In contrast to previous studies that have shown that the neutral phenol serves as the nucleophile for WT Csk-promoted phosphorylation of a tyrosine-containing substrate, the phenolate ion acts as primary nucleophile for the D314N Csk-catalyzed reaction. Rate comparisons of D314N Csk-promoted phosphotransfer using a series of fluorotyrosine-containing peptide substrates reveal a near zero beta(nuc), consistent with a dissociative mechanism of phosphotransfer. These combined results argue against a hydroxy nucleophile-to-phosphate proton transfer occurring prior to an associative transition state of phosphoryl transfer.
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