MAPPING THE TRANSITION-STATE FOR ATP HYDROLYSIS - IMPLICATIONS FOR ENZYMATIC CATALYSIS

Background: Phosphoryl transfer, typically involving high energy phosphate donors such as ATP, is the most common class of biological reactions. Despite this, the transition state fro phosphoryl transfer from ATP in solution has not been systematically investigated. Characterization of the transition state for the uncatalyzed hydrolysis of ATP would provide a starting point for dissection of enzyme-catalyzed reactions. Results: We examined phosphoryl transfer from ATP, GTP and pyrophosphate to a series of alcohols; these reactions are analogous to the phosphorylation of sugars and other biological alcohols and to the hydrolysis of ATP. The Bronsted beta(nucleophile) value of 0.07 is small, indicating that there is little bind formation between the incoming nucleophile and the electrophilic phosphoryl group in the transition state. Coordination of Mg2+ has no measurable effect on this value. The Bronsted beta(leaving group) value of -1.1 for phosphoryl transfer to water from a series of phosphoanhydrides is large and negative, suggesting that the bond between phosphorous and the leaving group oxygen is largely broken in the transition state. Conclusions: Uncatalyzed hydrolysis of ATP in solution occurs via a dissociative, metaphosphate-like transition state, with little bond formation between nucleophile and ATP and substantial cleavage of the bond between the gamma-phosphoryl moiety and the ADP leaving group. Bound Mg2+ does not perturb the dissociative nature of the transition state, contrary to proposals that enzyme-bound metal ions alter this structure. The simplest expectation fro phosphoryl transfer at the active site of enzymes thus entails a dissociative transition state. These results provide a basis for analyzing catalytic mechanisms for phosphoryl transfer.