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Influence of modified tRNATyr on the activation step of tyrosine catalyzed by tyrosyl-tRNA synthetase from Saccharomyces cerevisiae

Aminoacyl-tRNA synthetases (EC 6.1.1) catalyze the covalent attachment of amino acids to the 3� -ends of their cognate transfer RNAs. Tyrosyl-tRNA synthetase from yeast Saccharomyces cerevisiae (TyrRS, EC 6.1.1.1) is a homodimeric enzyme capable of binding only one molecule of its macromolecular substrate, tRNATyr. TyrRS catalyzes the aminoacylation of tRNATyr in a two step reaction. The reactive intermediate tyrosyl adenylate is formed from tyrosine and ATP in the first reaction step, which can be conveniently assayed by steady-state kinetics of pyrophosphate exchange. In order to determine the number of active sites per homodimer, the kinetics of pyrophosphate exchange was measured in the presence of tRNATyr analogue unable to accept the amino acid. The analogue was found to form the expected equimolar complex with dimeric enzyme. It was a competitive inhibitor of pyrophosphate exchange with respect to ATP and non-competitive with respect to tyrosine. Inhibition can not exceed 50%, sugesting the simplest model in which yeast TyrRS is a symmetrical dimer, possessing two identical active sites, both capable of catalyzing the formation of tyrosyl adenylate.

pyrophosphate exchange; MALDI-MS; modified tRNATyr; S. cerevisiae; tyrosyl-tRNA synthetase

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