Topic 3 of 3
Enzymes
Apply specificity and condition sensitivity.
A-Level 8873, revised syllabus (2026-2027)
An enzyme has a specific active site and suitable operating conditions
Use the lock-and-key model without confusing cooling with denaturation.
Enzymes in this course are protein molecules acting as biological catalysts. Their folded structures create active sites whose shape and chemical environment suit particular substrates. In the lock-and-key model, a matching substrate binds to the active site; the enzyme catalyses a particular reaction and releases the products to be used again.
- Matching substrate
Its shape and interactions suit the active site.
- Enzyme-substrate complex
Binding enables a lower-activation-energy pathway.
- Products released
The enzyme is available for another suitable substrate.
At lower temperatures, less kinetic energy and fewer effective collisions make activity slower. Raising temperature initially increases rate. At sufficiently high temperatures, disruption of interactions holding the protein's shape changes the active site: denaturation causes activity to fall. Cooling a still-folded enzyme usually just slows it; heating beyond its tolerance can damage function.
Changing pH can alter charged groups and interactions in the protein and at its active site. An enzyme therefore has a limited useful pH range; extreme pH can denature it. Specificity includes both the choice of substrate and the type of reaction catalysed. The same enzyme does not automatically catalyse every reaction its substrate could undergo.