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.
An enzyme's active-site shape depends on the precise folding of its molecule, held in place by weak bonds that break easily under too much heat or the wrong pH. Below the optimum, an enzyme simply works more slowly, because molecules move and collide less often — this is temporary, and speeds back up as temperature rises. Above a certain point, though, those weak bonds break, the enzyme's shape permanently distorts, and the active site no longer matches its substrate's shape at all — like a lock bent so badly no key, however correct, will ever turn it again. That is why "too cold" and "too hot" are not simply opposite ends of one slow-fast scale: one recovers, the other does not.