LETTER pubs.acs.org/jchemeduc
Pitfalls in Assuming a “Rate-Limiting Step” in a Mechanism Sidney Toby* Professor Emeritus, Department of Chemistry and Chemical Biology, Rutgers, the State University of New Jersey, Piscataway, New Jersey 08854, United States ABSTRACT: An anomaly in the article “The Effect of Temperature on the Enzyme-Catalyzed Reaction: Insights from Thermodynamics” (DOI: 10.1021/ed800088n) is discussed. KEYWORDS: Physical Chemistry, Kinetics n the article “The Effect of Temperature on the EnzymeCatalyzed Reaction: Insights from Thermodynamics”,1 there appears the statement “According to chemical kinetics, when there are several steps, the slow or rate-limiting elementary step (reaction 3 in our model) determines the reaction rate.” This is true for reactions in series; if we have the sequence
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AfBfCfD then if there is a slow step, it will be rate limiting. But if the system consists of parallel reactions AfB AfC AfD then if there is a fast step, it will govern the overall rate. If a kinetic system under consideration consists of many reactions, concepts like rate-limiting step have limited significance and may change with experimental conditions. The concept is an approximation that should be used only when computer simulations cannot be made. If the elementary steps in a mechanism and the rate constants for the steps are known (or may be estimated), then the kinetic behavior is better described by using computer simulation. This is in contrast with the reaction stoichiometry that always has significance and may be derived from kinetic considerations.2
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ REFERENCES (1) Aledo, J. C.; Jiminez-Riveres, S.; Tena, M. J. Chem. Educ. 2010, 87, 296–298. (2) Toby, S.; Tobias, I. J. Chem. Educ. 2003, 80, 520–523. Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.
Published: June 10, 2011 1028
dx.doi.org/10.1021/ed100288j | J. Chem. Educ. 2011, 88, 1028–1028
