Experimental assignment of the structure of the transition state for the association of barnase and barstar

Association of a protein complex follows a two step reaction mechanism, with the first step being the formation of an encounter complex which evolves into the final complex. Here we present new experimental data for the association of the bacterial ribonuclease barnase and its polypeptide inhibitor barstar which shed light on the thermodynamics and structure of the transition state and preceding encounter complex of association at diminishing electrostatic attraction. We show that the activation entropy at the transition state is close to zero, with the activation enthalpy being equal to the free energy of binding. This observation was independent of the magnitude of the mutual electrostatic attraction, which were altered by mutagenesis or by addition of salt. The low activation entropy implies that the transition state is mostly solvated at all ionic strengths. The structure of the transition state was probed by measuring pairwise interaction energies using double-mutant-cycles. While at low ionic strength all proximal charge-pairs form contacts, at high salt only a subset of these interactions are maintained. More specifically, charge-charge interactions between partially buried residues are lost, while exposed charged residues maintain their ability to form specific interactions even at the highest salt concentration. Uncharged residues do not interact at any ionic strength. The results presented here suggest that the barnase-barstar binding sites are correctly aligned during the transition state even at diminishing electrostatic attraction, although specific short range interactions of uncharged residues are not yet formed. Furthermore, most of the interface desolvation (which contributes to the entropy of the system) has not yet occurred. This picture seems to be valid at low and high salt. However, at high salt, interactions of the activated complex are limited to a more restricted set of residues which are easier approached during diffusion, prior to final docking. This suggest that the steering region at high salt is more limited, albeit maintaining its specificity.