What are enzymes?

Enzymes are biological catalysts - catalysts are substances that increase the rate of chemical reactions without being used up. Enzymes are alsoproteins [proteins: organic compounds made up of amino acid molecules. One of the three main food groups, proteins are needed by the body for cell growth and repair. ] that are folded into complex shapes that allow smaller molecules to fit into them. The place where these substrate molecules fit is called the active site.

The animation shows how this works. In this example, two small molecules join together to make a larger one.

If the shape of the enzyme changes, its active site may no longer work. We say the enzyme has been denatured [denatured: If a protein is denatured, its structure and function is altered. This can be caused by heat, altered pH or by chemical agents. ]. They can be denatured by high temperatures or extremes of pH [pH: scale of acidity/alkalinity. pH below 7 = acidic, pH above 7 = alkaline ]. Note that it is wrong to say the enzyme has been killed. Although enzymes are made by living things, they are proteins, and not alive.

Temperature and enzymes

As the temperature increases, so does the rate of reaction. But very high temperatures denature enzymes.

The graph shows the typical change in an enzyme's activity with increasing temperature. The enzyme activity gradually increases with temperature until around 37ºC, or body temperature. Then, as the temperature continues to rise, the rate of reaction falls rapidly, as heat energy denatures the enzyme.

Temperature and enzyme activity

pH and enzymes

Changes in pH alter an enzyme’s shape. Different enzymes work best at different pH values. The optimum pH for an enzyme depends on where it normally works. For example, intestinal enzymes have an optimum pH of about 7.5. Enzymes in the stomach have an optimum pH of about 2.

pH and enzyme activity

Enzymes and respiration

Enzymes in cells catalyse photosynthesis, protein synthesis - joining amino acids together, and aerobic respiration.

Aerobic respiration

Respiration is not the same thing as breathing. That is more properly called ventilation. Instead, respiration is a chemical process in which energy is released from food substances, such as glucose - a sugar.

Aerobic [Aerobic: 'With oxygen.' Aerobic exercise involves oxygen consumption by the body. If exercise is not too fast and is steady, the heart can supply all the oxygen the muscles need. ] respiration needs oxygen to work. Most of the chemical reactions involved in the process happen in tiny objects inside the cell cytoplasm, called mitochondria [mitochondria: structures in the cytoplasm of all cells where respiration takes place. Singular is mitochondrion ].

This is the equation for aerobic respiration:

glucose + oxygen    →    carbon dioxide + water (+ energy)

The energy released by respiration is used to make large molecules from smaller ones. In plants, for example, sugars, nitrates and other nutrients are converted into amino acids. Amino acids can then join together to make proteins. The energy is also used:

• to allow muscles to contract in animals
• to maintain a constant body temperature in birds and mammals

You may wish to check your understanding of how respiration affects the length of food chains, and the efficiency of food production. Visit the Food chains and cycles section for a reminder.

The digestive system

Digestion is the breakdown of large molecules into smaller, soluble molecules that can be absorbed into the body. Digestion happens inside the gut, and relies on enzymes. You learnt about the gut in your Key Stage 3 studies. Visit the section on Diet and digestion if you want to check back.

This interactive diagram will remind you of the main parts of the gut:

Enzymes and digestion

The enzymes involved in respiration, photosynthesis and protein synthesis work inside cells. Other enzymes are produced by specialised cells and released from them; the digestive enzymes are like this. They pass out into the gut, where they catalyse the breakdown of food molecules.

Different enzymes

Different enzymes catalyse different digestion reactions.

Enzymes and their reactions catalysed

Amylase is an example of a carbohydrase. Lipids are fats and oils.

Different parts of the gut

Different parts of the gut produce different enzymes.

Where enzymes are produced

Summary

Overall, this means that:

• Amylase catalyses the breakdown of starch into sugars in the mouth and small intestine.
• Proteases catalyse the breakdown of proteins into amino acids in the stomach and small intestine.
• Lipases catalyse the breakdown of fats and oils into fatty acids and glycerol in the small intestine.

Other substances in digestion

You should recall that different enzymes work best at different pH values. The digestive enzymes are a good example of this.

Enzymes in the stomach

The stomach produces hydrochloric acid. This helps to begin digestion, and it kills many harmful microorganisms that might have been swallowed along with the food. The enzymes in the stomach work best in acidic conditions - in other words, at a low pH.

Enzymes in the small intestine

After the stomach, food travels to the small intestine. The enzymes in the small intestine work best in alkaline conditions, but the food is acidic after being in the stomach. A substance called bile neutralises the acid to provide the alkaline conditions needed in the small intestine.

Bile and enzyme production in the liver and pancreas

Enzymes in industry

Enzyme names

The names of the different types of enzymes usually end in the letters -ase. Three of the most common enzymes with their chemical actions are:

• lipase - breaks down fats
• protease - breaks down proteins
• carbohydrase - breaks down carbohydrates

Enzyme uses

Enzymes allow certain industrial processes to be carried out at normal temperatures and pressures, thereby reducing the amount of energy and expensive equipment needed. Enzymes are also used in the home, for example, in 'biological' detergents. The table shows some common enzyme uses you should be familiar with.

Uses of enzymes

Now try a Test Bite