FREE-ENERGY OF BURYING HYDROPHOBIC RESIDUES IN THE INTERFACE BETWEEN PROTEIN SUBUNITS

We have obtained an experimental estimate of the free energy change as sociated with variations at the interface between protein subunits, a subject that has raised considerable interest since the concept of acc essible surface area was introduced by Lee and Richards [Lee, E. & Ric hards, F. M. (1971) J. Mol. Biol. 55, 379-400], We determined by analy tical ultracentrifugation the dimer-tetramer equilibrium constant of f ive single and three double mutants of human Kb, One mutation is at th e stationary alpha(1) beta(1) interface, and all of the others are at the sliding alpha(1) beta(2) interface where cleavage of the tetramer into dimers and ligand-linked allosteric changes are known to occur. A surprisingly good linear correlation between the change in the free e nergy of association of the mutants and the change in buried hydrophob ic surface area was obtained, after corrections for the energetic cost of losing steric complementarity at the ap dimer interface, The slope yields an interface stabilization free energy of -15 +/- 1.2 cal/mol upon burial of 1 Angstrom(2) of hydrophobic surface, in very good agre ement with the theoretical estimate given by Eisenberg and McLachlan [ Eisenberg, D. & McLachlan, A. D. (1986) Nature (London) 319, 199-203].