation containing the cross-linked dimer was observed afterheating above 85℃. Therefore，we focused on the sole cysteine (Cys168) in C-terminalregion，and evaluated theeffects 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 forthe enzyme activity and quatemary structure in wild type and all variants，while the other variants decreasedthem 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 fluorescenceexperiments. Therefore，we suggest that the small volume and less hydrophobicity of side chain at 168 positionmay contribute to the conformational thermostability, and substitution with Ala is the most suitable forthermostabilization and suppression ofthermal aggregation of Tth PPase.
徳島大学総合科学部自然科学研究 22, 75-84, 2008-12