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In my biochemistry laboratory class, I designed an experiment to study the effect of a His-tag on enzyme activity. First, I measured the activity of the His-tagged enzyme. Then, I cut off the His-tag (using enterokinase) and measured the activity again.

After data analysis, I was able to calculate the $K_\mathrm{M}$ values and $k_\mathrm{cat}$ values for both enzymes. How should I interpret the data? I hypothesize that the His-tag could interfere with kinetics. But, how do I use the data to show this? Should I compare $K_\mathrm{M}$ values or $k_\mathrm{cat}$ values?

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  • $\begingroup$ People would usually compare three numbers: $K_M$, $k_{cat}$, and $k_{cat}/K_M$. If any of those numbers are significantly different for the wild-type enzyme compared to the His-tagged enzyme, then you could conclude that the His tag interferes, at least somewhat. $\endgroup$ – Curt F. Apr 28 '16 at 15:58
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I assume the excised-off His-tag as well as the enterokinase protease are removed from the system.
I would use the catalytic efficiency, i.e. the ratio of kcat/KM as the metric to compare the results of two conditions.

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