[Prozac]

Hello,
       My lab partner and I purify some enzymes using different methods for comparison. The data we got is shown below:

                             My prep    partner’s prep
Vmax (μmoles/mg sec)    5,000    8,000
Km (mM)                            5.0    4.8

Why is my V_max so much lower than my lab partner's, yet our K_m is almost identical


Any Suggestions?

[Yggdrasil]

The Vmax you have listed is actually the specific activity of the enzyme prep (Vmax/mg of enzyme).  The specific activity is used to quantify the purity of your protein prep.

Consider an enzyme where 10ng of enzyme gives a Vmax of 100µmol/sec in a particular assay.  Here, the specific activity would be 10µmol/sec/mg.  Now consider what happens if you use a prep of enzyme that is only 50% pure.  10ng of this impure prep would contain only 5ng of enzyme, so the measured Vmax would be 50µmol/sec and the specific activity would be 5µmol/sec/mg.

Also, unless the impurities contain some type of inhibitor or other compounds that interfere with the reaction, the impurities will not affect the measurement of Km (b/c this value does not change with enzyme concentration).