A thermodynamic analysis is presented for correlating and predicting reaction equilibrium properties for enzyme-catalyzed synthesis of valuable chemical compounds, such as small-molecule pharmaceuticals. Utilization of group-contribution methods is shown for obtaining reference-state ideal gas and standard-state solution Gibbs energies of formation of reactants and products, as well as for estimating solution nonidealities. Several transaminase reactions have been treated. These cases were previously shown not to be accurately predicted by estimates of Gibbs energies of formation from group contributions. This work achieves accurate descriptions of the data by regressing some unknown component values with a method that accounts for uncertainties in property and data uncertainties. The results and methodology can be applied to many biosystems, but having some reaction equilibrium measurements, with uncertainties, that involve the species of interest are essential for reliable results.