Transition state analysis andrequirement of Asp-262 general acid/base catalyst for full activation of dual-specificity phosphataseMKP3 by extracellular regulated kinase

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Dual-specificity phosphatase MKP3 down-regulates mitogenic signaling through dephosphorylation of extracellular regulated kinase (ERK). Unlike a simple substrate−enzyme interaction, the noncatalytic, amino-terminal domain of MKP3 can bind efficiently to ERK, leading to activation of the phosphatase catalytic domain by as much as 100-fold toward exogenous substrates. It has been suggested that ERK activates MKP3 through the stabilization of the active phosphatase conformation, enabling general acid catalysis. Here, we investigated whether Asp-262 of MKP3 is the bona fide general acid and evaluated its contribution to the catalytic steps activated by ERK. Using site-directed mutagenesis, pH rate and Brönsted analyses, kinetic isotope effects, and steady-state and rapid reaction kinetics, Asp-262 was identified as the authentic general acid catalyst, donating a proton to the leaving group oxygen during P−O bond cleavage. Kinetic isotope effects [18(V/K)bridge, 18(V/K)nonbridge, and 15(V/K)] were evaluated for the effect of ERK and of the D262N mutation on the transition state of the phosphoryl transfer reaction. The patterns of the three isotope effects for the reaction with native MKP3 in the presence of ERK are indicative of a reaction where the leaving group is protonated in the transition state, whereas in the D262N mutant, the leaving group departs as the anion. Even without general acid catalysis, the D262N mutant reaction is activated by ERK through increased phosphate affinity (∼8-fold) and the partial stabilization of the transition state for phospho-enzyme intermediate formation (∼4-fold). Based on these analyses, we estimate that dephosphorylation of phosphorylated ERK by the D262N mutant is >1000-fold lower than by native, activated MKP3. Also, the kinetic results suggest that Asp-262 functions as a general base during thiol−phosphate intermediate hydrolysis.

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