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Enzymes: Class 9 Biology
What enzymes are, their properties, the lock and key and induced fit models of action, the effect of temperature, pH, substrate concentration and enzyme concentration, inhibitors, cofactors and the uses of enzymes.
Key points
Enzymes are biological catalysts. A catalyst speeds up a chemical reaction without being changed by it. Almost all enzymes are globular proteins made by living cells. Without enzymes, the reactions of life would be too slow to support the cell at body temperature.
Properties of enzymes:
- They are proteins, so they are denatured by high temperature and extreme pH.
- They are specific: each enzyme works on one substrate or one type of reaction.
- They are needed in very small amounts, as they are not used up in the reaction.
- They work at the temperature and pH that suit them, called the optimum.
- Most reactions they catalyse are reversible, and the enzyme speeds up both directions.
- They lower the activation energy of a reaction, which is the energy needed to start it.
The substance on which an enzyme acts is the substrate, and the substances formed are the products. The enzyme has a small region called the active site, which has a particular three dimensional shape. The substrate binds to the active site and forms an enzyme-substrate complex. The reaction takes place, the products leave, and the enzyme is free to be used again.
The lock and key model explains specificity. The active site has a fixed shape that fits only its own substrate, as a key fits one lock. The induced fit model is an improved idea. The active site is flexible, and its shape changes a little as the substrate binds, so that the fit becomes exact and the substrate is strained and reacts more easily.
Enzymes are named by adding -ase to the substrate or to the type of reaction. Amylase breaks down starch, protease breaks down proteins, lipase breaks down fats, maltase breaks down maltose, and catalase breaks down hydrogen peroxide into water and oxygen. Intracellular enzymes work inside cells (such as catalase), and extracellular enzymes are secreted outside, such as the digestive enzymes.
Factors that affect the rate of enzyme action:
- Temperature: the rate increases as temperature rises, because particles have more kinetic energy and collide more often. It is highest at the optimum, which is about 37 °C for human enzymes. Above about 45 to 50 °C the enzyme is denatured, because its shape changes and the active site is lost, and this change is permanent. At low temperature enzymes are inactive but not destroyed.
- pH: each enzyme has an optimum pH. Pepsin works best at about pH 2 in the stomach, while salivary amylase and pancreatic enzymes work best near pH 7 to 8. A large change in pH changes the shape of the active site and denatures the enzyme.
- Substrate concentration: the rate rises as more substrate is added, until all active sites are busy. Then the rate levels off, because the enzyme concentration is limiting.
- Enzyme concentration: with plenty of substrate, the rate increases in proportion to the amount of enzyme.
Inhibitors are substances that reduce the rate of enzyme action. A competitive inhibitor has a shape like the substrate and blocks the active site. A non-competitive inhibitor binds elsewhere and changes the shape of the active site. Heavy metals such as mercury and poisons such as cyanide are inhibitors. Many medicines also work by blocking enzymes of disease causing microbes.
Some enzymes need a non-protein helper to work. A cofactor may be a metal ion such as zinc or magnesium. A coenzyme is a small organic molecule, and many vitamins of the B group are parts of coenzymes. Lack of such a vitamin in the diet can stop an enzyme from working properly.
A simple experiment shows the effect of temperature on catalase. Pieces of liver are put in hydrogen peroxide at different temperatures. Bubbles of oxygen are counted or the height of foam is measured. The most bubbling is seen near 37 °C, and no bubbles are formed with boiled liver, since the enzyme is denatured.
Enzymes have many uses. Biological detergents contain proteases and lipases to remove stains. Amylase is used in making glucose syrup and in brewing, pectinase is used to clear fruit juice, rennin is used in cheese making, and proteases tenderise meat. Lactase is used to make milk safe for people who cannot digest lactose. Immobilised enzymes are fixed to a surface and can be reused.
Study tip: Learn to sketch the graphs for temperature, pH and substrate concentration and to explain the shape of each part in words. In every answer on denaturation say that the shape of the active site changes. Use the term active site correctly and never say that an enzyme is killed.
Definitions and flashcards
Say the meaning first, then open the card to check.
Enzyme
A protein that acts as a biological catalyst.
Substrate
The substance on which an enzyme acts.
Active site
The part of an enzyme where the substrate binds.
Activation energy
The energy needed to start a chemical reaction.
Denaturation
Permanent change in the shape of a protein, causing loss of its activity.
Optimum temperature
The temperature at which an enzyme works fastest.
Inhibitor
A substance that reduces the rate of an enzyme catalysed reaction.
Cofactor
A non-protein substance needed by an enzyme in order to work.
Coenzyme
An organic cofactor, often derived from a vitamin.
Short questions with model answers
These are the kind of questions asked for 2, 3 and 5 marks. Try each one before opening the answer.
What is an enzyme? Give two examples. (2 marks)
An enzyme is a protein that acts as a biological catalyst, speeding up chemical reactions in living things without being used up. Examples are amylase and catalase.
Why does a boiled enzyme not work? (2 marks)
High temperature breaks the bonds that hold the protein in shape. The shape of the active site is lost, the substrate no longer fits, and the denaturation is permanent.
Explain the lock and key model. (3 marks)
The active site of the enzyme has a specific shape. Only a substrate with a complementary shape can fit into it, as only the right key fits a lock. They form an enzyme-substrate complex, the reaction occurs, the products are released and the enzyme is unchanged. This explains why each enzyme is specific.
Describe how temperature affects the rate of an enzyme catalysed reaction. (3 marks)
At low temperature the rate is low because particles have little kinetic energy. As temperature rises, the rate rises since collisions between enzyme and substrate become more frequent and energetic. The maximum rate is at the optimum, about 37 °C in humans. Above this, the enzyme starts to denature and the rate falls rapidly to zero.
Why does the rate of an enzyme reaction stop increasing when more substrate is added? (3 marks)
When all the active sites of the enzyme molecules are occupied by substrate, adding more substrate cannot increase the rate. The enzyme concentration has become the limiting factor, so the graph levels off.
State the uses of enzymes in industry and at home. Give four examples. (5 marks)
Biological detergents contain proteases and lipases that digest protein and fat stains on clothes. Pectinase is used to break down pectin and to clear fruit juice. Rennin is used to curdle milk in making cheese. Amylase changes starch to sugar in brewing and in making syrups. Proteases are used to tenderise meat. Using enzymes saves energy because they work at low temperature.
Common mistakes
- Saying that low temperature kills or denatures enzymes. Cold only slows them, and they work again when warmed.
- Saying that enzymes are living. They are chemicals, so they are denatured, not killed.
- Writing that enzymes are used up in reactions.
- Confusing the substrate with the product, or the enzyme with the substrate.
- Drawing the temperature graph with a sudden fall at the start. The rate rises slowly to the optimum and then drops steeply.