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

The energetics of binding of MgADP to the isolated beta subunit of F(1)-ATPase from thermophilic Bacillus (Tbeta) was characterized by high-precision isothermal titration calorimetry. The reaction was enthalpically driven, with a DeltaCp of -36cal(molK)(-1). To gain insight into the molecular basis of this small DeltaCp, we analyzed the changes in accessible surface areas (DeltaASA) between the structures of empty and MgADP-filled beta subunits, extracted from the crystal structure of bovine heart F(1). Consistent with the experimental DeltaCp, the DeltaASA was small (-775A(2)). We used a reported surface area model developed for protein reactions to calculate DeltaCp and DeltaH from DeltaASA, obtaining good agreement with the experimental values. Conversely, using the same model, a DeltaASA of -770A(2) was estimated from experimental DeltaCp and DeltaH for the Tbeta-MgADP complex. Our structural-energetic study indicates that on MgADP binding the isolated Tbeta subunit exhibits intrinsic structural changes similar to those observed in F(1).