Substrate Specificity of an Active Dinuclear Zn(ll) Catalyst for Cleavage of RNA Analogues and a Dinucleoside

摘要

The cleavage of the diribonucleoside UpU (uridylyl-3'-5'-uridine) to form uridine and uridine (2',3')-cyclic phosphate catalyzed by the dinuclear Zn(ll) complex of 1,3-bis(1,4,7-triazacyclonon-1-yl)-2-hydroxypropane (Zn_2(1)(H_2O)) has been studied at pH 7-10 and 25 deg C.The kinetic data are consistent with the accumulation of a complex between catalyst and substrate and were analyzed to give values of k_c (s~(-1)),K_d (M),and K_c/K_d (M~(-1) s~(-1)) for the Zn_2(1)(H_2O)-catalyzed reaction.The pH rate profile of values for log k_c/K_d for Zn_2(1 )(H_2O)-catalyzed cleavage of UpU shows the same downward break centered at pH 7.8 as was observed in studies of catalysis of cleavage of 2-hydroxypropyl-4-nitrophenyl phosphate (HpPNP) and uridine-3'-4-nitrophenyl phosphate (UpPNP).At low pH,where the rate acceleration for the catalyzed reaction is largest,the stabilizing interaction between Zn_2(1)(H_2O) and the bound transition states is 9.3,7.2,and 9.6 kcal/mol for the catalyzed reactions of UpU,UpPNP,and HpPNP,respectively.The larger transition-state stabilization for Zn_2(1)(H_2O)-catalyzed cleavage of UpU (9.3 kcal/mol) compared with UpPNP (7.2 kcal/mol) provides evidence that the transition state for the former reaction is stabilized by interactions between the catalyst and the C-5'-oxyanion of the basic alkoxy leaving group.