Kinetic studies on the interactions of Escherichia coli K12 elongation factor Tu with GDP and elongation factor Ts

The kinetic parameters describing the dissociation of GDP from the elongation factor Tu (EF-Tu) . GDP complex in the absence and presence of elongation factor Ts (EF-Ts) have been characterized using an equilibrium isotope exchange technique. The rate constant for dissociation of GDP from EF-tu was found to be 1.7 x 10(-3) s-1. Since this dissociation rate is greatly enhanced by EF-Ts, it follows that the dissociation of GDP in the presence of EF-Ts proceeds via the formation of a ternary EF-Tu . GDP . EF-Ts complex as represented below: EF-Tu . GDP + EF-Ts in equilibrium EF-Tu . GDP . EF-Ts in equilibrium EF-Tu . EF-Ts + GDP. Analysis of the exchange kinetics according to this reaction scheme yields a rate constant for the dissociation of GDP from the ternary complex of greater than or equal to 1270 s-1. The equilibrium association constants for GDP and EF-Ts to form the ternary complex was found to be 6.4 x 10(4) M-1 and 1.8 x 10(5) M-1, respectively. These results demonstrate that the dissociation of GDP from EF-Tu in the presence of EF-Ts is not the rate-limiting process in protein synthesis.