So far, we have seen how chemically identical behavior of isotopes are exploited in elucidating enzyme mechanisms. Despite their remarkable chemical similarity, isotopic substitution does affect reaction rates that directly involve them. Isotopic substitution can influence the equilibrium position, affect reaction rate, or both. The former (affecting the equilibrium constant) is termed equilibrium isotope effect while the latter (affecting the rate constant) is termed kinetic isotope effect (KIE). Commonly encountered types of isotope effects in enzyme study are given with examples below.