Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

3.23 · Enzymes

Factors that Affect Enzyme Action — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 11 parts Lesson 8 of 17 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read
Lesson summary

3.2 Factors that Affect Enzyme Action — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Explain how temperature, pH, concentrations, inhibitors and immobilisation affect enzyme activity.Below the optimum — molecules gain kinetic energy, move faster and collide more of…

  • Above the optimum
  • Effect on charges
  • Effect on bonds
  • Extreme pH
  • Substrate in excess
  • Substrate limited

Section 1 of 11Temperature

Below the optimum
  • Below the optimum molecules gain kinetic energy, move faster and collide more often, so more enzyme–substrate complexes form.
  • The rate roughly doubles for each 10 °C rise (Q10 ≈ 2).
Above the optimum
  • Above the optimum vibration breaks hydrogen and ionic bonds, the active site changes shape and fewer complexes form.
  • Denaturation is permanent, so the rate falls steeply.

Section 2 of 11pH

Effect on charges
  • Effect on charges changing H+ concentration alters the charges on R groups, e.g. –COO-/–COOH and –NH3+/–NH2.
Effect on bonds
  • Effect on bonds ionic and hydrogen bonds break, so the active site changes shape and charge.
Extreme pH
  • Extreme pH causes denaturation.
Buffers
  • Buffers keep the pH constant in investigations.

Section 3 of 11Enzyme concentration

Substrate in excess
  • Substrate in excess the rate is proportional to enzyme concentration because each extra enzyme adds active sites.
Substrate limited
  • Substrate limited adding more enzyme eventually has no effect, so the graph levels off.

Section 4 of 11Substrate concentration

Increase
  • Increase higher concentration means more frequent collisions and a higher rate.
Saturation
  • Saturation at high concentration all active sites are occupied, the rate reaches Vmax and enzyme concentration becomes limiting.

Section 5 of 11Vmax and Km

Finding Km
  • Finding Km plot initial rate against substrate concentration and find Vmax.
Method
  • Method calculate ½Vmax, read across to the curve, then down to the axis.
Meaning
  • Km is the substrate concentration at ½Vmax; a lower Km means higher affinity.

Section 6 of 11Reversible inhibitors

Competitive
  • Competitive competes with the substrate for the active site.
  • Vmax can still be reached with enough substrate, but Km increases.
Non-competitive
  • Non-competitive binds elsewhere and changes the active site shape.
  • Vmax falls and Km is unchanged.
Reversibility
  • Reversibility they bind with weak non-covalent bonds and can leave.

Section 7 of 11Immobilised enzymes

Method
  • Method enzymes are trapped in alginate beads.
Advantages
  • Advantages reuse, uncontaminated product, greater stability and use in continuous flow columns.
Example
  • Example lactase making lactose-free milk.

Section 8 of 11Investigating a factor

1
Choose the independent variable, e.g. temperature, using thermostatically controlled water baths.
2
Equilibrate separate tubes of enzyme and substrate for about 5 minutes before mixing.
3
Keep volumes, concentrations and pH constant, using a buffer.
4
Start the timer and measure product formed or substrate left at fixed intervals.
5
Repeat at least three times and calculate a mean.
6
Calculate the initial rate and plot rate against the independent variable.
7
Use a boiled-enzyme control to show changes are due to enzyme activity.

Section 9 of 11More from the specification

1Temperature effect
  • Rate rises as temperature increases because molecules gain kinetic energy and have more successful enzyme-substrate collisions.
2Denaturation
  • Above the optimum, bonds in the tertiary structure break, the active site changes shape and the enzyme-catalysed reaction stops.
3Substrate concentration
  • Rate rises to a maximum, Vmax, when all active sites are saturated;
    • the Michaelis-Menten constant Km is the concentration giving half Vmax.
4Inhibitors
  • Competitive inhibitors compete for the active site; non-competitive bind elsewhere and change its shape.
5Measuring rate
  • Initial rate is found from the gradient of a product-against-time graph, with solutions kept at set temperatures.
6Buffer solution
  • A solution that holds the pH at a fixed value.
7Enzyme-substrate complex
  • The temporary structure formed when a substrate binds to an enzyme's active site.
8Successful collision
  • A collision with enough energy and correct orientation.

Section 10 of 11Key terms

TermMeaning
OptimumThe condition at which an enzyme works fastest.
VmaxThe maximum rate of an enzyme-catalysed reaction.
KmThe substrate concentration at half Vmax.
Competitive inhibitorAn inhibitor that competes for the active site.
Non-competitive inhibitorAn inhibitor that binds elsewhere and changes the active site.
Immobilised enzymeAn enzyme fixed to an inert insoluble support.
BufferA solution that resists changes in pH.

Section 11 of 11Exam technique

1
Saying low temperature denatures enzymes — it only slows the reaction.
2
Saying more enzyme always increases the rate — substrate must not be limiting.
3
Saying non-competitive inhibitors can be overcome by more substrate — they cannot.
4
Forgetting that pH can denature enzymes — extreme pH breaks ionic and hydrogen bonds.

Last updated: