The catalytic mechanism of amino acid:tRNA ligases. Synergism and formation of the ternary enzyme-amino acid-ATP complex.

Formation of binary and ternary enzyme-ligand complexes was investigated for amino acid:tRNA ligases specific for L-isoleucine, L-leucine, and L-phenylalanine. Each of the enzymes exhibited synergistic binding when a substrate was substituted by a structurally related compound. The strength of coupling between the sites binding the amino acid and ATP was strongly dependent on the structure of ligands. The phenomenon was observed with the L-leucine and L-phenylalanine-specific enzymes only in the presence of magnesium. Spermine was inhibitory for L-phenylalanine:tRNA ligase. From the variation which structure of the strength of the observed synergism a correlation scheme was derived considering the ammonium group, the carboxylate group and the side chain of the amino acid, and the adenosine and triphosphate moieties of ATP. The strength of coupling between the subsites binding various combinations of these moieties was evaluated. We found that binding of the subgroups of the amino acid exerts an intramolecular synergism. The strength intramolecular synergism was similar to the strength of the intermolecular synergism observed for the simultaneous binding of an amino alcohol and ATP (or MgATP-2-). We have derived a molecular mechanism for the formation of the ternary enzyme-amino acid-ATP (or MgATP-2-) complex taking into account the synergistic phenomena. The complex is considered to involve electrostatic repulsion between the amino acid carboxylate and the ATP triphosphate moieties. When one of the negatively charged groups have been eliminated, the enzymatic rearrangement which facilitates the formation of this complex may be seen as a synergistic coupling.