Cómo afecta la temperatura a la actividad enzimática (y por qué)

Here is how temperature affects enzyme activity: as it gets warmer, molecules move faster and collide more often, so the enzyme works faster, up to its optimum temperature (about 37 °C for most human enzymes). Above that, heat breaks the bonds that hold the enzyme in shape, its active site changes, and the rate drops sharply. An enzyme changed like this is denatured, and cooling it down doesn't bring it back.

What is an enzyme?

An enzyme is a protein that acts as a biological catalyst: it speeds up a chemical reaction in a living thing without being used up. Amylase in your saliva breaks starch into the sugar maltose; pepsin in your stomach breaks proteins into shorter chains called peptides. Without enzymes, these reactions would be far too slow to keep you alive at body temperature.

Each enzyme is a long chain of amino acids folded into a precise 3D shape. One small region, the active site, has exactly the right shape and charges to hold one kind of molecule, its substrate.

The lock and key model

In the lock and key model, the substrate fits the active site the way a key fits a lock. The two join to form an enzyme–substrate complex, the reaction happens, the products leave, and the enzyme is free to take the next substrate. One enzyme molecule can do this thousands of times a second.

The model explains why enzymes are specific: amylase only works on starch because only starch fits its active site. A refinement, the induced fit model, says the active site isn't completely rigid: it moulds slightly around the substrate as it binds, gripping it more tightly. Both models agree on the key point: shape is everything. Anything that changes the shape of the active site changes how well the enzyme works.

Why does warming an enzyme speed it up?

For a reaction to happen, a substrate molecule has to bump into an active site. At low temperatures molecules move slowly, so these collisions are rare and the rate is low. The enzyme isn't damaged by cold; it is just slow. Put it back in the warm and it recovers fully.

Warming gives the molecules more kinetic energy. They move faster, collide more often and collide harder, so more enzyme–substrate complexes form each second. Below the optimum, a 10 °C rise roughly doubles the rate (biologists call this a Q₁₀ of about 2).

What is the optimum temperature?

The optimum temperature is the temperature at which an enzyme works fastest. It is a balance: high enough for plenty of fast collisions, but not so high that the enzyme starts to lose its shape.

Different organisms have enzymes suited to where they live:

Enzyme	Where it works	Optimum temperature	Optimum pH
Salivary amylase	human mouth	about 37 °C	about 7
Pepsin	human stomach	about 37 °C	about 2
Taq polymerase	bacteria in hot springs	about 72 °C	about 8

Taq polymerase comes from Thermus aquaticus, a bacterium found in hot springs. Because it survives near-boiling temperatures, it is the enzyme used in PCR, the method that copies DNA in labs and in COVID tests.

What happens when an enzyme is denatured?

When an enzyme denatured by heat is examined, its chain of amino acids is still intact, but the weaker bonds (hydrogen bonds and others) that held it in its folded shape have broken. The protein unfolds and the active site loses its shape, so the substrate no longer fits and the enzyme can't work.

That's why the rate graph isn't symmetrical. It rises gradually to the optimum, then falls steeply, because denaturing speeds up dramatically with every degree. Most human enzymes are largely denatured by 50 to 60 °C. It is also why denaturing is permanent: you can't refold the protein just by cooling it, in the same way you can't un-fry an egg. Egg white is mostly protein, and frying it is denaturation you can see.

A useful way to describe the graph in an exam:

1. Below the optimum: rate rises with temperature (more kinetic energy, more frequent successful collisions).
2. At the optimum: rate is at its maximum.
3. Above the optimum: rate falls quickly, as the active site changes shape and the enzyme denatures.

What is the effect of pH on enzymes?

The effect of pH on enzymes works through shape too. Each enzyme has an optimum pH. Move away from it and the extra H⁺ or OH⁻ ions disturb the charges in the active site and the bonds that hold the protein folded, so fewer substrates fit. Pepsin is happiest at pH 2, in stomach acid, and stops working in the neutral small intestine. Amylase works best near neutral and is stopped by stomach acid. Extreme pH denatures an enzyme just as heat does.

Two other things change the rate:

• Substrate concentration: more substrate means more collisions, until every active site is busy. Then the rate levels off.
• Inhibitors: a competitive inhibitor looks like the substrate and blocks the active site; a non-competitive inhibitor binds elsewhere and bends the active site out of shape.

Common exam mistakes

• Saying enzymes are "killed". Enzymes aren't alive. Say "denatured", and say why: the active site changes shape so the substrate no longer fits.
• Saying cold denatures enzymes. Cold only slows them down; the effect is reversible.
• Saying the enzyme is used up. Catalysts are unchanged at the end of the reaction.
• Describing the graph without explaining it. Mention kinetic energy and collisions on the rising side, shape and denaturing on the falling side.

Enzymes are proteins, so their shape is ultimately written in DNA: see how DNA makes a protein. To see where enzymes are made inside a cell, open the Cell Explorer and find the ribosomes.

Frequently asked questions

What is the optimum temperature for human enzymes?

Most human enzymes work fastest at about 37 °C, which is normal body temperature. That's no coincidence: our enzymes evolved to work in the conditions inside our bodies. Above about 40 °C they start to lose their shape, which is one reason a very high fever is dangerous. Enzymes from other organisms can have very different optimums, from near freezing to above 70 °C.

Can a denatured enzyme work again?

Usually not. Denaturing breaks the bonds that hold the enzyme folded in its exact shape, so the active site no longer fits the substrate. Cooling it down doesn't refold it correctly, so the change is permanent. This is different from cold, which only slows an enzyme down. Warm a cold enzyme up again and it works normally.

Why does the rate fall so quickly above the optimum?

Two effects pull in opposite directions. Higher temperature still makes molecules collide faster, but it also makes the enzyme unfold, and unfolding speeds up very steeply with each extra degree. Just above the optimum, enzymes are being denatured faster than the extra collisions can make up for, so the rate drops sharply instead of tailing off gently.

Do enzymes get used up in a reaction?

No. Enzymes are catalysts, so they come out of each reaction unchanged and can immediately take another substrate molecule. A single enzyme can catalyse thousands of reactions every second. Your body still replaces enzymes over time, because they slowly wear out and get broken down, but not because the reaction itself uses them.

Try it yourself

Open the Enzyme Lab and watch amylase break down starch at 37 °C. Drag the temperature down to 10 °C and see the molecules slow down; then bring it back and watch the enzyme recover. Now push it up to 70 °C and watch "still folded" fall as the enzyme denatures. Cooling it won't help this time: press New enzyme to start again. Try pepsin at pH 7 and then at pH 2, and switch to the hot-spring enzyme to see an optimum far above body temperature.