Feruloyl esterase (rumen microorganism)

The Gibbs energy of reaction (Δ^Rg) is the key quantity in the thermodynamic characterization of biological reactions. Its calculation requires precise standard Gibbs energy of reaction (Δ^Rg⁺) values. The value of Δ^Rg⁺ is usually determined by measuring the apparent (concentration-dependent) equilibrium constants K, e.g., the molality-based K_m. However, the thermodynamically consistent determination of Δ^Rg⁺ requires the thermodynamic (activity-based) equilibrium constant K_a. These values (K_m and K_a) are equal only if the ratio of the activity coefficients of the reactants to the activity coefficients of the products (K_γ) is equal to unity. In this work, the impact of K_γ on the estimation of K_a for biological reactions was investigated using methyl ferulate (MF) hydrolysis as a model reaction. The value of K_γ was experimentally determined from K_m values that were measured at different reactant concentrations. Moreover, K_γ was independently predicted using the thermodynamic model ePC-SAFT. Both the experimentally determined and the predicted K_γ values indicate that this value cannot be assumed to be unity in the considered reaction. In fact, in the reaction conditions considered in this work, K_γ was shown to be in the range of 3 < K_γ < 6 for different reactant molalities (2 < mmol MF kg^-1 < 10). The inclusion of K_γ and thus the use of the thermodynamically correct K_a value instead of K_m lead to remarkable differences (almost 40%) in the determination of Δ^Rg⁺. Moreover, the new value for Δ^Rg⁺ increases the concentration window at which the reaction can thermodynamically occur. The influence of additives was also investigated both experimentally and theoretically. Both procedures consistently indicated that the addition of NaCl (0 to 1 mol kg^-1 water) moderately decreased the value of K_γ, which means that the values of K_m increase and that a higher amount of products is obtained as a result of the addition of salt. Additionally, K_m was found to strongly depend on pH. A ten-fold increase in the K_m values was observed in the pH range of 6 to 7; this increase corresponds to a change of more than 100% in the value of Δ^Rg⁺.