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Journal of Biochemistry 2005 138(4):343-353; doi:10.1093/jb/mvi133
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© 2005 The Japanese Biochemical Society

Regular Paper

Stabilization Mechanism of the Tryptophan Synthase {alpha}-Subunit from Thermus thermophilus HB8: X-Ray Crystallographic Analysis and Calorimetry

Yukuhiko Asada1, Masahide Sawano1, Kyoko Ogasahara2, Junji Nakamura3, Motonori Ota4, Chizu Kuroishi1, Mitsuaki Sugahara1, Katsuhide Yutani1 and Naoki Kunishima1,*

1 Advanced Protein Crystallography Research Group, RIKEN Harima Institute at SPring-8, 1-1-1 Kouto, Mikazuki-cho, Sayo-gun, Hyogo 679-5148; 2 Institute for Protein Research, Osaka University, 3-2 Yamadaoka, Suita, Osaka 565-0871; 3 Nihon SiberHegner KK, Tokyo Ryutsu Center, BE4-1, 6-1-1 Heiwajima, Oota, Tokyo 143-6591; and 4 Global Scientific Information and Computing Center, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguroku, Tokyo 152-8550

* To whom correspondence should be addressed. Tel: +81-791-58-2937, Fax: +81-791-58-2917, E-mail: kunishima{at}spring8.or.jp

In order to elucidate the thermo-stabilization mechanism of the tryptophan synthase {alpha}-subunit from the extreme thermophile Thermus thermophilus HB8 (Tt-{alpha}-subunit), its crystal structure was determined and its stability was examined using DSC. The results were compared to those of other orthologs from mesophilic and hyperthermophilic organisms. The denaturation temperature of the Tt-{alpha}-subunit was higher than that of the {alpha}-subunit from S. typhimurium (St-{alpha}-subunit) but lower than that of the {alpha}-subunit from P. furiosus (Pf-{alpha}-subunit). Specific denaturation enthalpy and specific denaturation heat capacity values of the Tt-{alpha}-subunit were the lowest among the three proteins, suggesting that entropy effects are responsible for the stabilization of the Tt-{alpha}-subunit. Based on a structural comparison with the St-{alpha}-subunit, two deletions in loop regions, an increase in the number of ion pairs and a decrease in cavity volume seem to be responsible for the stabilization of the Tt-{alpha}-subunit. The results of structural comparison suggest that the native structure of the Tt-{alpha}-subunit is better adapted to an ideally stable structure than that of the St-{alpha}-subunit, but worse than that of the Pf-{alpha}-subunit. The results of calorimetry suggest that the residual structure of the Tt-{alpha}-subunit in the denatured state contributes to the stabilization.


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