What Are the Effects of Boiling & Freezing on Enzyme Activity?

Enzymes are proteins with three-dimensional shapes that define their function. The active site is the most crucial part of the enzyme and is akin to a mouth with teeth that are properly in place. The atoms making up protein molecules can vibrate to different degrees depending on the temperature. Too much jiggling, due to boiling, or too little movement, due to freezing, will slow down an enzyme’s activity. Enzymes have comfort zones, outside of which they stop working. Boiling or freezing an enzyme completely stops its activity. However, it is interesting to note that what would be considered as boiling and freezing temperatures that are inhospitable to human life still have enzymes that function normally therein.

Enzymes are proteins with a three-dimensional (3D) shape that defines their activity. An enzyme’s 3D structure results from its tertiary structure. An enzyme has an active site, like a mouth, at which it binds its substrate -- the molecule that the enzyme will modify. This active site is formed by the tertiary structure of the enzyme, meaning it is formed by the presence of certain amino acid side chains (also known as amino acid R groups or function groups) that protrude, like teeth, into the space of the mouth. The side chains interact with the substrate just like teeth that bite into food, crushing, squishing or snapping the substrate.

Enzymes, like any other proteins, have specific 3D shapes that only exist at certain temperatures that they find comfortable. Enzymes of the human body maintain their normal shape and activity at around normal body temperature, about 37.5 degrees Celsius. Increasing the temperature increases the speed at which the atoms within an enzyme vibrate. Too much vibration causes the intermolecular interactions that hold the enzyme’s 3D shape to fall apart, which causes the enzyme to change its shape, including the shape of its active site. Thus, enzyme activity is completely abolished due to boiling. Most animal enzymes will denature, or unfold, starting at 40 degrees Celsius.

Boiling an enzyme adds so much heat to the system that atoms of the enzyme will vibrate to the extent of completely denaturing, or unfolding the enzyme. Instead of a specific 3D shape that properly positions the amino acid side chain teeth within the mouth that is the active site, there is no active site at all. Thus, enzyme activity is completely abolished when an enzyme is boiled. However, boiling only stops enzyme activity if the boiling temperature of the solution in which the enzyme is dissolved is outside the enzyme’s comfort zone. Thermophiles, bacteria that live in hot springs, have enzymes that function normally at boiling temperatures, so boiling those enzymes would not destroy enzyme activity. Hypothetically, even the thermophilic enzymes will denature if raised to a high enough temperature, but at that point the aqueous solvent might have evaporated, leaving solid enzyme crystals. One would need a solvent with a boiling point that is higher than the enzyme's comfort zone.

Just as atoms in an enzyme can vibrate too much at high temperatures, lowering the temperature too much can slow down atom vibrations to the point where the active site, or mouth, freezes in place. Thus, even if the amino acid side chain teeth are in the right positions in the active site, the mouth cannot bite down on the substrate because the whole enzyme is frozen. Freezing will also abolish enzyme activity. However, psycrophiles, bacteria that live in very cold places, have enzymes that can function at freezing temperatures. But psychrophile enzymes can still freeze at temperatures that cause the whole cell to freeze, regardless of its antifreeze defense mechanism.