Redistribution and loss of side chain entropy upon formation of a calmodulin-peptide complex

The response of the internal dynamics of calcium-saturated calmodulin to th e formation of a complex with a peptide model of the calmodulin-binding dom ain of the smooth muscle myosin light chain kinase has been studied using N MR relaxation methods. The backbone of calmodulin is found to be unaffected by the binding of the domain, whereas the dynamics of side chains are sign ificantly perturbed. The changes in dynamics are interpreted in terms of a heterogeneous partitioning between structure (enthalpy) and dynamics (entro py), These data provide a microscopic view of the residual entropy of a pro tein in two functional states and suggest extensive enthalpy/entropy exchan ge during the formation of a protein-protein interface.