How Do Enzymes Work? An Interactive Explainer

How do enzymes work? The short answer

An enzyme is a molecule that speeds up a chemical reaction without being used up. Almost all enzymes are proteins: long chains of amino acids folded into a particular 3D shape.¹⁵ The folding leaves a groove or pocket called the active site.¹¹ The molecule the enzyme works on, the substrate, fits into that pocket. The enzyme holds it in just the right way so that bonds break or form more easily, and the reaction needs far less energy to get started.¹ The finished products drift away, and the enzyme is ready to do it again.¹⁵

In one sentence: enzymes lower the "activation energy" hill that every reaction has to climb.

Try it at home: the sweet cracker test

Chew a plain, unsalted cracker and keep it in your mouth for a minute or so. It slowly starts to taste a little sweet. A cracker is mostly starch, a long chain of glucose sugar units that is too big to taste sweet. Your saliva contains an enzyme called amylase, which cuts starch into small pieces of two or three glucose units, such as maltose.⁹⁴ Those small sugars can reach your sweet taste buds. Britannica puts it simply: amylase in saliva "converts starch into sugar".¹¹

What is an enzyme made of?

Most enzymes are proteins, built from the same 20 kinds of amino acids as every other protein in your body.³ The order of the amino acids decides how the chain folds, and the fold decides the enzyme's job.¹¹ A few enzymes are made of RNA instead; these are called ribozymes.⁵¹¹ Some enzymes also need a helper to work: a cofactor, such as a metal ion, or a coenzyme, often made from a vitamin.¹¹¹ Catalase, for example, holds an iron atom in its active site.⁸

Enzymes are not alive. They are molecules, made by living cells from instructions in genes (see why DNA is important).⁵

How do enzymes lower activation energy?

Even reactions that release energy need a small push before they can start. That push is the activation energy, and the higher it is, the slower the reaction goes.² Picture a boulder that would happily roll downhill if it could just get over the bump in front of it. Sugar is a good real example: breaking it down releases energy, yet sugar can sit in a sugar bowl for years because the activation energy is too high for it to fall apart by itself.²

Heat can supply activation energy, because it makes molecules move and collide harder.² But heating a cell enough to speed up its reactions would kill it.¹¹ So cells use enzymes to make the hill lower instead. According to OpenStax Biology, they do it in a few ways:¹

Positioning: they bring two substrates together, lined up in exactly the right orientation.
Straining: they bend the substrate so its bonds are easier to break.
A better environment: the amino acids in the active site can make a tiny patch that is, say, slightly acidic, which suits the reaction.
Taking part: some briefly bond to the substrate as a step in the reaction, then return to their original state.

Enzymes don't change how much energy a reaction releases or absorbs overall. They only lower the hill in the middle.¹⁶

	Without an enzyme	With an enzyme
Activation energy	High hill	Lower hill
Overall energy change	Same	Same
Speed	Slow, sometimes extremely slow	Often millions of times faster
Is the catalyst used up?	n/a	No, it's reused

How much do enzymes speed up chemical reactions?

A lot. Enzymes let reactions in your body happen millions of times faster than they would without them.⁷ Some go far beyond that. Researchers at the University of North Carolina measured a reaction your body needs to build hemoglobin (and plants need to build chlorophyll). Without its enzyme, it has a half-life of 2.3 billion years, about half the age of Earth. With the enzyme, it happens in milliseconds.¹⁰

The active site: lock and key vs. induced fit

Each enzyme is choosy. The size, shape and chemistry of its active site suit only a particular substrate, so each enzyme usually speeds up just one kind of reaction.¹¹¹ A cell contains thousands of different enzymes, each with its own job.⁵

Scientists once described this as a lock and key: the substrate fits the active site perfectly, in one step. The modern picture is called induced fit. As the substrate binds, the enzyme shifts its shape slightly to grip it more tightly, a bit like a hug.¹⁶ The enzyme and substrate together are called the enzyme-substrate complex.¹

Because the shape matters so much, a look-alike molecule can jam the pocket. This is called competitive inhibition. Other molecules switch enzymes off by binding somewhere else and changing the enzyme's shape (allosteric inhibition). Cells use both tricks to control their chemistry.¹

Why do enzyme names end in "-ase"?

Many enzymes are named by adding "-ase" to the name of their substrate: urease breaks down urea, lactase breaks down lactose (milk sugar).¹¹ Not every enzyme follows the rule; pepsin and catalase are older names.¹¹

Examples of enzymes in your body

Enzyme	Where	What it does
Amylase	Saliva, and the pancreas	Cuts starch into smaller sugars⁴
Pepsin	Stomach	Cuts proteins into short pieces called peptides⁴
Lipase	Mostly the small intestine (from the pancreas)	Breaks down fats⁴
Lactase	Small intestine	Splits lactose, the sugar in milk⁴
Catalase	Inside cells	Breaks down harmful hydrogen peroxide into water and oxygen⁸

Most babies can digest milk, but many people make less lactase after childhood. Experts estimate that about 68 percent of the world's population has trouble digesting lactose (lactose malabsorption). When that leads to tummy symptoms, it is called lactose intolerance.¹² If they drink a lot of milk, the undigested sugar can cause bloating, gas and diarrhea.¹²⁷

What affects how enzymes work: temperature and pH

An enzyme works because of its shape, so anything that bends the shape out of place can slow it down or stop it.

Temperature. Warming things up generally speeds up reactions, with or without an enzyme, because molecules move and bump into each other more.¹² But past a certain point, the heat shakes the enzyme's folded shape apart. This is called denaturing, and a denatured enzyme stops working.¹³ You've seen proteins denature: when you fry an egg, the clear egg white turns solid and white, and it never turns back.³ Denaturing isn't always permanent, though; sometimes a protein can refold once conditions return to normal.³

Your body stays at about 37 °C (98.6 °F). With a very high fever, above about 40 °C (104 °F), some enzymes in your body can stop working properly.⁷ Not all proteins give up in the heat: bacteria that live in hot springs have proteins that work close to boiling.³

pH. The amino acids in the active site are sensitive to acid, so each enzyme has a pH range where it works best, and extreme pH can denature it.¹ Amylase works best at about pH 6.7 to 7, close to neutral, like your mouth. When it reaches the strong acid of your stomach, it stops.¹¹⁴ Pepsin is the opposite: it works best at pH 1.5 to 2.5, the acidity of stomach juice, and stops working once food moves on to the neutral intestine.¹¹

Each enzyme has a sweet spot · amylase ≈ pH 7 · pepsin ≈ pH 2

(The curves in the interactive above are simplified pictures of this idea, not lab measurements.)

Why are enzymes important?

Without enzymes, most chemical reactions in living things would be too slow to keep anything alive.¹¹¹⁰ They run digestion, turn food into energy, build new molecules, and even help blood clot.¹¹¹³ A few examples from across life:

Honeybees add an enzyme called invertase to nectar, which splits table sugar (sucrose) into glucose and fructose. See how bees make honey.¹⁴
Plants use the enzyme RuBisCO to grab carbon dioxide from the air in the Calvin cycle. See how photosynthesis works.¹⁵
Your cells use catalase to break down hydrogen peroxide. Each catalase molecule can destroy millions of hydrogen peroxide molecules every second.⁸
Food makers have used enzymes since ancient times to make bread, cheese, wine and beer, though no one understood what was happening until the 1800s.¹¹

When an enzyme is missing, things go wrong. In the rare inherited disease PKU, the body lacks an enzyme needed to process part of protein, so newborn babies are tested for it.⁷

Common misconceptions

"Enzymes get used up." They come out of the reaction unchanged and are reused over and over.¹⁵

"Enzymes add energy to a reaction." They don't add energy or change the overall energy of the reaction. They only lower the activation-energy hill.¹⁶

"Hotter is always faster." Only up to a point. Too much heat denatures the enzyme and the reaction slows or stops.¹

"Enzymes are alive." They're molecules, almost always proteins, made by living cells.⁵

"The substrate fits like a rigid key." The enzyme actually shifts to hug its substrate. That's induced fit.¹

FAQ

How do enzymes work, in simple words?

An enzyme is a folded protein with a pocket called the active site. The molecule it works on (the substrate) fits into the pocket, and the enzyme holds and bends it so the reaction needs much less energy to get started. The products are released and the enzyme is ready to work again.

How do enzymes speed up chemical reactions?

They lower the activation energy, the energy 'hill' every reaction has to get over before it can happen. With a lower hill, far more molecules make it over each second, so the reaction can run millions of times faster. Enzymes do this by bringing molecules together in the right position, bending or straining their bonds, and giving them a good chemical environment.

How do enzymes work in our body?

Your cells contain thousands of different enzymes, each speeding up one kind of reaction. Digestive enzymes like amylase, pepsin and lactase break food into small pieces you can absorb. Others build molecules, copy DNA, help blood clot, and clear away harmful chemicals like hydrogen peroxide.

What three things can destroy enzymes?

High temperature, extreme pH (too acidic or too basic) and certain chemicals. All three can make the enzyme lose its folded shape, which is called denaturing. Without the right shape, the active site no longer fits its substrate.

Are enzymes used up in a reaction?

No. Enzymes come out of the reaction unchanged and can be used again and again. A single catalase molecule can break down millions of hydrogen peroxide molecules every second.

How do digestive enzymes work?

Each one breaks a certain kind of food molecule. Amylase in your saliva starts cutting starch into smaller sugars, pepsin in your stomach cuts proteins into short pieces, and lactase in your small intestine splits the milk sugar lactose.

Why are enzymes important?

Without enzymes, most reactions in living things would be far too slow to keep anything alive. One reaction needed to make hemoglobin and chlorophyll has a half-life of 2.3 billion years without its enzyme; with it, it takes milliseconds.

How do enzymes work?

Run the enzyme yourself

Be the enzyme!

Chop the starch chain

Feed the machine

Lower the hill

Find the fit

Too hot, too sour

Meet the enzymes