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2.3.8Topic 2 · Genes and Health

The Role of Enzymes — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

5 min read 8 parts Lesson 22 of 35 1 quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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2.3.8 The Role of Enzymes — Edexcel A Level Biology A (SNAB) 9BI0 notes: Explain how enzymes act as biological catalysts by lowering activation energy and describe the factors that affect the rate of enzyme-controlled reactions.Enzymes are biological catalysts: globular proteins …

  • lower the activation energy
  • active site
  • initial rate
  • Exam-style question 1
  • Exam-style question 2
  • Exam-style question 3

Section 1 of 8What enzymes do

Enzymes are biological catalysts: globular proteins that speed up chemical reactions in cells without being used up.

What enzymes do
Without them the reactions of metabolism would be far too slow at body temperature.
They are needed for anabolic reactions (building up large molecules,
  • such as DNA polymerase making DNA) and catabolic reactions (breaking down,
  • such as amylase breaking down starch).
Some function inside cells (intracellular, such as catalase) and some outside (extracellular, such as digestive enzymes).

Section 2 of 8Activation energy

For a reaction to take place, reactant molecules must reach a minimum energy called the activation energy.

Activation energy
Lower the activation energy
Enzymes lower the activation energy by providing an alternative reaction pathway: they bring the substrate molecules together in the correct orientation,
  • or strain bonds in the substrate,
  • so the transition state is reached more easily.
More molecules have enough energy to react in a given time, so the rate rises.
The enzyme does not change the overall energy released or the position of equilibrium.

Section 3 of 8Specificity

Each enzyme catalyses a single reaction or a very limited range of reactions, because the shape of its active site, the region where the substrate binds, is determined by its tertiary structure.

Specificity
The substrate has a shape and charge distribution that are complementary to the active site.
Different enzymes have different amino acid sequences, and therefore different shapes.
Enzymes are named by the substrate and reaction type with the ending '-ase', for example lipase breaks down lipids.

Section 4 of 8Factors affecting rate

FactorEffect and explanation
TemperatureRate increases as kinetic energy rises, giving more frequent collisions with more energy. Above the optimum, bonds in the tertiary structure break and the active site changes shape, so the enzyme denatures and rate falls sharply. At low temperature enzymes are inactive but not denatured.
pHEach enzyme has an optimum pH (pepsin about 2, amylase about 7). Away from it, charges on R groups change, ionic and hydrogen bonds break, and the active site is distorted.
Substrate concentrationRate increases then levels off when all active sites are occupied (saturation); enzyme concentration becomes limiting.
Enzyme concentrationRate increases in proportion if there is excess substrate, until substrate becomes limiting.
InhibitorsReduce rate by competitive or non-competitive means.

Section 5 of 8Interpreting data and rates

The rate of reaction is measured as the amount of product formed, or substrate used up, per unit time.

Initial rate
On a graph of product against time, the gradient of a tangent at the start gives the initial rate, and the curve flattens as the substrate is used up.
When comparing curves, quote values with units, for example 'rate at 40 °C was 12 cm3 min-1 compared with 4 cm3 min-1 at 20 °C'.
A temperature coefficient Q10 compares rate at a temperature with that at 10 °C lower.

Enzymes also have commercial usesfor example, immobilised lactase to make lactose-free milk, proteases in washing powders and amylase in brewing.

Enzymes may be affected by other substances.

1
The activity of many enzymes is regulated by cofactors and by inhibitors, which are covered in the next lesson.
2
Remember the key vocabulary when you describe experiments: the independent variable is what you change, the dependent variable is what you measure (volume of product per unit time),
  • and control variables are what you keep constant.
3
For temperature questions, mention that enzymes from organisms living in hot springs,
  • such as Taq polymerase from Thermus aquaticus, have a higher optimum,
  • because their amino acid sequences give greater stability at high temperature, and this property is used in the polymerase chain reaction.
4
Examination answers should link denaturation to the loss of the active site shape, and never imply that peptide bonds are broken.

A final reminder concerns graphs.

Interpreting data and rates
In a rate versus temperature graph the rising part is limited by kinetic energy and the falling part is due to denaturation,
  • which is why the curve is asymmetric: it climbs gradually but drops sharply.
In a rate versus pH graph the curve is roughly symmetrical about the optimum.
Describing each section separately, with quoted values, gains the data-handling marks.

Section 6 of 8Exam-style questions

  1. Explain how an enzyme speeds up a reaction. [3]3 marks

  2. The rate of an enzyme-catalysed reaction increases with temperature to 45 °C and then falls to zero at 65 °C. Explain these results. [4]4 marks

  3. A reaction produces 36 cm3 of oxygen in 3 minutes. Calculate the mean rate. [2]2 marks

Section 7 of 8Key terms

TermMeaning
EnzymeA biological catalyst, usually a globular protein.
CatalystA substance that speeds up a reaction without being used up.
Activation energyThe minimum energy required for a reaction to occur.
Active siteThe region of an enzyme to which a substrate binds.
SubstrateThe molecule on which an enzyme acts.
ProductThe molecule formed by an enzyme-catalysed reaction.
OptimumThe temperature or pH at which enzyme activity is highest.
DenaturationPermanent change in shape of the enzyme.

Section 8 of 8Exam technique

1
Saying that enzymes are killed by high temperature. Enzymes are not alive; they are denatured.
2
Writing that enzymes provide energy for the reaction. They lower the activation energy.
3
Stating that the active site is the whole enzyme.
4
Forgetting that at low temperature enzymes are inactive, not denatured.

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