Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- An enzyme’s shape is held by weak interactions: hydrogen bonds, ionic interactions and hydrophobic interactions. Disturb them and the active site changes.
- Denaturation: loss of the working 3D shape, so the enzyme can no longer catalyse. Peptide bonds are not broken. Causes: heat, pH, chemicals.
- Temperature: faster molecules give more collisions, so the rate rises up to the optimum; above it, bonds in the enzyme break and the rate falls steeply.
- pH: changes the charges on R groups, disrupting the fold and the active site’s charge pattern. Each enzyme has its own optimum pH.
- Denaturation is sometimes reversible (small pH changes, removal of a chemical). Strong heating usually is not. Cold slows enzymes without denaturing them.
- Relative concentrations of substrate and product set the direction and efficiency of the net reaction. Removing product keeps the forward reaction going.
- Inhibitors bind reversibly: competitive at the active site; noncompetitive at an allosteric site.
Key relationships
| Change | Effect on rate | Reason |
|---|---|---|
| temperature up, below optimum | rises | more frequent, more energetic collisions |
| temperature up, above optimum | falls steeply | hydrogen bonds break; active site changes shape |
| pH away from optimum | falls | R-group charges change; fold and active site disrupted |
| more substrate (low levels) | rises | more active sites occupied |
| more substrate (high levels) | levels off | enzyme saturated; all active sites busy |
| product builds up | net forward rate falls | reverse reaction becomes more frequent |
| competitive inhibitor | falls, but recovers at high substrate | substrate outcompetes inhibitor for the active site |
| noncompetitive inhibitor | falls at all substrate levels | allosteric binding changes the active site |
| Calculation | Formula |
|---|---|
| percentage change | (new − old) ÷ old × 100 |
| percentage of control | inhibited rate ÷ uninhibited rate × 100 |
Assumptions behind the numbers
- Rates compared across conditions are initial rates, measured before substrate runs low.
- Only one factor changes at a time; the others are held constant.
- An optimum read from widely spaced points is only an estimate.
Mistakes to avoid
- Saying enzymes are “killed” by heat.
- Saying denaturation breaks peptide bonds.
- Saying cold denatures enzymes.
- Explaining the plateau at high substrate as “the enzyme is used up”.
- Saying a noncompetitive inhibitor binds the active site.
- Dividing by the new value, not the old one, in a percentage change.
Quick self-check
- With an inhibitor, the rate falls from 12.0 to 3.0 µmol min⁻¹. What percentage of normal is this? (3.0 ÷ 12.0 × 100 = 25%)
- Adding lots of substrate restores the rate. Which type of inhibitor? (Competitive)
- Why does pH 9 lower the activity of an enzyme with an optimum of pH 2? (R-group charges change, disrupting hydrogen bonds and ionic interactions; the active site loses its shape and charge pattern.)
Next: practice questions.