A novel alpha-proton exchange reaction catalyzed by Escherichia coli methionyl-tRNA synthetase.
The Escherichia coli truncated methionyl-tRNA synthetase (delta MTS) was shown to catalyze alpha-carbon hydrogen-deuterium exchange of L-selenomethionine, L-methionine, L-ethionine, and L-norleucine in the presence of deuterium oxide. The rate of alpha-proton exchange for L-methionine was shown to be linear with respect to delta MTS concentration. The exchange reaction showed saturation kinetics with apparent Km values of 21 and 4 mM in the absence and presence of saturating adenosine concentrations, respectively. As expected, delta MTS did not catalyze alpha-proton exchange of D-methionine since the enzyme has been shown to be specific for L-amino acids. In the absence of enzyme or in the presence of an equivalent concentration of Zn2+, no hydrogen-deuterium exchange was detected. The exchange reaction was not observed with L-methioninol, an analogue of L-methionine lacking the carboxylate group. These results suggest that the alpha-carboxylate group is a requirement for the delta MTS-catalyzed exchange reaction. The E. coli methionyl-tRNA synthetase (MTS) has previously been shown to be a zinc metalloprotein [Posorske, L. H., Cohn, M., Yanagisawa, N., & Auld, D. S. (1979) Biochim. Biophys. Acta 576, 128]. On the basis of the structural and mechanistic information available on MTS, we propose that the enzyme-bound zinc coordinates the carboxylate of the amino acid, while a base on the enzyme is responsible for exchange of the alpha-proton. The role of the enzyme-bound metal is to render the alpha-proton more acidic through coordination of the carboxylate group.(ABSTRACT TRUNCATED AT 250 WORDS)
Williams
J S
JS
Department of Biochemistry and Molecular Biology, University of Texas Medical School, Houston 77225.
Rosevear
P R
PR
D004926:Escherichia coli / Q000201:enzymology*; D008401:Gas Chromatography-Mass Spectrometry; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D008718:Methionine-tRNA Ligase / Q000378:metabolism*; D011522:Protons
D011522:Protons; D008718:Methionine-tRNA Ligase