Kinetic Isotope Effects in Enzymic Reactions

Abstract

During the conversion of one molecule to another, new bonds are formed, old bonds broken, and other bonds experience changes, either transient or permanent, in their hybridization. Isotopic substitution can have a profound influence on the energetics of these bonding changes. The effect of isotopic substitution on the force constant is usually minor; however, the changes in mass can be a relatively major factor. This chapter discusses the causes of primary and secondary kinetic effects for isotopes of hydrogen and gives some examples of the application of these effects to problems of biochemical interest. Division of isotope effects two categories—primary and secondary—that facilitates discussion. A primary kinetic isotope effect results when the bond to the isotopically substituted atom is broken in the transition state. A secondary isotope effect results when the bond to the isotopically substituted atom, though not broken, experiences significant changes in hybridization during the reaction.

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