Abstract:
Analysis of enzyme kinetic data requires more than just
comparisons of Kms and Vmaxs using the corresponding error estimates of the
parameters. This approach is often employed, but it can prompt contradictory
and misleading inferences that might be avoided using a confidence band. We
derive expressions for the confidence band for the Michaelis-Menten rate
equation that rely on estimates and variances of Km and Vmax. These can be
expressed in terms of the substrate concentration or the rate of reaction. While
these equations are simple, they are nonlinear, which reinforces the need to
consider both parameters simultaneously. The equations show that the
amplitude of the confidence interval () passes through a maximum if the
variance of the Km is sufficently large compared with the variance of Vmax.
We illustrate the value of the expressions by applying them to comparisons of
the kinetics of enzymes involved in nitrogen metabolism in parasites. These
examples confirm that (i) the variance of the estimate of the Km has a
particularly significant effect on and (ii) comparisons among Kms and among
Vmaxs are not necessarily sufficient to determine the significance of differences
in activity.
