Denaturation
Exam-ready definition
A permanent change in the shape of an enzyme's active site, caused by high temperature or unsuitable pH, so the substrate no longer fits and the enzyme stops working.
How this is marked
Never write that the enzyme is 'killed'. Enzymes are not alive, and that word loses the mark every time. The mark scheme wants: active site changes shape, substrate no longer fits/binds, reaction stops.
In an exam answer
Explaining why an enzyme stops working at 60 °C: 'The high temperature changes the shape of the enzyme's active site permanently, so the substrate no longer fits and the reaction stops.'
Full topic guide: Enzymes
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