Kinetic and thermodynamic characterization of the R17 coat protein-ribonucleic acid interaction

Abstract

A filter retention assay is used to examine the kinetic and equilibrium properties of the interaction between phage R17 coat protein and its 21 nucleotide RNA binding site. The kinetics of the reaction are consistent with the equilibrium association constant and indicate a diffusion-controlled reaction. The temperature dependence of Ka gives .DELTA.H [enthalpy change] = -19 kcal/mol. This large favorable .DELTA.H is partially offset by a .DELTA.S [entropy change] = -30 cal mol-l deg-1 to give a .DELTA.G [Gibbs free energy change] = -11 kcal/mol at 2.degree. C in 0.19 M salt. The binding reaction has a pH optimum centered around pH 8.5, but pH has no effect on .DELTA.H. While the interaction is insensitive to the type of monovalent cation, the affinity decreases with the lyotropic series among monovalent anions. The ionic strength dependence of Ka reveals that ionic contacts contribute to the interaction. Most of the binding free energy is a result of nonelectrostatic interactions.