Leaving-group effects on the ribonuclease T₁-catalyzed transphosphorylation of specific substrates

A series of alkyl esters of guanosine 3′ -phosphate has been synthesized, and the Michaelis-Menten parameters for the RNase T₁- catalyzed transphosphorylation of these substrates have been determined at 25°. Little variation in the parameter K_mi was observed for all substrates; an approximate linear relationship exists between the logarithms of the parameter k_cat/K_mi, which is a measure of the rate constant for transphosphorylation of the free RNase T₁by free substrate, and the Taft’s substituent constants, s*. A positive p* value obtained and the reactivity sequence arguments taken from the literature indicate an S_N2(P) mechanism for RNase T₁-transphosphorylation involving direct nucleophilic attack at phosphorus concerted with general acid catalysis of the formation of a pentacovalent intermediate or transition state.