Transition state stabilization by a phylogenetically conserved tyrosine residue in methionyl-tRNA synthetase.

The crystal structure of a fully biologically active monomeric form of Escherichia coli methionyl-tRNA synthetase (MetRS) complexed with ATP has recently been reported (Brunie, S., Zelwer, C., and Risler, J.-L., (1990) J. Mol. Biol. 216, 411-424), revealing details of the active site of the enzyme, including the location of amino acid residues potentially involved in substrate binding. In the present paper, the role of 3 active site residues in interaction with methionine, ATP, and tRNA(fMet) and in catalysis of methionyl-adenylate has been explored using site-directed mutagenesis. Lys142 is located near the ribose of ATP in the MetRS.ATP cocrystal. Mutation of this residue to Ala caused a 5-fold decrease in kcat/Km for ATP-PPi exchange, indicating some contribution of the lysine side chain to the specificity of the enzyme. Mutation of Tyr359 to Ala produced a 14-fold increase in the Km for ATP with only a small (2-3-fold) change in the other kinetic parameters, indicating that the major role of this residue is in formation of the initial complex with ATP and/or in stabilization of the methionyl-adenylate reaction intermediate. Mutation of the adjacent residue Tyr358 to Ala had no effect on the Km values for methionine or ATP but produced nearly a 2000-fold decrease in the rate of ATP-PPi exchange. This mutation also dramatically reduced the rate of pyrophosphorolysis of the isolated MetRS.Met-AMP complex on addition of pyrophosphate without increasing the Km for PPi. None of the mutations affected the Km for tRNAfMet in the aminoacylation reaction. The results suggest that Tyr358 may enhance the rate of methionyl-adenylate formation by binding to the alpha-phosphate of ATP in the transition state. Interaction of Tyr358 and Tyr359 with ATP during the course of the reaction requires a significant change in the conformation of this region of the active site compared to the structure found in the MetRS.ATP complex. Such a shift is consistent with an induced-fit mechanism for methionine activation. Primary sequence comparisons of methionine-specific enzymes from yeast and bacterial sources reveals that Tyr358 is conserved in all of the known MetRS sequences.