Which term describes an enzyme that acts on only a single substrate?

• A. Protease enzyme
• B. Enzyme specificity
• C. Substrate
• D. Immobilised enzyme

Answer: (B)

Enzyme specificity is the idea that an enzyme is selective for particular substrates because its active site has a shape and chemical environment that fit only certain molecules. When an enzyme acts on a single substrate, the substrate matches the active site like a key fits a lock, aligning the chemical groups just right to lower the activation energy and drive the reaction. This specificity can be described by the lock-and-key model or the induced-fit idea, where the enzyme may adjust slightly to snugly accommodate the substrate.

Other terms describe different concepts: a protease is simply a type of enzyme that cleaves proteins, not a description of how selective it is; a substrate is the molecule acted on by an enzyme, not the enzyme’s selectivity; and an immobilised enzyme is an enzyme fixed to a surface for reuse or control, not about substrate specificity.