Structural Dynamics of the Potassium Channel Blocker ShK: SRLS Analysis of (15)N Relaxation

The 35-residue ShK peptide binds with high affinity to voltage-gated potassium channels. The dynamics of the binding surface was studied recently with (microsecond to millisecond) (15)N relaxation dispersion and (picosecond to nanosecond) (15)N spin relaxation of the N-H bonds. Relaxation dispersion revealed microsecond conformational-exchange-mediated exposure of the functionally important Y23 side chain to the peptide surface. The spin relaxation parameters acquired at 14.1 and 16.45 T have been subjected to model-free (MF) analysis, which yielded a squared generalized order parameter, S(2), of approximately 0.85 for virtually all of the N-H bonds. Only a "rigid backbone" evaluation could be inferred. We ascribe this limited information to the simplicity of MF in the context of challenging data. To improve the analysis, we apply the slowly relaxing local structure (SRLS) approach, which is a generalization of MF. SRLS describes N-H bond dynamics in ShK in terms of a local potential, u, ranging from 10 to 18.5 kBT, and a local diffusion rate, D2, ranging from 4.2 × 10(8) to 2.4 × 10(10) s(-1). This analysis shows that u is outstandingly strong for Y23 and relatively weak for K22, whereas D2 is slow for Y23 and fast for K22. These observations are relevant functionally because of the key role of the K22-Y23 dyad in ShK binding to potassium channels. The disulfide-bond network exhibits a medium-strength potential and an alternating wave-like D2 pattern. This is indicative of moderate structural restraints and motional plasticity, in support of, although not directly correlated with, the microsecond binding-related conformational exchange process detected previously. Thus, new information on functionally important residues in ShK and its overall conformational stability emerged from the SRLS analysis, as compared with the previous MF-based estimate of backbone dynamics as backbone rigidity.