Enzymes

Essential Question

How do enzymes speed up the reactions that keep cells alive?

Digesting a meal would take days without help, and the reactions in your cells would be far too slow to keep you alive. Enzymes fix this problem. They are proteins that speed up chemical reactions by large amounts, and each one works best under certain conditions. In this lesson, you’ll learn how enzymes work and how temperature, pH, and the amount of substrate change how fast they work.

Activation Energy

• A reaction needs a push to get started
• That push is the activation energy
• Without help, many reactions in cells are too slow
• Heating the cell would also harm it

Many chemical reactions need a boost of energy before they can begin, even if they release energy in the end. This boost is called the activation energy. At the temperature of a living cell, most reactions would be far too slow without help, but raising the temperature enough to speed them up would damage the cell. Cells solve this problem with enzymes.

What Enzymes Are

• Enzymes are proteins that act as catalysts
• A catalyst lowers the activation energy
• It is not used up, so it can work again and again
• Each enzyme is specific to one kind of reaction

Enzyme	Substrate	Product	Where it works
Amylase	Starch	Sugars	Mouth, small intestine
Pepsin	Proteins	Short polypeptides	Stomach
Lipase	Fats	Fatty acids and glycerol	Small intestine

A catalyst is a substance that speeds up a reaction without being used up. Enzymes are catalysts made of protein. As the graph in the last section shows, an enzyme lowers the activation energy, but it does not change the energy of the reactants or the products. The molecule that an enzyme acts on is its substrate. Enzyme names often end in -ase, and each one usually matches its substrate, as in the table. An enzyme works on one substrate, or a very small group of similar ones.

The Active Site

• The active site is a pocket on the enzyme
• The substrate fits into the active site
• The enzyme-substrate complex forms
• Products leave and the enzyme is free to work again

An enzyme has a small pocket called the active site. The substrate fits into the active site, like a key into a lock, and they form an enzyme-substrate complex. In the active site, the substrate is held in a position that makes it easier to break or form bonds, which lowers the activation energy. The products then leave, and the enzyme is ready to work on another substrate molecule. Because the shape of the active site depends on the folded shape of the protein, anything that changes the shape of the enzyme can change how well it works.

Temperature and Enzyme Rate

• Rate = amount of product ÷ time
• A higher temperature makes molecules collide more often
• Each enzyme has an optimum temperature
• Above it, the enzyme denatures and the rate falls

The rate of a reaction tells how fast a product forms. It is found by dividing the amount of product by the time. The graph shows sample data for an enzyme. As the temperature rises from 0°C to 40°C, the rate increases because molecules move faster and collide more often. At 40°C the rate is at its highest. This is the optimum temperature for this enzyme. Above it, the rate falls sharply because heat denatures the enzyme, and the active site loses its shape.

pH and Enzyme Rate

• Each enzyme has an optimum pH
• A change in pH changes the charges on R groups
• This changes the shape of the active site
• Enzymes in different places work at different pH values

The graph shows sample data for three digestive enzymes. Pepsin works in the stomach, where the fluid is very acidic, and it is most active near pH 2. Amylase is most active near pH 7, which is close to neutral, and trypsin is most active near pH 8 in the small intestine. A change in pH changes the electric charges on parts of the protein, which can change its shape. Away from its optimum pH, an enzyme is less active, and at an extreme pH it can be denatured.

Substrate Concentration

• More substrate means more collisions with active sites
• The rate rises at first
• Then it levels off when all active sites are busy
• Adding more enzyme raises the maximum rate

When the amount of enzyme is fixed, increasing the amount of substrate speeds up the reaction at first, because substrate molecules find empty active sites more easily. Eventually, every enzyme molecule is working at full speed with a substrate in its active site. Adding more substrate cannot make the reaction go any faster, so the graph levels off. To raise the maximum rate, the cell needs more enzyme molecules.

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Worked Examples

Try each problem on your own first, then open the solution to check your work.

Example 1: Finding a Reaction Rate

(a) An enzyme produces 12 mL of gas in 4 minutes. What is the rate? (b) Use the temperature graph. At 40°C, how many mL of product would be made in 6 minutes? (c) How many times faster is the rate at 40°C than at 10°C?

Show the solution

1. (a) Rate: rate = amount ÷ time = 12 mL ÷ 4 min = 3 mL per minute.
2. (b) Product at 40°C: the rate at 40°C is 25 mL per minute. Amount = rate × time = 25 mL per min × 6 min = 150 mL.
3. (c) Comparison: the rate at 10°C is 3 mL per minute. 25 ÷ 3 = 8.3, so the reaction is about 8 times faster at 40°C.
4. Answer: (a) 3 mL per minute. (b) 150 mL. (c) About 8 times faster.

Example 2: Enzymes at Different pH Values

Use the graph of three enzymes. (a) What is the optimum pH of each enzyme? (b) Which enzyme could digest food in the stomach, where the pH is about 2? (c) Amylase from the mouth is swallowed into the stomach. Explain what happens to it, using the graph and what you know about protein shape.

Show the solution

1. (a) Optimum pH: pepsin, pH 2. Amylase, pH 7. Trypsin, pH 8.
2. (b) In the stomach: pepsin, because it has 100% activity at pH 2, and the other two enzymes are close to 0%.
3. (c) Amylase in the stomach: at pH 2 the activity of amylase is close to 0 on the graph. The acid changes the charges on the R groups, which changes the shape of the active site, so the amylase is denatured and stops working.
4. Answer: (a) Pepsin 2, amylase 7, trypsin 8. (b) Pepsin. (c) The acid changes its shape, so it stops working.

Example 3: Substrate and Active Sites

Use the graph of rate against substrate concentration for a fixed amount of enzyme. (a) What is the rate at a concentration of 6? (b) At about what concentration does the rate stop rising? (c) Explain why the rate levels off. (d) How could the cell increase the maximum rate?

Show the solution

1. (a) At 6: the rate is 44 units.
2. (b) Leveling off: the rate stops rising at about a concentration of 10, where it reaches 50 units.
3. (c) Explanation: at high substrate concentration, every active site is occupied all the time, so extra substrate has no free active sites to bind.
4. (d) Raise the maximum: make more enzyme molecules, which gives more active sites.
5. Answer: (a) 44 units. (b) About 10. (c) All active sites are in use. (d) Add more enzyme.

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Practice

Define the key terms of enzyme action, calculate a reaction rate, read graphs of reaction rate against pH and temperature, and explain how an enzyme lowers the activation energy. Try it on your own first, then check your work with the answer key.

• Enzymes Practice Worksheet (PDF)
• Enzymes Practice Answer Key (PDF)

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Lesson Slides

• 5.6 Enzymes Slides (PDF)

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Big Ideas

• Enzymes are protein catalysts that lower the activation energy
• A substrate fits into the active site
• Enzymes are not used up
• Temperature, pH, and substrate concentration change the rate
• Rate = amount of product ÷ time

Back to the essential question: enzymes speed up reactions by lowering the activation energy. The substrate fits into the active site, and the shape of the active site depends on the shape of the protein. That is why temperature and pH change how well an enzyme works, and why changing the amino acid sequence can stop it. Next, you’ll see how cells use energy released by reactions to make ATP.

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