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 Overhause r effect in the protein acyl-coenzyme A binding protein (ACBP) have be en measured in the presence and absence of the ligand, palmitoyl-coenz yme 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 determi ned from multidimensional heteronuclear NMR studies, and they have bee n shown to be essentially identical. However, the dynamics of the back bone is influenced by the presence of ligand in the binding site. The binding of ligand had significant and specific effects on the relaxati on time T-1 for many of the N-15 in the peptide backbone, in particula r those near residues with contacts to the ligand. Similarly, the nucl ear Overhauser effect at N-15 near such residues increased. There were no significant changes in the T-2 relaxation. T-1 values showing a si gnificant decrease and NOEs increasing in regions close to the binding site when the ligand was bound suggest two modes of action on the dyn amics of the protein when the ligand is binding. The reduced T-1 indic ates motion of lower amplitude in agreement with the structural constr aints introduced by protein-ligand interactions. The increased NOEs ma y be a consequence of shorter time constants for dynamics of the atoms close to the binding site. The Lipari-Szabo model could not be satisf actorily applied to the entire set of experimental data. In particular , many of the measured NOE values exceeded the theoretical maximum pre dicted in this model.