CORRELATED BOND ROTATIONS IN INTERACTIONS OF ARGININE RESIDUES WITH LIGAND CARBOXYLATE GROUPS IN PROTEIN LIGAND COMPLEXES

The H-1/N-15 HSQC NMR spectra of complexes of Lactobacillus casei dihy drofolate reductase containing methotrexate recorded at 1 degrees C sh ow four resolved signals for the four NHeta protons of the Arg(57) res idue, This is consistent with hindered rotation in the guanidino group resulting from interactions with the alpha-carboxylate of methotrexat e, Increasing the temperature causes exchange line-broadening and coal escence of signals, Rotation rates for the (NCzeta)-C-epsilon and (CNe ta)-N-zeta bonds have been calculated from lineshape analysis and from zz-HSQC exchange experiments. The interactions between the methotrexa te alpha-carboxylate group and the Arg(57) guanidino group decrease th e rotation rates for the (NCzeta)-C-epsilon bond by about a factor of 10 and those for the (CNeta)-N-zeta bonds by more than a factor of 100 with respect to their values in free arginine, Furthermore, the relat ive rates of rotation about these two bonds are reversed in the protei n complexes compared with their values in free arginine indicating tha t there are concerted rotations about the (NCzeta)-C-epsilon bond of t he Arg(57) guanidino group and the C'C-alpha bond of the glutamate alp ha-carboxylate group of methotrexate. (C) 1997 Federation of European Biochemical Societies.