Energetics of OCP1-OCP2 complex formation

OCP1 and OCP2, the most abundant proteins in the cochlea, are putative subunits of an SCF E3 ubiquitin ligase. Previous work has demonstrated that they form a heterodimeric complex. The thermodynamic details of that interaction are herein examined by isothermal titration calorimetry. At 25 °C, addition of OCP1 to OCP2 yields an apparent association constant of 4.0 × 107 M− 1. Enthalpically-driven (ΔH = − 35.9 kcal/mol) and entropically unfavorable (− TΔS = 25.5 kcal/mol), the reaction is evidently unaccompanied by protonation/deprotonation events. ΔH is strongly dependent on temperature, with ΔCp = − 1.31 kcal mol− 1 K− 1. Addition of OCP2 to OCP1 produces a slightly less favorable ΔH, presumably due to the requirement for dissociation of the OCP2 homodimer prior to OCP1 binding. The thermodynamic signature for OCP1/OCP2 complex formation is inconsistent with a rigid-body association and suggests that the reaction is accompanied by a substantial degree of folding.