The 4,4 '-dipyridyl disulfide-induced formation of GroEL monomers is cooperative and leads to increased hydrophobic exposure

The molecular chaperone, GroEL, is completely disassembled into monomers by the addition of 4,4 ' -dipyridyl disulfide. The dissociation leads to mono mers in a kinetically controlled process. The additions of functional ligan ds of GroEL such as Mg2+ or adenine nucleotides produced differences in the observed rates, but at the end of the kinetics, the dissociation was compl ete. In addition to the information obtained from native gels, the fluoresc ent probe bis-ANS was utilized to follow the monomer formation. The results demonstrate that the formation of monomers was associated with the exposur e of hydrophobic surfaces. This assessment was possible without the use of added chaotropes, such as urea, to dissociate GroEL. Dissociation kinetics were also followed by light scattering. The kinetics of dissociation of the 14mer are cooperative with respect to the concentration of 4,4 ' -DPDS. Th ermodynamic parameters for the kinetic process gave a free energy of activa tion (DeltaG*) of 19.3 +/- 1.2 kcal mol(-1), which was decomposed to an ent halpy of activation (DeltaH*) of 19.30 +/- 1.2 kcal mol-1 and an entropy of activation (DeltaS*) of -8.2 3.9 cal mol(-1) K-1. We conclude that the dis sociation of GroEL observed in this investigation is an enthalpy-controlled process.