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Electrostatic Contributions to the Kinetics and Thermodynamics of Protein Assembly

Department of Biophysical Chemistry, Lund University, Lund, Sweden

Correspondence: Address reprint requests to Sara Linse, Biophysical Chemistry, Chemical Centre, Lund University, S-221 00 Lund, Sweden. Tel.: 46-46-222-8246; Fax: 46-46-222-4543; E-mail: sara.linse{at}bpc.lu.se.

The role of electrostatic interactions in the assembly of a native protein structure was studied using fragment complementation. Contributions of salt, pH, or surface charges to the kinetics and equilibrium of calbindin D_9k reconstitution was measured in the presence of Ca²⁺ using surface plasmon resonance and isothermal titration calorimetry. Whereas surface charge substitutions primarily affect the dissociation rate constant, the association rates are correlated with subdomain net charge in a way expected for Coulomb interactions. The affinity is reduced in all mutants, with the largest effect (260-fold) observed for the double mutant K25E+K29E. At low net charge, detailed charge distribution is important, and charges remote from the partner EF-hand have less influence than close ones. The effects of salt and pH on the reconstitution are smaller than mutational effects. The interaction between the wild-type EF-hands occurs with high affinity (K_A = 1.3 x 10¹⁰ M^–1; K_D = 80 pM). The enthalpy of association is overall favorable and there appears to be a very large favorable entropic contribution from the desolvation of hydrophobic surfaces that become buried in the complex. Electrostatic interactions contribute significantly to the affinity between the subdomains, but other factors, such as hydrophobic interactions, dominate.

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