Kinetic evidence for half-of-the-sites reactivity in tRNATrp aminoacylation by tryptophanyl-tRNA synthetase from beef pancreas

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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 (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.

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