What are enzymes and how do they work?

Your body runs thousands of chemical reactions every second: breaking down food, building muscle, copying DNA. At body temperature most of these reactions would be far too slow to keep you alive. Enzymes fix that. They are biological catalysts: substances that speed up a reaction without being used up.

Almost every enzyme is a protein: a long chain of amino acids folded into a very particular 3D shape. On its surface is a small pocket called the active site. The molecule the enzyme works on is the substrate. When the substrate fits into the active site, the two form an enzyme–substrate complex, the reaction happens, and the products are released. The enzyme is free to take the next substrate molecule.

This is the lock-and-key model: only a substrate of the right shape fits, just as only one key fits a lock. That is why enzymes are specific. Amylase breaks down starch but does nothing to protein. A more modern idea, the induced-fit model, says the active site changes shape slightly as the substrate binds, gripping it more tightly, a bit like a glove fitting a hand.

How do enzymes speed reactions up? Every reaction needs a minimum amount of energy to get started, the activation energy. By holding the substrate in the right position and putting strain on its bonds, an enzyme gives the reaction an easier route with a lower activation energy. More molecules have enough energy to react, so the reaction goes faster.

Three things change how fast an enzyme works:

• Temperature. Warming speeds the reaction up at first, because molecules move faster and collide more often. Above the optimum temperature (about 37 °C for many human enzymes) the bonds holding the enzyme's shape start to break. The active site changes shape, the substrate no longer fits, and the enzyme is denatured.
• pH. Each enzyme has an optimum pH. Pepsin in the acidic stomach works best at about pH 2; amylase in saliva at about pH 7. Too far from the optimum and the enzyme denatures. (See acids and bases.)
• Substrate concentration. More substrate means more collisions, so the rate rises, until every active site is busy. Then the rate levels off.

Enzymes are at work in your digestive system, inside every one of your cells, and in the kitchen: enzymes in yeast and flour help bread rise, and the enzyme chymosin (in rennet) curdles milk to make cheese.

Calculating the rate: if you measure the amount of product (or substrate used) over a time, rate = amount ÷ time, for example in cm³/s. If you only time how long a reaction takes to finish, use rate = 1 ÷ time, with units of s⁻¹. A shorter time means a faster rate.

What is the simple definition of an enzyme?#

An enzyme is a protein that speeds up a chemical reaction in a living thing without being used up. Scientists call it a biological catalyst.

Are all enzymes proteins?#

Almost all are. A few RNA molecules, called ribozymes, can also act as catalysts, but at school "enzyme" means a protein.

How do enzymes lower activation energy?#

The active site holds the substrate in the best position and strains its bonds, giving the reaction an easier route. Less energy is needed to start it, so more molecules react each second.

How do enzymes help digestion?#

Digestive enzymes break big food molecules into small ones that can pass into the blood: carbohydrases such as amylase turn starch into sugars, proteases turn proteins into amino acids, and lipases turn fats into fatty acids and glycerol.

What are "liver enzymes" in a blood test?#

Liver cells contain enzymes, and some leak into the blood when cells are damaged, so doctors measure them as a clue. Only a doctor can interpret what a particular result means.

Do digestive enzyme supplements help you lose weight?#

There is no good scientific evidence that they do. Your pancreas and gut already make digestive enzymes; enzyme medicines are prescribed by doctors for specific conditions where the body does not make enough.

What happens when an enzyme is denatured?#

The protein unfolds and the active site loses its shape, so the substrate cannot fit. Denaturing by heat is usually permanent: a cooked egg white cannot be uncooked.