Conformational dynamics and kinetics of peptide antagonist interactions with interleukin-1 receptor. Fluorescence studies using the NBD-labelled peptide AF12415.

Interleukin-1 plays a key role in the inflammatory response provoked by various disease states and inhibition of its action can bring therapeutic benefits. Steady-state and time-resolved studies of the intrinsic tryptophan fluorescence of the free soluble Type I form of interleukin-1 receptor (IL-1R) reveal that the rotational motions of the three major domains are strongly associated. Bound peptide antagonists are buried in hydrophobic regions, but a flexible association permits access to species from the aqueous phase. Ligand binding does not lead to rigidification of the receptor structure. The kinetics and mechanism of complex formation and dissociation, involving IL-1R with receptor antagonist protein (IL-1ra) and with peptides AF11733 (15 aa) and AF10961 (21 aa) were determined with the aid of peptide AF12415 (15 aa) labeled at its N-terminus by the NBD fluorophore, which exhibits a five-fold increase in emission intensity at 540 nm on binding of the peptide to IL-1R. The magnitude of the ON rate constant, typically 1 x 10(6) M-1 s-1, implies the existence of an intermediate 'encounter complex' involving interactions of low specificity. Readjustments of the initial encounter complex leads to a final complex where very specific interactions dominate. The first-order rate constant for this latter process is the most sensitive indicator of the true peptide affinity for the receptor binding site, and thus provides a better criterion than the apparent OFF rate (typically 2 x 10(-3) s-1) for discrimination of peptide antagonists.