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[Comparison of ATP-pyrophosphate exchange reactions, catalyzed by two different "one-site" forms of tryptophanyl-tRNA-synthetase and by native enzyme].

Kinetic parameters of the reaction of ATP-PPi exchange, catalysed by "one-site" form of beef pancreas tryptophanyl-tRNA-synthetase are determined. The one-site form was obtained by affinity labelling of one active centre of the enzyme with alkylated analog of tryptophan. The comparison of kinetic parameters of this one-site form with the other one produced earlier by affinity labelling with analog of tryptophanyl-tRNA and the native enzyme was carried out. It was shown that the presence of tRNA improves the discrimination properties of the enzyme and diminishes the concentrations of "dead-end" complexes. Negative cooperativity in binding of two molecules of tryptophan is observed, which disappears in the presence of ATP, leading to an apparent synergistic effect. The molecule of tryptophan, which is present in the active centre, diminishes affinity of pyrophosphate.