Abstract
Adenosine (S)-5''-[.alpha.-17O, 18O2]triphosphate was synthesized and used to investigate the stereochemical course of activation of tryptophan by tryptophanyl-tRNA synthetase from beef pancreas. The reaction proceeds by displacement of Mg pyrophosphate from MgATP with inversion of configuration at P.alpha.. Tryptophanyl-tRNA synthetase catalyzes positional isotope exchange in adenosine 5''-[.beta.-18O2]triphosphate in the presence of tryptophan but not in its absence or in the presence of the competitive inhibitors tryptamine and tryptophanol. These observations eliminate a multi-adenylyl transfer or dissociative mechanism and in combination with the stereochemical study leave a direct associative in line displacement of P.alpha. of MgATP as the only tenable mechanism for the activation of tryptophan by tryptophanyl-tRNA synthetase.