Dimeric glutamyl‐tRNA synthetases from wheat Kinetic properties and functional structures

Abstract

The Km in the tRNA aminoacylation reaction were studied for the 3 dimeric glutamyl-tRNA synthetases C, P and E. The values were the following: for tRNA, 0.20 .mu.M, 0.20 .mu.M and 0.44 .mu.M for glutamic acid, 10 .mu.M, 83 .mu.M and 83 .mu.M; for MgATP, 0.46 mM, 0.38 mM and 0.26 mM. MgATP concentrations higher than 2 mM induce pronounced inhibition. The presence of the cognate tRNA was required for [32P]PPi-ATP isotopic exchange. In the absence of tRNA no hyperbolic saturation of the enzymes by glutamic acid occurred. Analysis of the enzyme activity as a function of enzyme concentration led to the conclusion that the active forms are dimers which are in equilibrium with inactive monomers. The values of the dissociation constants Kd were 43 nM, 53 nM and 87 nM for glutamyl-tRNA synthetases C, P and E, respectively.