COMPARISON OF BACKBONE DYNAMICS OF APO-ACYL-COENZYME-A AND HOLO-ACYL-COENZYME-A BINDING-PROTEIN USING N-15 RELAXATION MEASUREMENTS

N-15 magnetic relaxation parameters T-1, T-2 and the nuclear Overhauser effect in the protein acyl-coenzyme A binding protein (ACBP) have been measured in the presence and absence of the ligand, palmitoyl-coenzyme A, in order to obtain information about local and global dynamical properties of the peptide backbone with and without the ligand bound in the binding site. The three-dimensional structures of acyl-coenzyme A binding protein are known for both states of the protein as determined from multidimensional heteronuclear NMR studies, and they have been shown to be essentially identical. However, the dynamics of the backbone is influenced by the presence of ligand in the binding site. The binding of ligand had significant and specific effects on the relaxation time T-1 for many of the N-15 in the peptide backbone, in particular those near residues with contacts to the ligand. Similarly, the nuclear Overhauser effect at N-15 near such residues increased. There were no significant changes in the T-2 relaxation. T-1 values showing a significant decrease and NOEs increasing in regions close to the binding site when the ligand was bound suggest two modes of action on the dynamics of the protein when the ligand is binding. The reduced T-1 indicates motion of lower amplitude in agreement with the structural constraints introduced by protein-ligand interactions. The increased NOEs may be a consequence of shorter time constants for dynamics of the atoms close to the binding site. The Lipari-Szabo model could not be satisfactorily applied to the entire set of experimental data. In particular, many of the measured NOE values exceeded the theoretical maximum predicted in this model.