Apopular lab that can be performed by students is to test the reaction rate of catalase enzyme when it acts on the substrate hydrogen peroxide. catalase has the ability to break down hydrogen peroxide. catalase can be found in beef liver from the grocery store! however, if the beef liver is boiled first, the catalase will not be able to break down hydrogen peroxide. what is the variable affecting enzyme function

The variable affecting the enzyme action is temperature.  

Catalase is the enzyme which acts upon the "substrate hydrogen peroxide", breaking down them into water and oxygen gas.  Each and every enzyme is a protein, which has a tertiary structure which is essential for the enzyme action. The tertiary structure of the enzyme provides the site of enzymatic action where the substrate do gets attached for the enzymatic action to take place.  

Temperature, or heat, changes the protein structures irreversibly after a certain range. So if the beef liver is boiled, then the enzyme structure gets permanently changed, which is why the catalase cannot breakdown the peroxides, after boiling.  

there are so many examples for that in different areas, like biology experiment carried out in our lab recently.

here's one link: