ATP sulfurylase catalyzes and chemically links the hydrolysis of GTP and the synthesis of activated sulfate (APS). Like many GTPases, its GTPase activity is allosterically regulated, in this case, by APS-forming reactants and their analogues. Using these activators, we have been able to mimic many of the complexes that form in the native reaction, including an E-AMP intermediate. The effectsof each of these complexes on GTP hydrolysis are determined. The resultsof pre-steady-state and isotope trapping studies demonstrate that the binding of activator and substrate to the enzyme are near equilibrium and that the rate-determining step appears to be scission of the α, γ-bond of GTP. These properties of the system allow the energetic consequences of activator binding on the ground- and transition-state complexes to be evaluated. Activation occurs predominantly by transition-state stabilization, resulting in kcat increases. The values for kcat span a 180-fold range and vary with each activator. Km, or ground-state, effects are relatively small, ~3-fold, and are uniform throughout the activator series. These studies provide an indepth view of the energetic interactions between the two active sites at each step of the APS-forming reaction.
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