Constant Enthalpy Change Value during Pyrophosphate Hydrolysis within the Physiological Limits of NaCl
J. Biol. Chem_288_29247.pdf 1.06 MB
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.
This work was supported by Grant-in-aid for Scientific Research on Innovative Areas 20118005 from the Ministry of Education, Culture, Sports, Science, and Technology of Japan.
Journal of Biological Chemistry
The American Society for Biochemistry and Molecular Biology, Inc.
This research was originally published in the Journal of Biological Chemistry. Satoshi Wakai, Shun-ichi Kidokoro, Kazuo Masaki, Kaoru Nakasone, and Yoshihiro Sambongi. Constant Enthalpy Change Value during Pyrophosphate Hydrolysis within the Physiological Limits of NaCl. J Biol Chem. 2013; 288:29247-29251. © 2013 the American Society for Biochemistry and Molecular Biology, Inc.