HYDROLYSIS OF NUCLEOSIDE PHOSPHATES .11. SYNERGISM BETWEEN DIFFERENT METAL-IONS IN THE DEPHOSPHORYLATION OF ADENOSINE 5'-TRIPHOSPHATE (ATP) IN MIXED METAL-ION ATP SYSTEMS, AND INFLUENCE OF A DECREASING SOLVENT POLARITY (DIOXANE-WATER MIXTURES) ON THE DEPHOSPHORYLATION RATE - EFFECTS OF MG-2+, NA+, AND NH4+ IONS

The initial rates of the dephosphorylation (i.e., v0 = d[PO4]/dt) of adenosine 5′-triphosphate (ATP) (=10-3 M) in the mixed metal ion systems Cu2+/ATP/Mg2+, Ni2+, Zn2+, or Cd2+ with the ratios 1:1:1 and 1:1:5 have been measured and compared at pH 5.5 and 50°C (I = 0.1, NaClO4) with the corresponding binary ATP/M2+ 1:2 and 1:6 systems. The remarkable result is that addition of Mg2+ to a Cu2+/ATP 1:1 system accelerates the dephosphorylation rate significantly more than the same amounts of Mg2+ accelerate the reaction in a Mg2+/ATP 1:1 system. The same synergism, based also on the Cu2+/ATP 1:1 system, is observed with Ni2+, but not with Zn2+ and Cd2+. This observation is attributed to the formation of a prereactive state in the Cu2+/ATP 1:1 system, i.e., of a [Cu(ATP)]24- dimer which involves purine-stacking and a Cu2+/N-7 interaction; the inherent reactivity in this dimer may be triggered by the addition of Mg2+ or Ni2+. In Zn2+ or Cd2+/ATP 1:1 systems also a prereactive state is formed and therefore no synergism is observed in a comparison with the corresponding mixed Cu2+ systems. In agreement herewith, there is a rather pronounced synergism in the Zn2+/ATP Mg2+ system at pH 7.5, and a somewhat smaller one under the same conditions in the Zn2+/ATP/Na+ system. In the latter system the synergism may be considerably favored by reducing the solvent polarity, i.e., by changing the solvent from water to 50% (v/v) dioxane-water; similar effects, though less pronounced, are observed with Zn2+/UTP. In connection with the solvent effects it is recalled that the polarity in the active-site cavities of enzymes is also lower than in the bulk water. By experiments with ATP and Mg2+/ATP systems it is shown that Na+ and NH4+ have corresponding effects; this observation is important regarding cationic side chains of amino acid residues in proteins. Some further implications of the present results for biosystems are also indicated. The phosphate groups in TNP are labeled as α, β, and γ, where the latter refers to the terminal phosphate group (Fig. 1). If nothing else is specified, the formula PO4 reperesents all related species which may be present in solution, i.e., H3PO4, H2PO4-, HPO42-, and PO43-. The term "dephosphorylation" is used for the transfer of a phosphoryl group to a water molecule; the term "hydrolysis" can also refer to this, but for the most part we use it in connection with the formation of hydroxo complexes of metal ions. The terms monomeric or dimeric complexes mean that one or two NTP4- together with at least the corresponding equivalents of M2+ are within the considered complex; hence, e.g., M2(NTP) is a monomeric (but dinuclear) nucleotide complex whereas [M(NTP)]24- is a dimeric one. © 1990.