Boiling denatures the enzyme, but freezing has no effect.
At high temperatures (even if it’s not boiling, about 60°C is enough) hydrogen bonds and other interactions between different amino acids of an enzyme are broken. This is not degradation, no bonds are broken – only interactions (London, Van der Waals, etc). This makes the protein to denature, which means it will lose its 3D shape and become linear – no longer globular: it only preserves its primary structure. This makes the enzyme to become useless: it can’t make the substrate become product anymore. So the activity becomes practically zero.
Freezing DOES make a change in enzyme activity, but for other reasons which are not denaturing (the enzyme has the same structure as before):
-Concentration: Now some of the water has become ice and you have 2 phases, which are the solution and the ice. So the solution has less water, and so a higher concentration. Higher concentration means… higher activity! So it has the same structure, but it works more.
-Cell disruption: Membranes are broken and this will release molecules that used to be stored in an organelle. Sometimes this makes the enzyme find the substrate – so it actually increases activity! As I said for concentration, this means the enzyme has the same structure but it works more.