Catalysis of tRNATyr aminoacylation by tyrosyl-tRNA synthetase can be divided into two steps. In the first step, tyrosine is activated by ATP to form the tyrosyl-adenylate intermediate. In the second step, the tyrosyl moiety is transferred to the 3′-end of tRNA. To investigate the roles that enthalpic and entropic contributions play in catalysis by tyrosyl-tRNA synthetase, the temperature-dependence of the tyrosine activation step has been determined. A van't Hoff plot for dissociation of ATP from the E·Tyr complex reveals three distinct regions of linearity. Particularly striking is the change occurring at 25 °C, where the values of ΔH° and ΔS° go from −40.3 kcal/mol and −125 cal/mol-°K below 25 °C to +38.4 kcal/mol and +139 cal/mol-°K above 25°C. We propose that these changes are due to a temperature-dependent change in the catalytic mechanism of tyrosyl-tRNA synthetase. Previous investigations suggest that there is a synergistic interaction between tyrosine and ATP that stabilizes the transition state for the tyrosine activation reaction. We postulate that below 25°C, this synergistic interaction occurs during the initial binding of ATP, whereas above 25°C, it occurs in the transition state. To test this hypothesis we have determined the temperature-dependence for the binding of ATP to the unliganded enzyme. Preliminary results support the above hypothesis. This research was supported by NIH GM068070 to E.A.F.