Enzymes: the lock and key model
Biology · Key Stage 3 · The School
A catalyst that is not used up
An ENZYME is a biological CATALYST: a substance that speeds up a chemical reaction without being changed or used up itself. This means a single enzyme molecule can help many reactions happen, one after another, which is why the body needs only tiny amounts of most enzymes to work effectively.
The lock and key model
An enzyme has a precisely shaped ACTIVE SITE, like a lock. Only a SUBSTRATE molecule with a matching shape, like the right key, fits into it. Once it fits, the enzyme speeds up the reaction that changes the substrate into a product, which then leaves the active site free for the next substrate molecule.
Temperature and pH change the shape
Enzymes work fastest at an OPTIMUM temperature and pH — for most human enzymes, around body temperature and a near-neutral pH. Too hot, and the enzyme's shape is permanently destroyed, called DENATURING, so the active site no longer fits its substrate at all. Too far from the optimum pH does the same. Once denatured, an enzyme cannot work again, however much the temperature is later corrected.
Imagine a class tests amylase, a human enzyme that breaks starch into sugars. The times below are made up to show the method. Three tubes hold equal amounts of amylase and starch. Tube A sits in ice water at 0 °C. Tube B sits in a 37 °C water bath, close to body temperature. Tube C sits in a 60 °C water bath. Every minute, a drop from each tube goes onto iodine. Iodine solution is orange-brown. It turns blue-black while starch is left, and stays orange-brown once the starch has gone. At 1 minute, only B is orange-brown. At 5 minutes, A and C are still blue-black. Is each enzyme slowed or denatured? The class moves A and C into the 37 °C bath. One minute later A turns orange-brown. Ten minutes later C is still blue-black. If C were only working slowly, moving it to 37 °C would have sped it up, as it did for A. So A was only slowed, because molecules collide less often in the cold, and its shape was not damaged. C is denatured. The explanation is that heat broke the weak bonds that held its shape, so its active site no longer fits starch. A and B hold working enzyme, and C does not. Pupils doing this for real wear eye protection, because iodine can irritate eyes, and an adult supervises the hot water. Real results depend on the enzyme used, so check its label for the best temperature.