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

Yeast tyrosyl-tRNA synthetase (TyrRS, EC 6.1.1.1) is a homodimeric enzyme capable of binding only one molecule of its macromolecular substrate, tRNA(Tyr). The reactive intermediate tyrosyl adenylate is formed from tyrosine and ATP in the first reaction step, which can be conveniently assayed by pyrophosphate exchange. In order to determine the number of active sites per homodimer, the kinetics of pyrophosphate exchange was measured in the presence of the tRNA(Tyr) 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 cannot exceed 50%, suggesting 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.