Active site

In the syllabus and the exam

Active site is the structural feature examiners use to explain both enzyme specificity and denaturation, and candidates are expected to describe it as a small region on the enzyme's surface with a shape complementary to a particular substrate, formed by the way the enzyme's chain of amino acids is folded. Structured questions frequently ask candidates to use the active site to explain why an enzyme works on only one substrate (specificity), why increasing substrate concentration beyond a point no longer increases reaction rate (all active sites are occupied, or 'saturated'), and why extreme temperature or pH stops an enzyme from working (the active site's shape is permanently altered, so the substrate can no longer bind). A common error is describing the active site as if it were the entire enzyme molecule, when it is only one specific region on it; another frequent mistake is describing substrate binding without mentioning shape-matching at all, simply stating that the substrate 'enters' the enzyme without explaining why only that particular substrate can do so. The lock-and-key model, where the active site is described as rigid, and the induced-fit model, where the active site is described as slightly flexible and moulding around the substrate as it binds, are both used in SPM answers, and candidates should be able to describe at least the basic lock-and-key idea clearly. A useful memory check for candidates is that any answer describing enzyme specificity, saturation, or denaturation that does not mention the active site's shape at some point is almost certainly incomplete, since the active site is the structural link connecting all three of these commonly tested ideas.

Example

The active site of catalase has a shape that fits only hydrogen peroxide, allowing the enzyme to break it down quickly into water and oxygen.

Related terms

EnzymeSubstrateDenaturation

Source:SRC-DSKP-EN