Abstract
Summary Frieden [Biochem. Biophys. Res. Commun. (1976) 68 , 914–917] recently identified limitations on using initial velocity data to distinguish certain ordered and random mechanisms. He found that the rapid equilibrium ordered mechanism with EB and EP abortive complexes cannot be distinguished from the random addition case, except by equilibrium exchange measurements. Reexamination of the rapid equilibrium assumption clearly demonstrates, however, that isotope exchange methods are also ineffective. Nonetheless, it is demonstrated that the rapid equilibrium assumption does not hold when the kcat exceeds 30–50 sec−1. Thus combined use of initial rates and isotope exchange approaches offer reliable tests for rigorously defining the mechanism of many, and possibly most, enzymes.
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