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Enzymes speed up reactions. Why do they stop working when it gets too hot?
Enzyme Activity lab · Last updated: 16 October 2026
You watch enzymes speed up a reaction, then turn up the temperature. At first the enzymes work faster, but past a certain point they suddenly stop. They find out why heat can ruin these tiny machines, which digest our food, copy our DNA and are even used in washing powders.
What you'll discover
- Enzymes are proteins that speed up chemical reactions in living things without being used up.
- Each enzyme has an active site with a precise shape that only fits certain molecules.
- Too much heat changes that shape for good, so the enzyme stops working: it has been denatured.
The science in plain words
Enzymes are biological catalysts. They speed up the chemical reactions that keep living things alive, such as breaking down food during digestion, often making them millions of times faster. Without enzymes, most of these reactions would be far too slow to support life.
Each enzyme is a protein, a long chain folded into a very particular three-dimensional shape. Part of that shape forms a pocket called the active site. The molecule the enzyme works on, called its substrate, fits into this site like a key in a lock. Once it is held there, the reaction happens easily and the products are released, so the enzyme can work again. Amylase in saliva, for example, breaks starch down into sugar.
Warming things up at first makes enzymes work faster, because molecules move faster and meet more often. But each enzyme has an optimum temperature where it works best, around 37 °C for most enzymes in the human body. Above that, the extra heat makes the protein vibrate so strongly that the weak bonds holding its shape break. The active site changes shape and the substrate no longer fits. The enzyme is denatured, and usually it cannot recover, just as a cooked egg white cannot turn clear again. Strong acid or alkali can denature enzymes too, although some, such as pepsin in the stomach, work best in acid.
Where it fits
- India, NCERT: Class 10 science, Chapter 5: Life Processes
- United States: Grades 9 to 12
Common questions
Why do enzymes stop working at high temperatures?
Enzymes are proteins held in a precise shape by weak bonds. High temperatures make the molecule vibrate so much that these bonds break and the shape changes. The active site no longer fits its substrate, so the enzyme stops working. This change is called denaturing.
What is the optimum temperature for human enzymes?
Most enzymes in the human body work best at about 37 °C, which is normal body temperature. Enzymes from other living things can be different: some bacteria that live in hot springs have enzymes that work well at temperatures that would denature ours.
What does denatured mean?
A denatured protein has lost its normal folded shape, usually because of heat or strong acid or alkali. For an enzyme, this means its active site no longer fits the molecules it works on. Cooking an egg is an everyday example of proteins being denatured.
Related labs
- Chemical Reactions: Molecules must crash hard enough to react. What makes that happen more often?
- Protein Folding: A protein is a long chain. How does it fold itself into exactly the right shape?
- Cellular Respiration: Living things get energy from food. Does it make a difference whether they have oxygen?
See all chemistry labs or every Taz science lab.
Try it yourself
The Enzyme Activity lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.