Tryptophanyl adenylate formation by tryptophanyl-tRNA synthetase from Escherichia coli

Publisher Website
Google Scholar
Abstract
By gel filtration and titration on DEAE-cellulose filters we show that Escherichia coli tryptophanyl-tRNA synthetase forms tryptophanyl adenylate as an initial reaction product when the enzyme is mixed with ATP-Mg and tryptophan. This reaction precedes the synthesis of the tryptophanyl-ATP ester known to be formed by this enzyme. The stoichiometry of tryptophanyl adenylate synthesis is 2 mol per mole of dimeric enzyme. When this reaction is studied either by the stopped-flow method, by the fluorescence changes of the enzyme, or by radioactive ATP depletion, three successive chemical processes are identified. The first two processes correspond to the synthesis of the two adenylates, at very different rates. The rate constants of tryptophanyl adenylate synthesis are respectively 146 .+-. 17 s-1 and 3.3 .+-. 0.9 s-1. The third process is the synthesis of tryptophenyl-ATP, the rate constant of which is 0.025 s-1. The Michaelis constants for ATP and for tryptophan in the activation reaction are respectively 179 .+-. 35 .mu.M and 23.9 .+-. 7.9 .mu.M, for the fast site, and 116 .+-. 45 .mu.M and 3.7 .+-. 2.2 .mu.M, for the slow site. No synergy between ATP and tryptophan can be evidenced. The data are interpreted as showing positive cooperativity between the subunits associated with conformational changes evidenced by fluorometric methods. The pyrophosphorolysis of tryptophanyl adenylate presents a Michaelian behavior for both sites, and the rate constant of the reverse reaction is 360 .+-. 10 s-1 with a binding constant of 196 .+-. 12 .mu.M for inorganic pyrophosphate (PPi). Major differences appear between tryptophanyl-tRNA synthetases from E. coli and from beef. In the procaryotic enzyme, the two subunits present nearly half-of-the-sites reactivity but exhibit kinetic positive cooperativity toward tryptophan. In the eucaryotic enzyme, the two subunits are identically active at substrate saturation but exhibit negative cooperativity toward tryptophan. The bacterial enzyme makes tryptophan derivatives such as tryptophanamide. The mammal enzyme does not. The E. coli enzyme does not cross-react with antibodies raised against the beef protein.

This publication has 1 reference indexed in Scilit: