The solvent effect is the effect solvation has on the reaction rate. When a substrate is solved in water hydrogen bonds stabilize the grounds state thereby making it less reactive.

Many enzymes have a hydrophobic (apolar, aprotic) pocket in their catalytic centre, thereby desolvating the substrate and reducing the dG‡.

Solvation stabilizes charged compounds more than neutral compounds, so a protonated compound will be less stabilized inside the hydrophobic cavities. This leads to a shift of the acid-base equilibrium inside these cavities, i.e. to a decreased pKa.