Solvent Effects on Enzyme Activity

It was early discovered that enzyme activity in organic solvents depends very much on the nature of the solvent. It was realized that the polarity or hydrophobicity of the solvent had a large influence, with non-polar hydrophobic solvents often providing higher reaction rates than more polar, hydrophilic solvents. When the kinetics of enzymatic reactions is studied, it is often found that Km values in organic solvents are much higher than those in water for the corresponding 

The same kind of reasoning can be used even though the aqueous phase is small or even non-existent. What really matters is the partitioning of the substrate between the bulk solvent and the active site of the enzyme, and this is not influenced by the introduction of an aqueous phase in between. 

Partitioning between water and water-immiscible organic solvents is thus a straightforward way to get quantitative data for predicting solvent effects on the conversion of a certain substrate. However, the method is not applicable to water-miscible solvents. An alternative way to quantify solvation is to carry out theoretical calculation of interactions between the various components in the reaction mixture. 

When substrate activities are used instead of substrate concentrations in studies of enzyme kinetics in organic media, solvent effects due to substrate solvation disappear. Remaining solvent effects should be due to direct interactions between the enzyme and the solvent. In a study of lipase-catalyzed esterification reactions, it was found that Km values based on activities were indeed more similar tban those based on concentrations in different solvents, but still some differences remained [49]. 

As discussed in Section 1.2.3, it is crucial tbat the effects of solvents are studied at fixed water activity, or else indirect effects due to competition for water between enzyme and solvent will cause strong effects and mask the true solvent effects. In general, when correcting for substrate solvation, hydrophobic solvents seem to give higher rates than other solvents [5]. 

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