Effects of Local Conditions on Enzyme Activity
High-Yield Summary
- Enzyme rate roughly doubles per 10°C rise — up to the optimal temperature, beyond which the enzyme denatures and activity crashes.
- Each enzyme has its own optimal pH matched to its normal environment: pepsin (stomach) ~1.5–2, trypsin (small intestine) ~8.
- pH affects activity two ways: altering active-site amino acid ionization (disrupts binding) and outright denaturation at extremes.
- High salinity disrupts ionic/hydrogen bonds, altering shape and reducing activity — mainly an in vitro concern, not physiological.
Key Terms
- Denaturation
- Loss of 3D structure (especially active-site shape) from extreme temperature, pH, or salinity — abolishes function.
- Optimal temperature/pH
- Condition at which an enzyme's activity peaks, matched to its physiological environment.
- In vitro
- In a test tube/lab setting, as opposed to within a living organism (in vivo).
Pepsin vs. Trypsin — pH Adaptation
| Pepsin (stomach) | Trypsin (small intestine) |
|---|---|
| Optimal pH ~1.5–2 (strongly acidic) | Optimal pH ~8 (mildly basic) |
| Adapted to gastric acid | Adapted to bicarbonate-neutralized intestinal contents |
Common MCAT Trap
- "More heat/farther from neutral pH = always faster" is false — activity rises only up to the optimum, then denaturation collapses it.
- pH can hurt activity WITHOUT denaturing the enzyme (reversible ionization-state changes at the active site) — don't assume every pH effect is permanent denaturation.
Quick Recall
Roughly how much does enzyme rate increase per 10°C rise, before denaturation?
Why does trypsin's optimal pH differ so much from pepsin's?
Name two distinct ways pH can reduce enzyme activity.
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