Fluorescence Studies on the Interaction between Yeast Seryl‐tRNA Synthetase and Its Substrates

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Abstract
Substrate‐induced variations in the native fluorescence of seryl‐tRNA synthetase from yeast have been used to evaluate the binding equilibria with its ligands. Binding of l‐serine to the enzyme can be detected by equilibrium dialysis but not by fluorescence quenching. Hence, in contrast to ATP and tRNASer, l‐serine does not induce any fluorescence‐sensitive conformational change of the synthetase. A comparison of the binding constants for ATP (2 × 104 M−1) and seryl adenylate (1 × 106 M−1) indicates that the activation reaction is mainly driven by the higher affinity of seryl adenylate to the enzyme. tRNASer and seryl‐tRNASer though causing rather different maximal fluorescence quenching are bound to the enzyme with similar stoichiometry and association constant. The implications of this result are discussed with respect to the mechanism of the aminoacylation reaction. It is also shown that the enzyme clearly is capable of discriminating l‐serine from other amino acids, ATP from other nucleotidetriphosphates and AMP, and tRNASer from other tRNA species in the binding processes. The spectral properties, the effects of outside quenchers and the fluorescence decay times of the free enzyme and the enzyme‐substrate complexes are discussed with respect to the intercalation hypothesis of Hélène.