Cambridge O Level5090

Enzyme action

Biology 5090 Chapter Notes

What this chapter covers

Enzyme actionEffects of temperature and pH
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In this chapter

  1. 1Introduction to Enzymes
  2. 2How Enzymes Work: The Lock and Key Model
  3. 3Effect of Temperature on Enzyme Activity
  4. 4Effect of pH on Enzyme Activity

1. Introduction to Enzymes

Enzymes are essential for life. They are proteins that act as biological catalysts. A catalyst is a substance that speeds up a chemical reaction without being changed or used up in the process. This means a small amount of enzyme can be used over and over again. In living organisms, thousands of chemical reactions, known as metabolic reactions, are happening every second. Without enzymes, these reactions would be too slow to sustain life. Enzymes ensure that processes like digestion, respiration, and DNA replication happen at a sufficient speed.

Enzyme + Substrate → Enzyme-Substrate Complex → Enzyme + Product

Key term

Enzyme: A protein that functions as a biological catalyst, speeding up a specific chemical reaction without being consumed by the reaction.

Examiner insight

Examiners award full marks for definitions that include both 'protein' and 'biological catalyst'. Simply calling it a 'catalyst' is often not enough.

Common pitfall

Stating that enzymes are 'used up' or 'consumed' during a reaction. They are catalysts and remain unchanged, ready to be reused.

Fun fact

The enzyme catalase, found in liver and potato cells, is one of the fastest known enzymes. A single molecule can break down over 40 million molecules of hydrogen peroxide per second.

Worked example 13 marks

Define the term enzyme and describe the function of enzymes in living organisms. [3]

  1. 1

    Step 1: Define enzyme. An enzyme is a protein that acts as a biological catalyst. [1 mark]

  2. 2

    Step 2: Describe its function. It increases the rate of chemical reactions (metabolic reactions) in cells. [1 mark]

  3. 3

    Step 3: Add detail about its role. It does this without being used up, allowing vital processes like digestion and respiration to occur at a speed that can sustain life. [1 mark]

Recap

  • Enzymes are proteins that act as biological catalysts.
  • A catalyst speeds up a reaction without being used up.
  • Enzymes are vital for all metabolic reactions in living organisms.
  • Without enzymes, reactions in the body would be too slow to support life.
  • Enzymes are reusable because they are not changed by the reaction they catalyse.

Quick check

  1. State two properties of enzymes.2 marks
  2. Why are enzymes described as 'biological' catalysts?1 mark

2. How Enzymes Work: The Lock and Key Model

Enzymes are highly specific, meaning each type of enzyme usually catalyses only one type of reaction. This specificity is explained by the 'lock and key' model. Each enzyme has a unique, three-dimensional shape which includes a special region called the active site. The substance the enzyme acts upon is called the substrate. The shape of the active site is complementary to the shape of its specific substrate, much like a key fits into a specific lock. The substrate binds to the active site, forming an enzyme-substrate complex. Inside the complex, the reaction occurs rapidly, and the substrate is converted into products. The products then detach from the active site, leaving the enzyme free to bind with another substrate molecule.

Key term

Active Site: The specific region on the surface of an enzyme where the substrate binds and the chemical reaction is catalysed.

Examiner insight

Examiners look for clear use of the terms 'active site', 'substrate', and 'complementary shape' when explaining enzyme specificity.

Common pitfall

Confusing the active site with the entire enzyme molecule. The active site is just a small part of the enzyme's total structure.

Worked example 13 marks

Using the lock and key model, explain why the enzyme amylase, which digests starch, cannot digest protein. [3]

  1. 1

    Step 1: State the principle of specificity. Enzymes are specific, meaning each enzyme has a uniquely shaped active site. [1 mark]

  2. 2

    Step 2: Apply this to the example. The active site of amylase has a shape that is complementary to its substrate, starch. [1 mark]

  3. 3

    Step 3: Explain the lack of fit. Protein molecules have a different shape that is not complementary to the active site of amylase. Therefore, the protein cannot bind to the active site, and no reaction occurs. [1 mark]

Recap

  • The 'lock and key' model explains enzyme specificity.
  • The enzyme's active site has a complementary shape to its substrate.
  • The substrate binds to the active site to form an enzyme-substrate complex.
  • After the reaction, products are released and the enzyme is unchanged.
  • An enzyme that acts on one substrate will not act on another with a different shape.

Quick check

  1. What is the name of the substance that an enzyme acts on?1 mark
  2. What is formed when a substrate binds to an enzyme's active site?1 mark

3. Effect of Temperature on Enzyme Activity

Temperature has a major effect on the rate of enzyme-catalysed reactions. At low temperatures, enzymes are inactive but not damaged. As temperature increases, both enzyme and substrate molecules gain kinetic energy and move faster. This leads to more frequent collisions between the active site and the substrate, increasing the rate of reaction. The rate continues to increase until it reaches its maximum at the optimum temperature. For most human enzymes, this is around 37°C. Beyond the optimum temperature, the rate of reaction drops sharply. The high temperature provides too much energy, causing the enzyme to vibrate vigorously. This breaks the bonds holding the protein in its specific 3D shape. The active site changes shape and is no longer complementary to the substrate. The enzyme is said to be denatured, and this change is irreversible.

Key term

Optimum Temperature: The temperature at which an enzyme exhibits its maximum rate of activity.

Examiner insight

When explaining denaturation, you must link the high temperature to the change in the 3D shape of the enzyme, which then leads to a change in the active site's shape.

Common pitfall

Stating that enzymes are 'killed' by heat. Enzymes are not living organisms; the correct term is 'denatured'.

Worked example 14 marks

The graph shows the effect of temperature on the rate of an enzyme reaction.(i) State the optimum temperature for this enzyme.(ii) Describe and explain the effect of increasing the temperature from 40°C to 60°C. [4]

  1. 1

    (i) The optimum temperature is the peak of the graph, which is 40°C. [1 mark]

  2. 2

    (ii) Description: As the temperature increases from 40°C to 60°C, the rate of reaction decreases rapidly. [1 mark]

  3. 3

    (ii) Explanation 1: At temperatures above the optimum, the high heat energy causes the bonds in the enzyme to break. [1 mark]

  4. 4

    (ii) Explanation 2: This changes the specific 3D shape of the active site, so the substrate can no longer bind. The enzyme is denatured. [1 mark]

Worked example 23 marks

An enzyme sample was kept at 10°C and another at 80°C. Both were then brought to the optimum temperature of 40°C. Predict and explain the activity of each sample. [3]

  1. 1

    Step 1: Predict the activity of the 10°C sample. The sample from 10°C will become active and work effectively. [1 mark]

  2. 2

    Step 2: Explain why. At low temperatures, the enzyme is inactive but not damaged. Warming it up restores its activity. [1 mark]

  3. 3

    Step 3: Predict and explain the activity of the 80°C sample. The sample from 80°C will show little or no activity because it has been irreversibly denatured by the high temperature. Its active site is permanently damaged. [1 mark]

Recap

  • Increasing temperature increases reaction rate up to the optimum by increasing kinetic energy and collision frequency.
  • The optimum temperature is where the enzyme is most active.
  • High temperatures cause the enzyme to denature, changing the active site's shape.
  • Denaturation is an irreversible change that stops the enzyme from working.
  • Low temperatures make enzymes inactive, but this is reversible.

Quick check

  1. What is the term for an enzyme that has lost its shape due to high heat?1 mark
  2. Why does reaction rate increase between 10°C and 30°C?2 marks

4. Effect of pH on Enzyme Activity

Just like temperature, pH also affects enzyme activity. Each enzyme has an optimum pH at which it works most effectively. For example, pepsin, an enzyme in the stomach, works best at an acidic pH of around 2, while trypsin in the small intestine works best at an alkaline pH of about 8. If the pH is too far above or below the optimum, the rate of reaction decreases. Extreme changes in pH can alter the chemical bonds that maintain the enzyme's three-dimensional shape. This changes the shape of the active site, so the substrate no longer fits. This process is also called denaturation, and it is usually irreversible. A graph of enzyme activity against pH typically shows a bell-shaped curve, with the peak at the optimum pH.

Key term

Denaturation: The irreversible change in the three-dimensional structure of a protein, such as an enzyme, caused by factors like extreme pH or heat, resulting in a loss of function.

Examiner insight

A good explanation for the effect of pH will state that the pH alters the forces holding the protein together, which changes the active site shape and reduces its efficiency.

Common pitfall

Assuming all enzymes have an optimum pH of 7. Many enzymes work in highly acidic or alkaline environments and have optimums to match.

Worked example 13 marks

The enzyme pepsin digests protein in the stomach (optimum pH 2), and the enzyme trypsin digests protein in the small intestine (optimum pH 8). Explain why pepsin would not work effectively in the small intestine. [3]

  1. 1

    Step 1: State the condition in the small intestine. The small intestine has an alkaline environment, with a pH of around 8. [1 mark]

  2. 2

    Step 2: Relate this to the enzyme's optimum. This pH is far from pepsin's optimum pH of 2. [1 mark]

  3. 3

    Step 3: Explain the consequence. The alkaline pH would alter the bonds in the pepsin enzyme, changing the shape of its active site. This would denature the enzyme, preventing it from binding to and digesting protein. [1 mark]

Worked example 22 marks

An investigation measured the rate of an enzyme reaction at different pH values. The rate was highest at pH 7 and decreased at pH 6 and pH 8. Explain the decrease in rate at pH 8. [2]

  1. 1

    Step 1: Identify the cause. The pH of 8 is above the enzyme's optimum pH of 7. [1 mark]

  2. 2

    Step 2: Explain the effect. The change in pH begins to alter the shape of the enzyme's active site, making it less complementary to the substrate. This reduces the rate at which enzyme-substrate complexes can form, slowing the reaction. [1 mark]

Recap

  • Each enzyme has an optimum pH at which its activity is highest.
  • Deviating from the optimum pH causes the reaction rate to decrease.
  • Extreme pH values (either too acidic or too alkaline) cause irreversible denaturation.
  • Denaturation by pH changes the shape of the active site, preventing the substrate from binding.
  • Different enzymes in the body have different optimum pH values suited to their location.

Quick check

  1. What is the general shape of a graph plotting enzyme activity against pH?1 mark
  2. State the term for the pH at which an enzyme works fastest.1 mark

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. Define the terms 'enzyme' and 'active site'.3 marks
  2. Explain, using the lock and key theory, why enzymes are described as specific.3 marks
  3. A student investigated the effect of temperature on the activity of the enzyme amylase. Sketch a graph to show the expected results, labelling the axes and the optimum temperature.4 marks
  4. Describe what happens to an enzyme molecule when it is denatured by high temperature.3 marks
  5. Compare the effect of boiling an enzyme with the effect of freezing it, and then returning both to their optimum temperature.4 marks
  6. The rate of a catalase reaction was measured by collecting 10 cm³ of oxygen. At pH 6, this took 25 seconds. At pH 5, it took 50 seconds. Calculate the rate of reaction in cm³/s for both pH values.2 marks
  7. Explain why biological washing powders containing protease enzymes are effective at removing blood stains, and why they work best at warm, not hot, temperatures.4 marks
  8. A student claims that 'all catalysts are proteins'. Explain whether this statement is correct or incorrect.3 marks
  9. Describe an experiment you could carry out to find the optimum pH for the enzyme lipase, which breaks down fat into fatty acids and glycerol. Your answer should include the variables you would control.6 marks
  10. A graph of reaction rate against substrate concentration for an enzyme shows that the rate increases initially and then plateaus, even if more substrate is added. Explain this observation.3 marks

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