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J Biol Chem, Vol. 273, Issue 30, 19102-19107, July 24, 1998

Structural and Energetic Factors of the Increased Thermal Stability in a Genetically Engineered Escherichia coli Adenylate Kinase

Simona Burlacu-Miron, Véronique Perrier, Anne-Marie Gilles, Elisabeth Pistotnik, and Constantin T. Craescu

From INSERM U350, Institut Curie-Recherche, 91405 Orsay Cedex, France and  Laboratoire de Chimie Structurale des Macromolécules (URA D1129), Institut Pasteur, 7524 Paris Cedex 15, France

Several variants of Escherichia coli adenylate kinase, designed to bind a Zn²⁺ ion, were produced by site-directed mutagenesis. The metal binding and enzymatic properties of the engineered variants have been described (Perrier, V., Burlacu-Miron, S., Bourgeois, S., Surewicz, W. K., and Gilles, A.-M. (1998) J. Biol. Chem. 273, 19097-19101). Here we report the structural properties and stability changes in a 4-Cys variant which binds a Zn²⁺ ion and has an increased thermal stability. CD studies indicate a very similar secondary structure content in the wild type and the engineered variant. NMR analysis revealed that the topology of the parallel -sheet, belonging to the protein core, and of the peripheral antiparallel -sheet are also conserved. The small local changes observed in the neighborhood of the substitution sites reflect a more compact state of the metal-binding domain. The Zn²⁺-bound quadruple mutant shows an increased thermal stability, reflected in a 9 °C increase of the mid-temperature of the first cooperative unfolding step. Binding of a bisubstrate analog P¹,P⁵-di(adenosine-5')-pentaphosphate increases, by about 7 °C, the midpoint of this transition in both wild type and modified variant. The NMR data suggest that the peripheral domains involved in substrate binding unfold during the first denaturation step. Urea denaturation experiments indicate an increased resistance against chemical unfolding of the Zn²⁺-binding variant. In contrast, the Gibbs free energy of unfolding (at physiologically relevant conditions) of the quadruple mutant is lower than that of the wild type.

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