ABSTRACT
The existence of a rate-limiting step in a reaction sequence has important implications for the response of the entire pathway to changes in reactant concentrations or to changes in the rate of individual steps. Metabolic pathways can be regulated efficiently by accelerating rate-limiting steps. The identification of rate-limiting steps in a reaction sequence is important for the dissection of the reaction mechanism, and to understand the behavior of the overall system to changes in concentrations. Rate-limiting steps in biochemical reactions can be identified by the kinetic isotope effect. Sometimes a kinetic isotope effect is measured even if the bond where the H-D substitution has been introduced is not the bond that breaks in the rate-limiting step. When the hydrogen in the methyl group of methylmalonyl-CoA is substituted by deuterium, a strong kinetic isotope effect is observed, pointing to abstraction of a hydrogen radical from this methyl group as the rate-limiting step of the catalyzed reaction.
