The previous sections of this text have discussed the kinetic behavior of individual enzymes in a controlled environment. The need for careful control of the reaction conditions has been stressed. An enormous amount of useful information concerning the mode of action of enzymes has been obtained during this century from these types of studies. Useful as these studies have been, the concept of an enzyme catalyzing a reaction in isolation runs counter to the purpose for which enzymes have been provided in nature. The purpose of an enzyme in nature is to catalyze a reaction in concert with the other enzymes in the metabolic pathway, and the purpose of a metabolic pathway is to catalyze a series of reactions in concert with the many other pathways with which it interacts. Anyone who has given even cursory thought to this matter must have concluded that, in the normal living organism, the action of myriad enzymes is a beautifully coordinated process. On the other hand, if coordination of the action of these enzymes becomes flawed, it is certain that the living organism is going to encounter serious difficulty.

In the view of this author, the investigation of the coordination of multienzyme systems is the most exciting challenge to the enzymologist. A multitude of questions present themselves. However, it is essential that the enzymologist follow the example of a judicious detective and ask purposeful questions and interpret the answers obtained in an insightful manner. For example, it is not sufficient to ask which enzyme catalyzes a rate-limiting step in a pathway.