INVIVO KINETICS AND THE REVERSIBLE MICHAELIS-MENTEN MODEL

The Michaelis-Menten model accounts for many of the characteristics of enzyme reactions (1). Although it is obviously an oversimplification-that does not include cooperativity, allosteric effects, multiple intermediate complexes, or multiple substrates or products-it is used extensively as an introduction to enzyme kinetics in biochemistry and molecular biology. Because the textbook presentations usually emphasize the enzyme assay in vitro, in which the initial product concentration is zero, it is easy for students to get the impression that the model is less applicable in vivo than it actually is. Most textbooks introduce the kinetic structure of metabolic pathways in a qualitative manner, if they consider it at all. Application of quantitative relationships is usually undertaken only when considering regulatory steps. Although there are shortcomings in using the model for any one specific enzyme, values derived from the model are frequently applied successfully in modeling metabolic networks (2, 3), for example, the kinetic constants obtained from in vitro determinations based on the Michaelian behavior of individual enzymes. The expanding literature on the kinetics of metabolic reactions in vivo has not yet been extensively incorporated in biochemistry textbooks. The purpose of this article is to illustrate that this popular teaching model for enzyme kinetics can be successfully applied to metabolic reactions in vivo.