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For tRNA-dependent protein biosynthesis, amino acids are first activated by aminoacyl-tRNA synthetases (aaRSs) yielding the reaction intermediates aminoacyl-AMP (aa-AMP). Stable analogues of aa-AMP, such as aminoacyl-sulfamoyl-adenosines, inhibit their cognate aaRSs. Glutamyl-sulfamoyl-adenosine (Glu-AMS) is the best known inhibitor of  Escherichia coli  glutamyl-tRNA synthetase (GluRS). Thermodynamic parameters of the interactions between Glu-AMS and  E .  coli  GluRS were measured in the presence and in the absence of tRNA by isothermal titration microcalorimetry. A significant entropic contribution for the interactions between Glu-AMS and GluRS in the absence of tRNA or in the presence of the cognate tRNA Glu  or of the non-cognate tRNA Phe  is indicated by the negative values of –TΔS b , and by the negative value of ΔC p . On the other hand, the large negative enthalpy is the dominant contribution to ΔG b  in the absence of tRNA. The affinity of GluRS for Glu-AMS is not altered in the presence of the non-cognate tRNA Phe , but the dissociation constant  K   d  is decreased 50-fold in the presence of tRNA Glu ; this result is consistent with molecular dynamics results indicating the presence of an H-bond between Glu-AMS and the 3’-OH oxygen of the 3’-terminal ribose of tRNA Glu  in the Glu-AMS•GluRS•tRNA Glu  complex. Glu-AMS being a very close structural analogue of Glu-AMP, its weak binding to free GluRS suggests that the unstable Glu-AMP reaction intermediate binds weakly to GluRS; these results could explain why all the known GluRSs evolved to activate glutamate only in the presence of tRNA Glu , the coupling of glutamate activation to its transfer to tRNA preventing unproductive cleavage of ATP.

plos one

tRNAGlu Increases the Affinity of Glutamyl-tRNA Synthetase for Its Inhibitor Glutamyl-Sulfamoyl-Adenosine, an Analogue of the Aminoacylation Reaction Intermediate Glutamyl-AMP: Mechanistic and Evolutionary Implications