Slow conformational changes in the rigid and highly stable chymotrypsin inhibitor 2

Slow conformational changes are often directly linked to protein function. It is however less clear how such processes may perturb the overall folding stability of a protein. We previously found that the stabilizing double mutant L49I/I57V in the small protein chymotrypsin inhibitor 2 from barley led to distributed increased nano second and faster dynamics. Here we asked what effect this mutant and the two individual mutants L49I and I57V have on the slow conformational dynamics of CI2. We used 15N CPMG spin relaxation dispersion experiments to measure the kinetics, thermodynamics and structural changes associated with slow conformational change in CI2. These changes result in an excited state that is populated to 4.3% at 1 °C. As the temperature is increased the population of the excited state decreases. Structural changes in the transition to the excited state are associated with residues that interact with water molecules that have well defined positions and are found at these positions in all crystal structures of CI2. The mutations in CI2 have only little effect on the structure of the excited state whereas the stability of the excited state to some extent follows the stability of the main state. The minor state is thus most populated for the most stable CI2 variant and least populated for the least stable variant. We hypothesize that the interactions between the mutated residues and the well-ordered water molecules links subtle structural changes around the mutated residues to the region in the protein that experience slow conformational changes.