Backbone and methyl dynamics of the regulatory domain of troponin C:: Anisotropic rotational diffusion and contribution of conformational entropy to calcium affinity

The N-terminal domain (residues 1 to 90) of chicken skeletal troponin C (NTnC) regulates muscle contraction upon the binding of a calcium ion to each of its two calcium binding loops. In order to characterize the backbone dynamics of NTnC in the apo state (NTnC-apo), we measured and carefully analyzed N-15 NMR relaxation parameters T-1, T-2 and NOE at H-1 NMR frequencies of 500 and 600 MHz. The overall rotational correlation time of NTnC-apo at 29.6-degrees-C is 4.86 (+/-0.15) ns. The experimental data indicate that the rotational diffusion anistropy D-2/D-1 of 1.10. Additionally, the dynamic properties of side-chains having a methyl group were derived from H-2 relaxation data of CH2D groups of a partially deuterated sample. Based on the dynamic characteristics of TnC, two different levels of "fine tuning" of the calcium affinity are presented. Significantly lower backbone order parameters (S-2), were observed for calcium binding site I relative to site II and the contribution of the bond vector fluctuations to the conformational entropy of sites I and II. This is consistent with different dissociation constants previously measured for sites I and II of 16 muM and 1.7 muM, respectively. In addition to the direct role of binding loop dynamics, the side-chain methyl group dynamics play an indirect role through the energetics of the calcium-induced structural change form aclosed to an open state. Our results show thatn the side-chains which will be exposed upon calcium binding have reduced motion in the apo state, suggesting that conformational entropic contributions can be used to offset the free energy cost of exposing hydrophobic groups. It is clear from this work that a complete determination of their dynamic characteristics is necessary in order to fully understand how TnC and other proteins are fine tuned to appropriately carry out their function.