B3.5 Factors Affecting Enzyme Action

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: how temperature, pH and substrate concentration change the rate of enzyme-controlled reactions. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Explain how temperature affects the rate of an enzyme-controlled reaction
• Explain what happens when an enzyme is denatured, using the lock and key model
• Describe and explain how pH affects enzymes, with examples of optimum pH
• Explain the effect of substrate concentration on rate
• Calculate rates from times and read rate graphs

Key words

• Optimum: The temperature or pH at which an enzyme works fastest.
• Denatured: The shape of the active site has changed so much that the substrate no longer fits; this is permanent.
• Rate: How fast a reaction happens: amount of change per unit time.
• Collision: When substrate and enzyme molecules meet. More collisions mean more enzyme–substrate complexes.
• Kinetic energy: The energy a particle has because it is moving.

Temperature

• Up to the optimum: molecules have more kinetic energy, so they move faster and collide more often. More enzyme–substrate complexes form each second, so the rate increases.
• At the optimum (about 37 °C for most human enzymes): the rate is fastest.
• Above the optimum: bonds holding the enzyme’s shape break. The active site changes shape, so the substrate no longer fits – in lock and key terms, the “lock” has changed and the “key” no longer fits. The enzyme is denatured and the rate falls quickly to zero.

Key fact: Denaturing is permanent. At low temperatures enzymes are not denatured – they just work slowly because molecules have less kinetic energy, and they work again when warmed.

pH

Each enzyme has an optimum pH. Away from it, the forces holding the protein in shape are disrupted, the active site changes shape, and the rate falls. Very acidic or very alkaline conditions denature the enzyme.

• Stomach protease (pepsin); Where it works Stomach (hydrochloric acid present); Optimum pH about 2 (acidic)
• Salivary amylase; Where it works Mouth; Optimum pH about 7 (neutral)
• Pancreatic protease (trypsin); Where it works Small intestine (alkaline bile and pancreatic juice); Optimum pH about 8–9 (slightly alkaline)

Substrate concentration

At first, more substrate means more collisions with active sites, so the rate rises. Eventually all active sites are occupied at any moment, so adding more substrate cannot raise the rate. Now the amount of enzyme limits the rate; adding more enzyme would raise it again.

Calculating rates

Often we time how long a reaction takes. A shorter time means a faster rate, so rate = 1 ÷ time. Because these numbers are small, they are often multiplied by 1000.

Worked example: rate from time

At 25 °C, amylase takes 80 s to digest the starch. At 35 °C it takes 50 s.
Rate at 25 °C = 1 ÷ 80 = 0.0125 s⁻¹ (× 1000 = 12.5).
Rate at 35 °C = 1 ÷ 50 = 0.020 s⁻¹ (× 1000 = 20).
Percentage increase = (20 − 12.5) ÷ 12.5 × 100 = 60%.

Answer: the rate is 60% higher at 35 °C

Exam tip: When describing a graph, quote numbers from it: “The rate increases from 10 °C to a peak at 40 °C, then falls to zero at 60 °C.”

Common mistakes

• Saying enzymes are “killed”. Enzymes are not alive; they are denatured.
• Saying enzymes are denatured at low temperatures. They work slowly but recover when warmed.
• Saying the substrate “cannot fit” without saying why: the active site has changed shape.
• Thinking all enzymes have an optimum pH of 7. The stomach protease works best at about pH 2.
• Forgetting that a longer time means a slower rate.

Revision questions

Try these, then check your answers below each one.

Question 1. The graph shows how temperature affects an enzyme. At which point is the enzyme being denatured? (Choose from: A, B, C.)

Answer: C

Working: At C the temperature is above the optimum, so the active site is changing shape and the rate is falling. A is below the optimum; B is the optimum.

Question 2. What word describes an enzyme whose active site has permanently changed shape so the substrate no longer fits?

Answer: denatured

Working: The enzyme is denatured.

Question 3. True or false? Enzymes are denatured at low temperatures such as 4 °C.

Answer: False

Working: False. At low temperatures molecules have less kinetic energy and collide less often, so the rate is slow, but the enzyme is not denatured.

Related lessons

• Previous lesson: B3.4 Catalysts and Enzymes
• Next lesson: B3.6 How the Digestive System Works
• In this chapter: B3.1 Tissues and Organs
• In this chapter: B3.2 The Human Digestive System
• In this chapter: B3.3 The Chemistry of Food

Continue in the full course

This lesson is section B3.5 (AGCSC0305) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.