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Protein Engineering, Vol. 14, No. 12, 961-966, December 2001
© 2001 Oxford University Press

High thermal stability of 3-isopropylmalate dehydrogenase from Thermus thermophilus resulting from low {Delta}{Delta}Cp of unfolding

Chie Motono, Tairo Oshima and Akihiko Yamagishi,1

Department of Molecular Biology, Tokyo University of Pharmacy and Life Science, 1432 Horinouchi, Hachioji, Tokyo 192-0392, Japan

To characterize the thermal stability of 3-isopropylmalate dehydrogenase (IPMDH) from an extreme thermophile, Thermus thermophilus, urea-induced unfolding of the enzyme and of its mesophilic counterpart from Escherichia coli was investigated at various temperatures. The unfolding curves were analyzed with a three-state model for E.coli IPMDH and with a two-state model for T.thermophilus IPMDH, to obtain the free energy change {Delta}{Delta}G° of each unfolding process. Other thermodynamic parameters, enthalpy change {Delta}{Delta}H, entropy change {Delta}{Delta}S and heat capacity change {Delta}{Delta}Cp, were derived from the temperature dependence of {Delta}{Delta}G°. The main feature of the thermophilic enzyme was its lower dependence of {Delta}{Delta}G° on temperature resulting from a low {Delta}{Delta}Cp. The thermophilic IPMDH had a significantly lower {Delta}{Delta}Cp, 1.73 kcal/mol.K, than that of E.coli IPMDH (20.7 kcal/mol.K). The low {Delta}{Delta}Cp of T.thermophilus IPMDH could not be predicted from its change in solvent-accessible surface area {Delta}{Delta}ASA. The results suggested that there is a large structural difference between the unfolded state of T.thermophilus and that of E.coli IPMDH. Another responsible factor for the higher thermal stability of T.thermophilus IPMDH was the increase in the most stable temperature Ts. The {Delta}{Delta}G° maximum of T.thermophilus IPMDH was much smaller than that of E.coli IPMDH. The present results clearly demonstrated that a higher melting temperature Tm is not necessarily accompanied by a higher {Delta}{Delta}G° maximum.


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