General Considerations on Transphosphorylations: Mechanism of the Metal Ion Facilitated Dephosphorylation of Nucleoside 5'-Triphosphates, Including Promotion of ATP Dephosphorilation by Addition of Adenosine 5'-Monophosphate

Article abstract of DOI:10.1021/ja00337a050

First-order rate constants (50 deg C; I=0.1, NaClO4) for the dephosphorylation of uncomplexed nucleoside 5'-triphosphates (=NTP=ATP, GTP, ITP, CTP, UTP, or TTP) are virtually identical at the same pH, reaction occuring by water attack as is evident from the specific rate constants; the reactivity decreases in the phosphate-protonated series H2(NTP)^2- > H(NTP)^3- > NTP^4-.Rate constants are also compared for NTP systems containing metal ions 2+ = Mg²⁺, Mn²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺> by combining new results with previous data.The effectiveness of the metal ions to promote ATP dephosphorylation decreases in the order Cu²⁺ > Cd²⁺ > Zn²⁺ > Ni²⁺ > Mn²⁺ > Mg²⁺, when the rates at the pH of the maximum promotion are compared.This pH, beginning with Cu²⁺ at pH 6.7, increases within the series Cu²⁺ < Zn²⁺ < Ni²⁺ < Cd²⁺ < Mn²⁺ (< Mg²⁺).The latter order reflects the tendency of these ions to form hydroxo complexes (which occurs in the reverse order) and implies that the nucleophilic attack occurs in an intramolecular fashion via an M-OH unit.This view is supported by the calculated specific rate constants and by othe experimental results.The sum of all experimental data for the determination of the initial rate of the dephosphorylation (v₀ = d/dt) gives evidence that the most reactive species for the pyrimidine-NTP systems has the composition M2(R-TP)(OH)^-, where R-TP represents any thiophosphate (including methyltriphosphate) with a noncoordinating terminal organic residue.The most reactive species for the purine-NTP/Cu²⁺ systems at pH <6 has the composition 2(OH)^-; this holds also (including the higher pH range) for the ATP systems with Ni²⁺, Zn²⁺, and Cd²⁺.The most reactive species for Cu²⁺/ATP at pH =/>6.7 is also a dimer with the composition 2(OH)5-.For 1 mM Cu²⁺/NTP 1:1 systems the reactivity decreases in the pH range 2-8 in the order ATP > GTP > ITP > CTP ca.UTP ca.TTP; this can only be explained by the decreasing stacking tendency in this series.An explanation based on the N-7/metal ion interaction, which is crucial for the reactivity of the dimers as shown by NMR experiments, is not applicable because the N-7 coordination tendency follows the order adenosine < inosine < guanosine.The larger dephosphorylation rates of the purine-NTPs compared to the pyrimidine-NTPs in the presence of M²⁺ have their origin in the additional M²⁺/N-7 interaction in purine-NTPs which facilitates the formation of the reactive species.Experiments at pH 5.5 with increasing amounts of Cu²⁺, Zn²⁺, Cd²⁺, Ni²⁺, Mg²⁺, Cu(dien)²⁺, or Cu(dpa)²⁺ added to 1 mM Cu²⁺/ATP mixture 2- is formed to a high degree> indicate that an intermolecular attack by water on 2 is also possible if M²⁺ = Mg²⁺, Ni²⁺, Cd²⁺, Cu(dien)²⁺, or Cu(dpa)²⁺.Addition of ...