Enzyme

In the syllabus and the exam

Enzymes anchor an entire chapter of structured questions and practical investigations, and candidates are expected to explain how factors such as temperature, pH and substrate concentration affect the rate of an enzyme-controlled reaction, usually by describing and interpreting a rate-against-factor graph with a clear optimum point. A common exam format gives a graph or a table of results and asks candidates to identify the optimum temperature or pH, describe the trend before and after that point, and explain the trend in terms of the enzyme's active site and, at the higher end, denaturation. A frequent error is describing an enzyme itself as being 'killed' by heat, when the correct terminology is that the enzyme is denatured, meaning its three-dimensional shape, particularly the active site, is permanently altered so it can no longer bind its substrate, while the enzyme molecule is not literally destroyed. Another common confusion is between the lock-and-key model and the induced-fit model of enzyme action, and between low temperature (which merely slows an enzyme's rate reversibly, without changing its shape) and high temperature (which denatures it irreversibly); candidates who describe a cold enzyme as 'denatured' rather than simply 'less active' lose marks for this distinction. Beyond digestion, candidates are expected to recognise enzymes at work in other syllabus contexts too, such as catalase breaking down hydrogen peroxide in liver cells and amylase acting in the germinating seed to mobilise stored starch, so the term should not be studied only as a digestion topic. Candidates should also remember that a single enzyme molecule can be reused repeatedly to catalyse a large number of substrate molecules one after another, which is why only a small quantity of enzyme is needed relative to the amount of substrate present.

Example

The enzyme amylase, found in saliva, breaks down starch into maltose, working best at body temperature and a slightly acidic to neutral pH.

Related terms

SubstrateActive siteDenaturation

Source:SRC-DSKP-EN