title: Kinetic evidence for half-of-the-sites reactivity in tRNATrp aminoacylation by tryptophanyl-tRNA synthetase from beef pancreas.
abstract: The aminoacylation reaction catalyzed by the dimeric tryptophanyl-tRNA synthetase from beef pancreas was studied under pre-steady-state conditions by the quenched-flow method. The transfer of tryptophan to tRNATrp was monitored by using preformed enzyme-bis(tryptophanyl adenylate) complex. Combinations of either unlabeled or L-[14C]tryptophan-labeled tryptophanyl adenylate and of aminoacylation incubation mixtures containing either unlabeled tryptophan or L-[14C]tryptophan were used. We measured either the formation of a single labeled aminoacyl-tRNATrp per enzyme subunit or the turnover of labeled aminoacyl-tRNATrp synthesis. Four models were proposed to analyze the experimental data: (A) two independent and nonequivalent subunits; (B) a single active subunit (subunits presenting absolute "half-of-the-sites reactivity"); (C) alternate functioning of the subunits (flip-flop mechanism); (D) random functioning of the subunits with half-of-the-sites reactivity. The equations corresponding to the formation of labeled tryptophanyl-tRNATrp under each labeling condition were derived for each model. By use of least-squares criteria, the experimental curves were fitted with the four models, and it was possible to disregard models B and C as likely mechanisms. Complementary experiments, in which there was no significant excess of ATP-Mg over the enzyme-adenylate complex, emphasized an activator effect of free L-tryptophan on the rate of aminoacylation. This result disfavored model A. Model D was in agreement with all data. The analyses showed that the transfer step was not the major limiting reaction in the overall aminoacylation process.
mesh_terms: D000604:Amino Acyl-tRNA Synthetases / Q000378:metabolism*; D000818:Animals; D001665:Binding Sites; D002417:Cattle; D007700:Kinetics; D046911:Macromolecular Substances; D008954:Models, Biological; D010179:Pancreas / Q000201:enzymology*; D011485:Protein Binding; D012346:RNA, Transfer, Amino Acyl / Q000378:metabolism*; D014369:Tryptophan-tRNA Ligase / Q000378:metabolism*
chemical_list: D046911:Macromolecular Substances; D012346:RNA, Transfer, Amino Acyl; D000604:Amino Acyl-tRNA Synthetases; D014369:Tryptophan-tRNA Ligase