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Cambridge IGCSE0610

Enzymes

Biology 0610 Chapter Notes

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Enzymes
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In this chapter

  1. 1Catalysts and enzymes
  2. 2Active site, enzyme-substrate complex and specificity
  3. 3Temperature and pH: optimum and denaturation
  4. 4Temperature: kinetic energy, shape and denaturation
  5. 5pH: shape and fit

1. Catalysts and enzymes

A catalyst is a substance that increases the rate of a chemical reaction and is not changed by the reaction.

Enzymes are proteins that are involved in all metabolic reactions, where they function as biological catalysts.

Enzymes are important in all living organisms because they maintain the rate of reactions needed to sustain life.

Examiner insight

“Some candidates were not precise enough in their responses, simply referring to catalysts as affecting or changing the rate of reaction rather than increasing it.” “Candidates also needed to describe a catalyst as being unchanged or not being used up in the reaction rather than not taking part in the reaction.”

Worked example 12 marks

Enzymes are biological catalysts. Describe the meaning of the term catalyst.

  1. 1

    (a substance that) increases the rate of a (chemical) reaction [1]

  2. 2

    is not changed by the reaction / is not used up [1]

2. Active site, enzyme-substrate complex and specificity

An enzyme has an active site. The substrate fits into it because the shape of the active site is complementary to the shape of the substrate.

The enzyme and substrate join to form an enzyme-substrate complex. The substrate is changed into products, which leave the active site. The enzyme is then free to work on another substrate molecule.

So an enzyme is specific: only a substrate whose shape fits the active site can bind.

enzyme + substrate → enzyme-substrate complex → enzyme + products

Examiner insight

“A few candidates thought that the substrate and the active site are the same shape rather than complementary shapes to each other.” “Some candidates incorrectly believed the product binds to an enzyme with a complementary shape and a substrate is formed.”

Worked example 15 marks

A student investigated the effect of two enzymes, amylase and pepsin, on starch.

Two test-tubes were set up and left for 10 minutes.

Test-tube 1 contained a mixture of starch suspension and amylase solution.

Test-tube 2 contained a mixture of starch suspension and pepsin solution.

After 10 minutes, Benedict's solution was added to both test-tubes and they were heated.

Predict and explain the expected results for test-tube 1 and test-tube 2.

  1. 1

    test-tube 1 turns red and test-tube 2 stays blue / does not change [1]

  2. 2

    amylase breaks down starch into a (named) reducing sugar [1]

  3. 3

    (shape of the) active site of amylase is complementary to starch [1]

  4. 4

    enzyme-substrate complexes are formed [1]

  5. 5

    (shape of the) active site of pepsin is not complementary to starch [1]

3. Temperature and pH: optimum and denaturation

Enzyme activity changes with temperature and with pH. Each enzyme has an optimum temperature and an optimum pH, where its activity is greatest.

Above the optimum temperature the activity falls. At high enough temperatures the enzyme is denatured and its activity is zero.

Away from the optimum pH the activity also falls. An extreme pH denatures the enzyme. To investigate one factor, change only that factor. Keep the other constant: a thermostatically controlled water-bath for temperature, a pH buffer for pH.

Examiner insight

“Some candidates appeared unfamiliar with the difference in the shape of graphs showing the effect of temperature and the effect of pH on enzyme action. The sudden drop to zero by the enzyme that is denatured by heat rather than pH was appreciated by relatively few candidates.” “While many candidates understood that raising the temperature denatures enzymes, a common misconception was that enzymes can be killed by heat.”

Worked example 16 marks

Figure 1.1 shows the activity of pectinase at different temperatures.

Complete the sentences using the information in Figure 1.1.

Enzymes are ........ catalysts.

At 0 °C the enzyme activity is ........ arbitrary units.

The optimum temperature for enzyme activity is ........ °C.

At the optimum temperature, the shape of the ........ site of the pectinase enzyme is ........ to the shape of the substrate pectin.

At 70 °C, all of the enzymes have ........ .

  1. 1

    Enzymes are biological catalysts. [1]

  2. 2

    At 0 °C the activity is 10 arbitrary units. [1]

  3. 3

    The optimum temperature is 50 °C. [1]

  4. 4

    the shape of the active site [1]

  5. 5

    is complementary to the shape of the substrate [1]

  6. 6

    At 70 °C all of the enzymes have been denatured. [1]

4. Temperature: kinetic energy, shape and denaturation

As the temperature rises towards the optimum, particles gain kinetic energy and move faster. The frequency of effective collisions between enzyme and substrate increases, so more enzyme-substrate complexes form.

Above the optimum, the shape of the active site changes. The substrate no longer fits, so the enzyme is denatured.

Above the optimum the kinetic energy does not decrease. Molecules still collide, but the collisions are not effective. At low temperatures the enzyme is not denatured: the molecules have less kinetic energy, so there are fewer effective collisions.

Examiner insight

“Common misconceptions included that kinetic energy decreased past the optimum temperature and that enzymes denatured at low temperatures.” “Denaturation was often explained in terms of the whole enzyme changing shape without recognising the importance of the shape of the active site for enzyme activity.”

Worked example 11 mark

Increasing temperature above the optimum for an enzyme results in loss of enzyme activity.

How is this explained?

A There are less frequent collisions between the enzyme and the substrate. · B The kinetic energy of the enzyme molecule is reduced. · C Substrate molecules move faster and effective collisions are less likely. · D The shape of the active site is changed and the substrate will no longer fit into it.

  1. 1

    D: the shape of the active site is changed and the substrate will no longer fit into it. [1]

5. pH: shape and fit

An enzyme has an optimum pH. There the most substrate binds to the active sites, so the most enzyme-substrate complexes form.

Away from the optimum, the shape of the active site changes. The active site is no longer complementary to the substrate, so fewer enzyme-substrate complexes form. At an extreme pH the enzyme is denatured.

The explanation for pH is about shape and fit. Increased kinetic energy belongs to the explanation for temperature.

Examiner insight

“It was evident that some candidates confused the effect of temperature on enzyme action with pH, referring to increased kinetic energy and frequency of collisions.” “Some described the active site changing when becoming denatured but did not refer to the shape changing.”

Worked example 13 marks

Figure 6.1 shows the effect of pH on the enzyme activity of two types of lipase, type P and type Q.

Explain the effect of pH on the enzyme activity of type P lipase shown in Figure 6.1.

  1. 1

    the enzyme is denatured at low and high / extremes of pH [1]

  2. 2

    the active site changes shape [1]

  3. 3

    the active site is no longer complementary to the substrate [1]

Recap

  • A catalyst increases the rate of a chemical reaction and is not changed by the reaction.
  • Enzymes are proteins and biological catalysts. They maintain the rate of reactions needed to sustain life.
  • The active site is complementary in shape to the substrate. The substrate fits, forms an enzyme-substrate complex, then products are released.
  • Each enzyme has an optimum temperature and an optimum pH. Too hot or too far from the optimum pH: the enzyme is denatured.
  • Extended: to the optimum temperature, more kinetic energy gives more effective collisions. Above it, kinetic energy keeps rising, but the shape of the active site changes and the substrate no longer fits.
  • Extended: pH changes the shape of the active site, so fewer enzyme-substrate complexes form. It does not change the kinetic energy.
  • Heat denatures an enzyme. It never kills it.

End-of-chapter exercise

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

  1. A student writes: "A catalyst is a substance that changes a reaction and is used up." Correct the two errors.2 marks
  2. Lipase breaks down fat. State the type of molecule that lipase is.1 mark
  3. Describe why enzymes are important in all living organisms.2 marks
  4. Complete the sentence. The shape of the active site of an enzyme is ........ to the shape of its ........ .2 marks
  5. A student measured the activity of an enzyme at six temperatures. (a) State the optimum temperature. (b) Describe how the activity changed between 40 °C and 60 °C. (c) State what happened to the enzyme at 60 °C. | temperature / °C | enzyme activity / arbitrary units | | --- | --- | | 10 | 2 | | 20 | 5 | | 30 | 9 | | 40 | 14 | | 50 | 4 | | 60 | 0 |3 marks
  6. A student investigates the effect of pH on the activity of an enzyme. She mixes the enzyme with its substrate in test-tubes at different pH values and times how long the reaction takes in each one. State the independent variable, the dependent variable and one variable that must be kept constant.3 marks
  7. The steps of enzyme action are listed out of order. A the products are released B the substrate binds to the active site, forming an enzyme-substrate complex C the enzyme is free to work on another substrate molecule D the substrate is changed into products Write the letters in the correct order.2 marks
  8. Catalase is an enzyme found in potato tissue. It breaks down hydrogen peroxide, forming water and oxygen. A student adds catalase to a starch suspension and finds that the starch is not broken down. Explain why.3 marks
  9. A bacterium lives in a hot spring. A student measured the activity of one of its enzymes at six temperatures. (a) Explain the change in activity between 20 °C and 80 °C. [2] (b) A sample of the enzyme is kept at 20 °C for one hour with no substrate. It is then warmed to 80 °C and substrate is added. Predict the activity and explain your prediction. [2] (c) Compare the kinetic energy of the molecules at 100 °C with their kinetic energy at 80 °C. Then explain why the activity at 100 °C is zero. [3] | temperature / °C | enzyme activity / arbitrary units | | --- | --- | | 20 | 3 | | 40 | 9 | | 60 | 21 | | 80 | 36 | | 90 | 12 | | 100 | 0 |7 marks
  10. A student extracted two enzymes, X and Y, from a fungus. Both break down the same protein. She measured the activity of each enzyme at six pH values. Every test-tube was kept at 30 °C. (a) Explain why the activity of enzyme Y is greatest at pH 9. [2] (b) Explain why enzyme Y has no activity at pH 3. [3] (c) Another student writes: "Enzyme X works at pH 3 but not at pH 7 because the acid gives the molecules more kinetic energy, so they collide more often." Explain why this is wrong and give the correct reason. [2] | pH | activity of enzyme X / arbitrary units | activity of enzyme Y / arbitrary units | | --- | --- | --- | | 2 | 8 | 0 | | 3 | 20 | 0 | | 5 | 8 | 2 | | 7 | 0 | 11 | | 9 | 0 | 24 | | 11 | 0 | 5 |7 marks

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