Polyphosphates, hydrolysis rates

KAR7HUB] Karweik, D. H., Huber, C. 0., Polyphosphate currents at the lead dioxide electrode for hydrolysis rate and instability constant measurements, y4 a/. Chem., 50, (1978), 1209-1212. Cited on pages 207, 209, 384. 

Ta 1.5 X 10 2, K3 2.1 X 10 and 2.4 x and the corresponding negative logarithms are pA" 1.0, pA"2 1.8, pA"3 6.57 and pA"4 9.62. The P—O—P linkage is kinetically stable towards hydrolysis in dilute neutral solutions at room temperature and the reaction half-life can be of the order of years. Such hydrolytic breakdown of polyphosphate is of considerable importance in certain biological systems and has been much studied. Some factors which affect the rate of degradation of polyphosphates are shown in Table 12.10. 

Catalytic Action of Various Cations on the Rate of Hydrolysis of Polyphosphates... 

The effect of cations on the rate of hydrolysis of cycio-tri- and cvc/o-tetra-phosphates has also been studied.260 Alkali metal cations retarded the hydrolysis of both cyclo-phosphates in acidic solution but accelerated the reaction in basic solution. The catalytic activity followed the sequence Li+ > Na+> K+. Mg11, Ca11 and Ni11 retarded the hydrolysis of the cyclo-phosphates in the pH range 1.0-2.0 or 1.0-2.7, but accelerated the reaction at pH>3. Catalysis by Cu11 is observed above pH 2, while Alni has a marked catalytic effect at pH 1 and 2. A variety of other papers have appeared dealing with metal ion-promoted hydrolysis of polyphosphates.261-264 However, the mechanistic details of these reactions remain unclear and the catalytically active complexes remain undefined. 

The linear polyphosphates and cyclophosphates are hydrolysed extremely slowly at neutral pH and room temperature in comparison with other polyacids such as polyarsenates and polyvanadates, and are unique in this respect. The half-hydrolysis time for the P-O-P bonds in linear polyphosphates at pH 7 and 25 °C is several years (Van Wazer, 1958). The rate of hydrolysis of these bonds is increased by raising the temperature, reducing the pH, and by the presence in the solution of colloidal gels and complex cations. The hydrolysis of these bonds is dependent on the ionic strength of the solutions (Van Wazer, 1958). 

The products of the PI3-kinase reaction are different phosphoinositide derivatives phosphorylated at the 3 position, of which PtdIns(3,4,5)P3 has the greatest regulatory importance. PtIns(3,4,5)P3, like cAMP, has the function of a messenger substance that activates effector molecules in the sequence for further signal conduction. In contrast to cAMP, PtdIns(3,4,5)P3 is localized in the cell membrane and performs its function in close association with processes at the cell membrane. The concentration of PtdIns(3,4,5)P3 in the cell depends both on the rates of synthesis by PI3-kinases and the rates of hydrolysis of its phosphate residues. Several inositol polyphosphate phosphatases have been identified that remove the phosphates at position 3 or 5 of the inositol moiety. Among the inositol polyphosphate phosphatases with specificity for the 3-position, the PTEN phosphatase has been identified as a tumor supressor protein (see below). 

The effect of metal ions on the rates of hydrolysis of polyphosphates is of great biochemical interest. Calcium and magnesium ions probably form six-membered chelate rings with a long-chain polyphosphate, although it... 

The rate of hydrolysis is influenced by pH, temperature and calcium concentration. Calcium shifts the chemical equilibrium of this reaction to the right as it bonds the orthophosphate ions formed. In addition to chemical hydrolysis, biochemical hydrolysis also takes place in waters, particularly in sewage with a dense biological population. A considerable part of polyphosphates in sewage fed into wastewater treatment plants is hydrolysed. The half-time of polyphosphates in surface waters is given in days and tens of days. 

Hydrolysis of triphosphate at pH 9 results in the formation of diphosphate (pyrophosphate) and monophosphate (orthophosphate). In a parallel step, the diphosphate is split, at a slower rate, into monophosphate. A tetraphosphate is thus cleaved into one triphosphate and one monophosphate anion, followed by cleavage of the triphosphate as just described (5.151). In general, hydrolytic degradation of long-chain polyphosphates proceeds primarily by scission of the end PO4 groups and there is a rapid buildup of monophosphate during such reactions. 

As usual there have been a number of papers on the hydrolysis of polyphosphates. Complexes of [VO] + or [VO] + with polyphosphates give species in which phosphate hydrolysis is much more rapid than for the uncomplexed species. We may also note the appearance of further evidence that the oxidation reactions of peroxodiphosphate involve hydrolysis in the rate-determining stage. Studies on the oxidation of antimony(iii), hydrazine, nitrite, and hypophosphite have appeared. ... 

Rates of decomposition of the phosphododecavanadate ion [H4PVi2036] depend on the concentration of acid or of alkali present. The vanadium is stated to end up in the form of [VO2] in acid solution, [HV04] and [HV207] in alka-line. Vanadium(v) is known to catalyse hydrolysis of polyphosphates. The maximum catalytic effect corresponds to the participation of a [V2P30io] anion (or some appropriately protonated form thereof), whose enthalpy of activation for dissociation must be unusually low. ... 

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