Structural energetics of MgADP binding to the isolated β subunit of F1-ATPase from thermophilic Bacillus PS3

The energetics of binding of MgADP to the isolated β subunit of F1-ATPase from thermophilic Bacillus (Tβ) was characterized by high-precision isothermal titration calorimetry. The reaction was enthalpically driven, with a ΔCp of −36 cal (mol K)−1. To gain insight into the molecular basis of this small ΔCp, we analyzed the changes in accessible surface areas (ΔASA) between the structures of empty and MgADP-filled β subunits, extracted from the crystal structure of bovine heart F1. Consistent with the experimental ΔCp, the ΔASA was small (−775 Å2). We used a reported surface area model developed for protein reactions to calculate ΔCp and ΔH from ΔASA, obtaining good agreement with the experimental values. Conversely, using the same model, a ΔASA of −770 Å2 was estimated from experimental ΔCp and ΔH for the Tβ–MgADP complex. Our structural–energetic study indicates that on MgADP binding the isolated Tβ subunit exhibits intrinsic structural changes similar to those observed in F1.