title: Constant enthalpy change value during pyrophosphate hydrolysis within the physiological limits of NaCl.
abstract: A decrease in water activity was thought to result in smaller enthalpy change values during PPi hydrolysis, indicating the importance of solvation for the reaction. However, the physiological significance of this phenomenon is unknown. Here, we combined biochemistry and calorimetry to solve this problem using NaCl, a physiologically occurring water activity-reducing reagent. The pyrophosphatase activities of extremely halophilic Haloarcula japonica, which can grow at ∼4 M NaCl, and non-halophilic Escherichia coli and Saccharomyces cerevisiae were maximal at 2.0 and 0.1 M NaCl, respectively. Thus, halophilic and non-halophilic pyrophosphatases exhibit distinct maximal activities at different NaCl concentration ranges. Upon calorimetry, the same exothermic enthalpy change of -35 kJ/mol was obtained for the halophile and non-halophiles at 1.5-4.0 and 0.1-2.0 M NaCl, respectively. These results show that solvation changes caused by up to 4.0 M NaCl (water activity of ∼0.84) do not affect the enthalpy change in PPi hydrolysis. It has been postulated that PPi is an ATP analog, having a so-called high energy phosphate bond, and that the hydrolysis of both compounds is enthalpically driven. Therefore, our results indicate that the hydrolysis of high energy phosphate compounds, which are responsible for biological energy conversion, is enthalpically driven within the physiological limits of NaCl.
authors: From the Graduate School of Biosphere Science, Hiroshima University, Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8528, Japan.
authors: Satoshi
authors: S
authors: Wakai
authors: Shun-ichi
authors: S
authors: Kidokoro
authors: Kazuo
authors: K
authors: Masaki
authors: Kaoru
authors: K
authors: Nakasone
authors: Yoshihiro
authors: Y
authors: Sambongi
mesh_terms: D019843:Archaeal Proteins / Q000378:metabolism; D055162:Biocatalysis; D002151:Calorimetry / Q000379:methods; D011756:Diphosphates / Q000737:chemistry* / Q000378:metabolism*; D029968:Escherichia coli Proteins / Q000378:metabolism; D019613:Haloarcula / Q000201:enzymology; D006868:Hydrolysis / Q000187:drug effects; D043564:Inorganic Pyrophosphatase / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins / Q000378:metabolism; D012965:Sodium Chloride / Q000737:chemistry* / Q000494:pharmacology; D012997:Solvents / Q000737:chemistry / Q000494:pharmacology; D013816:Thermodynamics
chemical_list: D019843:Archaeal Proteins; D011756:Diphosphates; D029968:Escherichia coli Proteins; D029701:Saccharomyces cerevisiae Proteins; D012997:Solvents; D012965:Sodium Chloride; D043564:Inorganic Pyrophosphatase
keywords: Bioenergetics; Biophysics; Enthalpy Change; Enzyme Catalysis; Halophile; Isothermal Titration Calorimetry; NaCl; Pyrophosphate; Thermodynamics; Water Activity
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