8 - Analysis of Progress Curves

Abstract

This chapter explores the process of analysis of progress curves for enzyme kinetic studies. A kinetic equation can be written in two distinct forms, either as an expression that shows how the concentration of a reactant changes with time, or as one that shows how the rate of reaction varies with the concentrations of the reactants. The enzymologist normally considers the second of these to be the usual form, whereas to the chemist the first is the usual form. Much information is contained in a progress curve (for an enzyme kinetic study) than merely the extrapolated rate at zero time. In the early stages of an enzyme kinetic study, when the main problem is to discover what equation is obeyed and what the approximate values of the kinetic parameters are, the usual initial rate studies are appropriate. It is then possible, in principle, to obtain accurate values of the kinetic parameters from a fairly small number of experiments. There is also a major advantage in being able to avoid the subjective nature of estimating initial rates from a curved plot: it is almost impossible to do this reproducibly and without bias. The analysis of progress curves is fraught with traps for the unwary, and it is perhaps for this reason that so many enzymologists have taken the safe but unadventurous course of confining their attention to initial rates.