What is the name given to biological catalysts?

What is a biological catalyst?

A catalyst is a substance that speeds up a reaction without being used up, so it can be used again and again. Living cells need thousands of reactions to happen quickly at body temperature, and enzymes are the catalysts that make that possible. Without them, digestion, respiration and building new cells would be far too slow to keep you alive.

Enzymes work by lowering the activation energy of a reaction, the energy needed to get it started, as OpenStax Biology explains. Catalysts in chemistry do the same thing; our Reaction Kinetics course shows it on an energy profile.

How do enzymes work?

Almost all enzymes are proteins: long chains of amino acids folded into a precise 3D shape. Part of that shape is a pocket called the active site. The AQA GCSE specification puts it like this: enzymes catalyse specific reactions in living organisms due to the shape of their active site.

1. The substrate (the molecule the enzyme acts on) fits into the active site.
2. They form an enzyme-substrate complex.
3. The reaction happens: the substrate is broken down, or two substrates are joined.
4. The products leave the active site, and the enzyme is free to work again.

The lock and key model

GCSE courses use the lock and key theory as a simplified model. The active site is the lock and the substrate is the key: only a key with exactly the right shape fits. That explains why each enzyme is specific to one substrate. At A-level, and in OpenStax, you meet the more refined induced fit model, where the active site changes shape slightly as the substrate binds, gripping it more tightly.

Digestive enzymes

Digestive enzymes break large food molecules into small ones that can be absorbed into the blood. These are the three groups AQA expects you to know:

Bile helps too, though it isn't an enzyme. AQA notes it is made in the liver, stored in the gall bladder, and is alkaline to neutralise stomach acid. It also breaks fat into tiny droplets, and the alkaline conditions and large surface area speed up lipase. Try it: this question asks you to match each enzyme to its job.

How temperature and pH affect enzymes

Every enzyme has an optimum temperature and pH at which it works fastest.

• Rising temperature, up to the optimum: molecules move faster, so enzyme and substrate collide more often and the rate goes up. For many human enzymes the optimum is around body temperature, 37 °C.
• Above the optimum: the extra energy shakes the enzyme's shape apart. The pocket loses its exact shape and the substrate can't bind. The enzyme is denatured, and the rate drops quickly. Denaturing is usually permanent.
• pH: a pH far from the optimum also changes the active site's shape and can denature the enzyme. Different enzymes suit different places: proteases in the stomach work best in strong acid, while amylase in the mouth works best near neutral.

Here x is the temperature in °C and y is the rate of reaction as a percentage of the fastest rate. This is a sketch of the typical shape for a human enzyme, not data from one experiment: the curve rises gradually, peaks at the optimum, then drops steeply once the enzyme starts to denature.

The amylase practical

AQA's required practical investigates the effect of pH on the rate of reaction of amylase. Amylase and starch are mixed in buffer solutions of different pH, and a drop of the mixture is tested with iodine every 30 seconds. Iodine turns blue-black with starch, so when it stays orange-brown, all the starch has been broken down. The shorter that time, the faster the enzyme.

In these example results, the rate is highest at pH 7, so the optimum is around pH 7. Calculating 1000 ÷ time turns "shortest time" into "biggest number", which makes a clearer graph. To find the optimum more exactly, you would repeat with pH values closer together around 7, keeping the temperature and the volumes the same. Plot your own results in our Practical Data Lab.

Enzymes outside the body

• Biological washing powders contain proteases and lipases that digest food and grease stains at low temperatures.
• Food production uses enzymes to make syrups from starch, soften baby foods and help cheese making.
• Medicine uses enzymes in tests, such as the enzyme strips that measure blood glucose.

Common mistakes

• Saying enzymes are killed. Enzymes aren't alive. They are denatured.
• Saying the substrate is denatured. It is the enzyme's active site that changes shape.
• Saying the active site and substrate are the same shape. They are complementary: they fit together like a lock and key.
• Saying enzymes are used up. Like all catalysts, they can be used again.
• Calling bile an enzyme. It emulsifies fat and neutralises acid, but it doesn't catalyse a reaction.
• Thinking low temperature denatures enzymes. Cold slows them down, but they usually work again when warmed.

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