9477 / 2027

Lesson 2 of 4 / Enzymes and investigations

Temperature and pH change enzyme rate

Why is a temperature-rate curve not symmetrical?

I can explain and investigate temperature and pH effects on catalysis.

  • I can investigate and explain temperature, pH and concentration effects on enzyme rate.Syllabus

    Syllabus 9477, Core 1(q). Investigate and explain temperature, pH and concentration effects on enzyme rate.

About 6 min

Make a guess. You are not marked.

A temperature-rate graph for an enzyme rises slowly to a peak and then falls. Is the fall as gradual as the rise?

  1. No. It falls steeply as the enzyme denatures.
  2. No. It falls slowly, because molecules slow down.
  3. Yes. The curve is symmetrical about the optimum.
Show the answer

No. It falls steeply as the enzyme denatures.

Below the optimum, rate rises with kinetic energy. Above it, bonds holding the active site break and activity drops fast.

The key idea

Below the optimum, more kinetic energy raises the frequency of effective collisions. Above it, the enzyme is denatured and the 3D shape of the active site changes. pH changes the charges that hold the shape and bind the substrate.

Explore the idea

Read the shape, then explain it

102030405060Relative rateTemperature (degrees C)0

Relative rate: 6 in this original data set. Increasing temperature can increase productive molecular encounters while the enzyme remains stable.

These are invented practice measurements. Joining points guides the eye; it does not establish the exact optimum between tested conditions.

Explanation

Enzymes act only in solution, where enzyme and substrate molecules can move and collide. An effective collision is one in which the substrate binds to the active site and an enzyme-substrate complex forms. As temperature rises towards the optimum, molecules gain kinetic energy, so the frequency of effective collisions increases and more enzyme-substrate complexes form per unit time.

Above the optimum, the enzyme vibrates enough to break the hydrogen bonds and ionic bonds that maintain its 3D shape. The 3D shape of the active site changes, so the substrate is no longer complementary to it. The enzyme is denatured and the number of functional active sites falls. The decline is steep because denaturation is not the mirror image of the gain in kinetic energy.

An optimum is the highest measured rate under the stated conditions and assay duration. Prolonged heating can inactivate an enzyme that seemed efficient during a short initial measurement. Do not treat one graph as a universal fixed optimum for all enzymes.

Changing pH alters the ionisation of R groups in the active site and in the substrate. It can also break ionic bonds and hydrogen bonds that maintain the 3D shape of the active site. Binding or catalysis can decline even before complete unfolding. Use a suitable buffer so that changes in reaction products do not cause an unintended pH change.

For a temperature investigation, equilibrate separate enzyme and substrate solutions before mixing, keep concentrations and pH fixed, and measure initial rate with repeats. A water bath controls temperature more reliably than assuming room conditions remain constant. Include a no-enzyme or denatured-enzyme control where suitable.

Step by step
  1. 1

    Control other variables

    Hold enzyme, substrate, volume and pH constant.

  2. 2

    Measure the early rate

    Avoid confounding substrate depletion with temperature.

  3. 3

    Interpret both sides

    Explain low-temperature kinetics and high-temperature structural loss separately.

Worked example

Work through the evidence

A catalase sample is slow at 5 C but recovers at 25 C. Another heated to 80 C does not recover. Explain.

One way to explain it

Cooling reduced molecular motion without necessarily denaturing the enzyme; warming restored rate. Strong heating disrupted functional structure, and failed refolding or aggregation prevented recovery.

Why this answer works
  • Recovery after treatment helps distinguish kinetic from structural effects.
  • Do not describe cold enzyme as dead.

Watch out for this

A student says: "Every fall in rate means the enzyme has denatured." What is wrong with this?

Show the answer

Rate can fall because of lower temperature, limited substrate or altered ionisation without complete denaturation.

Check your understanding

Why buffer an enzyme pH investigation?

  1. To maintain the intended pH during the reaction
  2. To prevent the substrate binding to the active site
  3. To keep the reaction temperature constant
Show the answer

To maintain the intended pH during the reaction

This reduces changes caused by the reaction itself.

Exam-style questions

Check your understanding

Which is NOT a reason why enzyme activity falls when pH moves far from the optimum?

  1. Ionic bonds holding tertiary structure are broken
  2. Enzyme molecules collide less often with substrate
  3. Charges on R groups in the active site change
  4. The substrate itself may change its charge
Show the answer

Enzyme molecules collide less often with substrate

Collision frequency depends mainly on temperature and concentration, not pH. pH acts on charges and bonds.

Quick recall

Card 1 of 3. Answer in your head, then check.

Why is a temperature-rate curve not symmetrical?

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