We report the effect of conformational dynamics on the fluctuations of electric fields in the active site of the enzyme ketosteroid isomerase (KSI). While KSI is considered to be a rigid enzyme with little conformational variation to support different stages of the catalytic cycle, we show that KSI utilizes cooperative side chain motions of the entire protein scaffold outside the active site to modulate electric fields in the active site. We find that while the active site residues Asp-40 and Tyr-16 maintain their electric field contributions at all effective time scales, the conformational dynamics of a single active residue, Asp-103, promotes large electric field fluctuations that contribute to different stages of the catalytic cycle, including the catalytic step and product release.