15N NMR relaxation studies of free and ligand-bound human acidic fibroblast growth factor

N-15 NMR relaxation data have been used to characterize the backbone dynamics of the human acidic fibroblast growth factor (hFGF-1) in its free and sucrose octasulfate (SOS)-bound states. N-15 longitudinal (R-1), transverse (R-2) relaxation rates and {H-1}-N-15 steady-state nuclear Overhauser effects were obtained at 500 and 600 MHz (at 25 degreesC) for all resolved backbone amide groups using H-1- detected two-dimensional NMR experiments. Relaxation data were fit to the extended model free dynamics for each NH group. The overall correlation time (tau (m)) for the free and SOS-bound forms were estimated to be 10.4 +/- 1.07 and 11.1 +/- 1.35 ns, respectively. Titration experiments with SOS reveals that the Ligand binds specifically to the C-terminal domain of the protein in a 1:1 ratio. Binding of SOS to hFGF-1 is found to induce a subtle conformational change in the protein. Significant conformational exchange (R-ex) is observed for several residues in the free form of the protein. How ever, in the SOS-bound form only three residues exhibit significant R-ex values, suggesting that the dynamics on the micro- to millisecond time scale in the free form is coupled to the cts-trans-proline isomerization. hFGF-1 is a rigid molecule with an average generalized parameter (S-2) value of 0.89 +/- 0.03. Upon binding to SOS, there is a marked decrease in the overall flexibility (S-2 = 0.94 +/- 0.02) of the hFGF-1 molecule. However, the segment comprising residues 103-111 shows increased flexibility in the presence of SOS. Significant correlation is found between residues that show high flexibility and the putative receptor binding sites on the protein.