Standard Gibbs energy of metabolic reactions: V. Enolase reaction

The glycolytic pathway is one of the most important pathways for living organisms, due to its role in energy production and as supplier of precursors for biosynthesis in living cells. This work focuses on determination of the standard Gibbs energy of reaction Δ^Rg'⁰ of the enolase reaction, the ninth reaction in the glycolysis pathway. Exact Δ^Rg'⁰ values are required to predict the thermodynamic feasibility of single metabolic reactions or even of metabolic reaction sequences under cytosolic conditions. So-called "apparent" standard data from literature are only valid at specific conditions. Nevertheless, such data are often used in pathway analyses, which might lead to misinterpretation of the results. In this work, equilibrium measurements were combined with activity coefficients in order to obtain new standard values Δ^Rg'⁰ for the enolase reaction that are independent of the cytosolic conditions. Reaction equilibria were measured at different initial substrate concentrations and temperatures of 298.15 K, 305.15 K and 310.15 K at pH 7. The activity coefficients were predicted using the equation of state electrolyte Perturbed-Chain Statistical Associating Fluid Theory (ePC-SAFT). The ePC-SAFT parameters were taken from literature or fitted to new experimentally determined osmotic coefficients and densities. At 298.15 K and pH 7, a Δ^Rg'⁰(298.15 K, pH 7) value of -2.8 ± 0.2 kJ mol^-1 was obtained. This value differs by up to 5 kJ mol^-1 from literature data. Reasons are the poorly defined "standard" conditions and partly undefined reaction conditions of literature works. Finally, using temperature-dependent equilibrium constants and the van 't Hoff equation, the standard enthalpy of reaction of Δ^Rh'⁰(298.15 K, pH 7) = 27 ± 10 kJ mol^-1 was determined, and a similar value was found by quantum-chemistry calculations.