# Effect of His-tag on enzyme activity

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?

• 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. – Curt F. Apr 28 '16 at 15:58