Figure 6.

Gibbs free-energy diagram depicting the transition-state stabilizing effect of the H-bond between Asp¹⁰⁹ and His²². Gibbs free-energy diagram for the two reactions described by the equilibrium constants K₂ and K₄ (Scheme 1) of the WT (black) and D109A mutant (gray) of EF-Tu, respectively. Removal of the H-bond between His²² and Asp¹⁰⁹ results (as calculated from the experimentally determined rate constants summarized in Table 1) in destabilization of the Tu•Ts complex and the two transition states (Tu•Ts^≠ and Tu•GDP•Ts^≠) to a similar extent, as reflected by the comparable ΔΔGs for both reactions (ΔΔG₄ = 6.4 kJ/mol and ΔΔG₂ = 7.0 kJ/mol) consistent with the energy for a single H-bond. This increase in energy of the transition states and the Tu•Ts complex results in decreased rate constants for k_-4 and k₂, and no changes for the rate constants of k₄ and k_-2, as indicated by the length of the respective arrows (gray versus black).