Evaluation of cooperative interactions between substructures of Iso-1-cytochrome c using double mutant cycles

A double mutant cycle has been used to evaluate interaction energies between the global stabilizer mutation asparagine 52 → isoleucine (N52I) in iso-1-cytochrome c and mutations producing single surface histidines at positions 26, 33, 39, 54, 73, 89, and 100. These histidine mutation sites are distributed through the four cooperative folding units of cytochrome c. The double mutant cycle starts with the iso-1-cytochrome c variant AcTM, a variant with no surface histidines and with asparagine at position 52. Isoleucine is added singly at position 52, AcTMI52 variant, as are the surface histidines, AcHX variants, where X indicates the histidine sequence position. The double mutant variants, AcHXI52, provide the remaining corner of the double mutant cycle. The stabilities of all variants were determined by guanidine hydrochloride denaturation and interaction energies were calculated between position 52 and each histidine site. Six of the seven double mutants show additive (AcH33I52, AcH39I52, AcH54I52, AcH89I52, and AcH100I52) stability effects or weak interaction energies (AcH73I52) of the histidine mutations and the N52I mutation, consistent with cooperative effects on protein folding and stability being sparsely distributed through the protein structure. The AcH26I52 variant shows a strong favorable interaction energy, 2.0 ± 0.5 kcal/mol, between the N52I mutation in one substructure and the addition of His 26 to an adjacent substructure. The data are consistent with an entropic stabilization of the intersubstructure hydrogen bond between His 26 and Glu 44 by the Ile 52 mutation.