Heats and Entropies of Reaction of Alkaline-earth Metal Ions with Pyrophosphate and Triphosphate Ions

Half-of-Sites Binding of Orotidine 5‘-Phosphate and α-d-5-Phosphorylribose 1-Diphosphate to Orotate Phosphoribosyltransferase from Saccharomyces cerevisiae Supports a Novel Variant of the Theorell−Chance Mechanism with Alternating Site Catalysis

“…The heat effects associated with the dilution of OPRTase and PRPP were measured and found to be negligible. Because the binding of Mg 2+ to PP i is known to be endothermic (24), it was hypothesized that the endothermic event was the binding of Mg 2+ to PRPP. Indeed, titrations of PRPP (no Mg 2+ ) into Mg 2+ gave a strong endothermic binding isotherm with a curve characteristic of a two-sequential-site binding model.…”

Thermochemistry of the biochemical reaction: { pyrophosphate(aq) + H2O(l) = 2 phosphate(aq)}

“…The adjustments for ionic strength were done by using the extended Debye-Hückel equation (2,12) with the "ion-size" parameter set at 1.6 kg 1/2 · mol −1/2 . The value r H o m = −13.3 kJ · mol −1 was used in performing temperature adjustments to the pK s for reaction (13) reported by Karweik and Huber (30) and Miyajima et al (32) (23) and Sugano et al (29) and will also be used in all subsequent calculations. We judge the electrochemical study of Wolhoff and Overbeek (24) to yield the most reliable pK value for reaction (13).…”