By the end of this lesson
- Explain how pH affects active-site shape and enzyme activity.
- Interpret enzyme activity–pH curves.
- Relate different optima to enzyme environments without treating location as the mechanism.
BIG QUESTION
Why can a change of a few pH units stop an enzyme even when temperature and substrate concentration stay constant?
pH changes proton concentration and therefore molecular charge, altering the interactions required for binding and catalysis.
pH changes molecular charge
pH measures hydrogen-ion concentration on a logarithmic scale. Changing pH changes protonation and charge on amino-acid side chains in a protein.
Those charge changes can alter ionic and hydrogen-bond interactions that maintain tertiary structure or participate directly in catalysis.
Activity peaks near an enzyme-specific optimum
At the optimum pHThe pH giving the highest measured enzyme activity under stated conditions, because the active site then has its most favourable shape for binding the substrate., the active site has a charge distribution and shape favourable for substrate binding and catalysis. Moving away can reduce productive binding and slow the reaction.
At sufficiently extreme pH, enough stabilising interactions are disrupted for the active site to lose its functional shape; the enzyme is described as denatured.
Different environments favour different enzymes
Pepsin functions in the acidic stomach and has an acidic optimum. Enzymes acting in the small intestine often function best in neutral to alkaline conditions. This does not mean acid “activates all stomach enzymes”; each protein has its own structure.
An enzyme transferred far from its normal pH may lose activity, while a different enzyme remains active. Evolution has selected proteins stable and catalytic in their usual cellular or extracellular environment.
Reading a pH curve
Locate the pH at the maximum measured rate, then describe how activity changes on both sides. Avoid assuming the curve must be symmetric.
A graph shows performance under those assay conditions. Substrate concentration, temperature, ionic strength and measurement duration can alter the measured curve.
Graph skills: SLAPUK and choosing a scaleVolume of oxygen against pH: an even scale on each axis, then plot with crosses.
pH effects from particles to graph
The causal chain links chemical environment to measured rate.
| Level | Change | Consequence |
|---|---|---|
| Solution | Hydrogen-ion concentration changes. | Amino-acid side-chain protonation changes. |
| Protein | Charges and stabilising interactions change. | Active-site shape or chemistry changes. |
| Binding/catalysis | Fewer productive enzyme–substrate complexes form. | Product formation per second decreases. |
| Extreme pH | Tertiary structure is substantially disrupted. | The enzyme denatures and may not recover. |
Language that earns precision
- pH
- A logarithmic measure related to hydrogen-ion concentration.
- optimum pH
- The pH producing the highest measured activity under specified conditions.
- tertiary structure
- The overall three-dimensional folding of one protein chain.
- denaturation
- Loss of the functional three-dimensional structure of a protein.
Common misconceptions
Claim: pH changes the substrate into an enzyme.
Correction: pH changes molecular charge and can alter enzyme structure or binding.
Claim: Every enzyme works best at pH 7.
Correction: Optimum pH depends on the enzyme and its usual environment.
Claim: Any small pH change permanently denatures an enzyme.
Correction: Moderate changes may reduce binding reversibly; sufficiently extreme pH can cause denaturation.
Claim: Pepsin works in acid because acid digests the protein for it.
Correction: Pepsin itself catalyses protein hydrolysis; acidic conditions support its active-site structure and chemistry.
Match the answer to the command word
Quote the optimum and describe both sides of the curve using the data.
Link pH → charge/interactions → active site → complex formation → rate.
Use the enzyme’s environment as evidence, while stating that optimum is enzyme-specific.
Recall, apply, then justify
Explain how a large change in pH can reduce enzyme activity.
Hint
Build the full molecular chain.
Suggested marking points
- pH changes hydrogen-ion concentration and amino-acid charges.
- Interactions maintaining protein shape are disrupted.
- The active site changes shape or chemical properties.
- Fewer productive enzyme–substrate complexes form, so rate decreases.
Pepsin has maximum activity at pH 2. State its optimum pH and suggest why this is biologically useful.
Hint
Use the graph maximum and stomach environment.
Suggested marking points
- Optimum pH is 2.
- This matches the acidic conditions of the stomach where pepsin functions.
Explain why location alone is not a complete explanation of an enzyme’s optimum pH.
Hint
Name the direct molecular cause.
Suggested marking points
- Location describes the environment but does not cause catalysis directly.
- pH-dependent charges, bonding and active-site structure determine activity.
Two enzymes have the same active-site shape at pH 7 but different pH optima. Suggest molecular reasons for the difference.
Planning hint
Consider side-chain identity, charge, stability and catalytic residues.
What a strong design includes
- They may contain different amino acids outside or within the active site.
- Side chains can have different acid–base properties.
- Different ionic interactions may stabilise each protein at different pH values.
- Catalytic residues may require different protonation states.
- The same static shape at pH 7 does not prove the same dynamics or stability across pH.