Energetics of OCP1–OCP2 complex formation Original Research Article

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.