NMR studies of the structure and dynamics of peptide E, an endogenous opioid peptide that binds with high affinity to multiple opioid receptor subtypes.

Structural and dynamic properties of opioid peptide E have been examined in an sodium dodecyl sulfate (SDS) micelle. Structural and dynamic studies both indicate that this peptide exhibits greater segmental mobility than typical structured proteins. An nmr structural analysis of adrenal peptide E in SDS micelles indicated the presence of two well-defined beta-turns, one at the N-terminus encompassing residues 3 to 6, and the second in the region between residues 15 and 18. Certain side chain dihedral angles were also remarkably well defined, such as the chi 1 angle of F4, which exhibited a trans configuration. These calculated structures were based on a set of 9.5 restraints per residue. The backbone dynamics of peptide E in SDS micelles were examined through an analysis of 15N-relaxation parameters. An extended model-free analysis was used to interpret the relaxation data. The overall rotational correlation time is 19.7 ns. the average order parameter S2 is 0.66 +/- 0.15. The N-terminal loop region residues including G3 to R6 have an average order parameter of 0.70 +/- 0.23. The average order parameter lies somewhere between that observed for a random coil (e.g., S2 = 0.3) and that of a well-defined tertiary fold (e.g., S2 = 0.86). This suggests that peptide E in SDS micelles adopts a restricted range of conformations rather than a random coil. Based on the helical structure recently obtained for the highly homologous kappa-agonist dynorphin-A(1-17) and the beta-turn in the same region of peptide E, it is reasonable to assume that these two elements of secondary structure reflect different receptor subtype binding geometries. The intermediate order parameters observed for peptide E in an SDS micelle suggest a degree of dynamic mobility that may enable facile interconversion between helical and beta-turn geometries in the N-terminal agonist domain.