Factors affecting enzyme activity
CIE A-Level Biology· 9700/32 3.2 Enzymes· 15 min read
1. Effect of Temperature★★☆☆☆⏱ 3 min
Temperature affects enzyme activity through two key mechanisms: changing the kinetic energy of reacting molecules, and altering the stability of the enzyme's tertiary structure.
Optimum Temperature
The temperature at which an enzyme-catalysed reaction reaches its maximum rate, balancing increased collision frequency and the risk of denaturation.
Example:
Most human enzymes have an optimum temperature of ~37°C.
Explain why enzyme activity increases from 10°C to 37°C, then drops to almost zero at 80°C.
- 1
Between 10°C and 37°C, increasing temperature raises the kinetic energy of both enzyme and substrate molecules.
- 2
More kinetic energy leads to more frequent, higher-energy successful collisions between enzymes and substrates, so more enzyme-substrate complexes form per second, increasing reaction rate.
- 3
Above 37°C up to 80°C, increased molecular vibration breaks the weak hydrogen and ionic bonds holding the enzyme's tertiary structure.
- 4
The enzyme denatures: the active site loses its complementary shape to the substrate, can no longer form enzyme-substrate complexes, so reaction rate drops to near zero.
Exam tip:
Always link rate changes to mechanism (collision frequency or bond breaking/denaturation) to access full marks in CIE.
2. Effect of pH★★☆☆☆⏱ 3 min
pH measures hydrogen ion concentration, which changes the charge of amino acid R-groups in the enzyme's polypeptide chain. This alters the ionic bonds that maintain tertiary structure and active site shape.
Optimum pH
The pH at which an enzyme's active site is in its correct, functional conformation, giving maximum reaction rate.
Example:
Stomach protease pepsin has an optimum pH of ~2, while intestinal trypsin has an optimum pH of ~7.5.
Explain why an enzyme with optimum pH 7 shows almost no activity at pH 12.
- 1
pH 12 is highly alkaline, with an extremely low concentration of hydrogen ions (H⁺) and high concentration of hydroxide ions (OH⁻).
- 2
The change in H⁺ concentration alters the ionization of R-groups on the amino acids that make up the enzyme.
- 3
This breaks the ionic bonds that maintain the enzyme's tertiary structure and active site shape.
- 4
The active site loses complementarity to the substrate, so no enzyme-substrate complexes can form, leading to near-zero activity.
3. Effect of Enzyme & Substrate Concentration★★★☆☆⏱ 4 min
When temperature and pH are kept constant, the concentration of enzyme and substrate limits how frequently enzyme-substrate complexes can form. Rate is always limited by whichever factor is furthest from its saturation point.
Saturation
A point where all available enzyme active sites are occupied by substrate at any given time, so reaction rate cannot increase further.
Describe and explain the shape of a reaction rate vs substrate concentration graph at fixed enzyme concentration.
- 1
At low substrate concentrations: rate increases linearly with increasing substrate concentration. At this point, there are more empty active sites than substrate molecules, so adding more substrate increases the number of complexes formed per second.
- 2
At medium substrate concentrations: the rate of increase slows, as a growing proportion of active sites are now occupied.
- 3
At high substrate concentrations: rate plateaus at Vmax (maximum rate). All active sites are saturated, so enzyme concentration becomes the limiting factor, and adding more substrate does not increase rate.
Exam tip:
CIE often asks you to identify the limiting factor at different points on this graph, make sure you can state it clearly for each region.
4. Enzyme Inhibition★★★☆☆⏱ 5 min
Inhibitors are molecules that reduce or stop enzyme activity by binding to the enzyme. CIE A-Level requires you to distinguish between two main types: competitive and non-competitive.
Competitive Inhibition
Inhibitor has a similar shape to the substrate, binds to the enzyme's active site and blocks substrate from binding.
Non-competitive Inhibition
Inhibitor binds to an allosteric site (not the active site), altering the enzyme's tertiary structure so the active site can no longer bind substrate.
Compare the effect of increasing substrate concentration on competitive vs non-competitive inhibition (fixed inhibitor concentration).
- 1
Competitive inhibition: At low substrate concentration, the inhibitor outcompetes substrate for active sites, so rate is lower than the uninhibited reaction.
- 2
As substrate concentration increases, more substrate molecules are available to outcompete the inhibitor for active sites. Rate eventually reaches the same Vmax as the uninhibited reaction, just at a higher substrate concentration.
- 3
Non-competitive inhibition: Inhibitor does not bind to the active site, so increasing substrate concentration cannot displace it from the allosteric site.
- 4
Vmax is permanently reduced, because a proportion of enzyme molecules are always inactivated. Rate never reaches the original Vmax, no matter how high substrate concentration gets.
Check your understanding:
Which type of inhibition can have its effect fully overcome by increasing substrate concentration?
A) Competitive only
B) Non-competitive only
C) Both
D) Neither
Reveal answer
A) Competitive only —Correct! Competitive inhibitors bind to the active site, so high substrate concentration can outcompete them. Non-competitive inhibitors alter the enzyme's shape permanently, so substrate concentration cannot reverse inhibition.
5. Common Pitfalls
Wrong move:
Stating that enzymes 'die' at high temperatures.
Why:
Enzymes are proteins, not living organisms, so this term is incorrect and loses marks in CIE exams.
Correct move:
State that the enzyme denatures, meaning its tertiary structure and active site shape is permanently destroyed.
Wrong move:
Claiming low temperature denatures enzymes.
Why:
Low temperature only reduces kinetic energy, it does not break the weak bonds that hold enzyme structure together.
Correct move:
Explain low activity at low temperature as fewer successful collisions between enzyme and substrate, not denaturation.
Wrong move:
Stating that the reaction stops at Vmax.
Why:
Vmax is the maximum sustained reaction rate, not a stopped reaction. All active sites are occupied, but reaction continues.
Correct move:
State that reaction rate plateaus at Vmax, it does not stop.
Wrong move:
Claiming all enzymes have an optimum temperature of 37°C.
Why:
This is only true for human/mammalian enzymes. Thermophilic bacteria have enzymes with optima above 70°C.
Correct move:
Refer to the optimum temperature of the specific enzyme given in the question, unless it is stated to be human.
Wrong move:
Confusing Vmax with the point where all substrate is used up.
Why:
Vmax is determined by enzyme concentration, not substrate concentration. It is the maximum rate when substrate is in excess.
Correct move:
Remember Vmax is the maximum rate when all active sites are saturated with substrate.
6. Quick Reference Cheatsheet
Factor | Effect on Reaction Rate | Key Mechanism |
|---|---|---|
Temp (below optimum) | Increases with temperature | More kinetic energy → more successful collisions |
Temp (above optimum) | Drops to zero | Bonds break → permanent denaturation |
pH away from optimum | Drops from maximum | Change in R-group charge breaks ionic bonds |
[Substrate] (fixed enzyme) | Increases then plateaus at Vmax | Plateaus when all active sites are saturated |
[Enzyme] (fixed substrate) | Increases proportionally | More active sites available for substrate |
Competitive inhibition | Same Vmax, lower rate at low [S] | Inhibitor competes for active site |
Non-competitive inhibition | Vmax permanently lowered | Inhibitor alters active site shape |
7. Frequently Asked
Why doesn't low temperature denature enzymes?
Low temperature only reduces molecular kinetic energy, it does not break the weak hydrogen and ionic bonds that hold enzyme tertiary structure together. Enzymes are just inactive, not permanently denatured.
What is a limiting factor in enzyme experiments?
A limiting factor is any factor that is in short supply and restricts the rate of reaction. For example, when all active sites are saturated, enzyme concentration becomes the limiting factor.
When this came up on past exams
AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2022 · 2
Effect of pH on amylase activity
- 2023 · 1
Compare competitive/non-competitive inhibition
- 2021 · 3
Practical temperature enzyme investigation
Going deeper
- practical guideCIE A-Level Biology required practicals: enzyme experimentsFor practical paper preparation
What's Next
Understanding factors affecting enzyme activity is foundational for all further topics in metabolism, and is heavily assessed in both theory and practical CIE A-Level papers. Mastery of this sub-topic allows you to interpret experimental data, explain control of metabolic pathways, and understand real-world applications of enzymes in biotechnology. The core concepts of denaturation and inhibition also underpin key topics like respiration, photosynthesis and pharmacology.