Over the last two decades, hundreds of circular proteins (with N- and C- termini linked together) have been discovered in nature with a wide range of biological activities. While much progress has been made in elucidating the structure and corresponding biological activities of circular proteins, the physical basis of how the circular topology may modulate the functions of proteins (e.g. molecular recognitions) remains elusive. In this study, we aim to understand the effect of cyclization on conformational entropy of an artificial circular protein. We modified Calmodulin (CaM) at its N- and C-termini and used the formation of disulfide bond to facilitate its cyclization. Our ITC studies revealed that in contrast to its linear counterpart, binding of cyclic CaM with its ligand is contributed more from the favorable change in entropy of binding than the enthalpy of binding. This change can be attributed to loss of conformational entropy upon cyclization. To further qualify this change, we characterized the backbone dynamics of modified CaM using NMR relaxation techniques. Here we will present our ITC and NMR data, which indicate that changes in protein conformational entropy upon cyclization can be used to modulate the free energy of protein-ligand interactions.

Effect of cyclization on protein's conformational entropy and internal dynamics

248th National Meeting (2014)

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