Answer: Denaturing
Explanation: Enzymes are made of amino acids that are linked by bonds, amongst those bonds are hydrogen bonds that are very weak. High temperatures and low pH(acidity) breaks those bonds changing the molecular conformation of the enzymes. That way they denature and loss the ability to perform their functions. High temperatures and acidity alter the structure of their active site preventing the substrate from binding to them. While high temperatures denatures enzymes, low temperature renders them inactive.