Enzymes explained: induced fit, inhibition and the marks examiners want

Induced fit, not lock and key

AQA expects the induced-fit model. The active site is not a rigid complementary shape: when the substrate binds, the active site changes shape so that it moulds around the substrate. This distortion puts strain on the substrate's bonds, lowering the activation energy of the reaction.

A common lost mark is writing that the enzyme 'fits' the substrate. Say instead that the active site is complementary to the substrate, then that binding induces a conformational change forming an enzyme–substrate complex.

Temperature and pH

• Rising temperature increases kinetic energy, so more successful collisions and more enzyme–substrate complexes form per second.
• Above the optimum, hydrogen and ionic bonds break, the tertiary structure changes, the active site is no longer complementary and the enzyme is denatured.
• pH changes alter the charge on R-groups, breaking ionic bonds and again changing the active site shape. Small shifts are reversible; large ones are not.

Competitive vs non-competitive inhibition

A competitive inhibitor has a shape similar to the substrate and binds to the active site, so fewer enzyme–substrate complexes form. Increasing substrate concentration overcomes the effect, so the same maximum rate is reached, just more slowly.

A non-competitive inhibitor binds to an allosteric site away from the active site, changing the tertiary structure so the active site is no longer complementary. Increasing substrate concentration does not overcome it, so the maximum rate falls.

Answer template that scores

• Name the interaction: 'binds to the active site / an allosteric site'.
• State the structural consequence: 'active site no longer complementary'.
• State the measurable consequence: 'fewer enzyme–substrate complexes form, so rate decreases'.