3.1.3—Enzyme action hypotheses

The lock-and-key model is a useful first explanation of enzyme specificity because it shows a complementary substrate and active site. The induced-fit hypothesis extends this model: binding causes a small adjustment in the enzyme’s active site, and sometimes in the substrate, so the final complex is better arranged for catalysis.

• Initial recognition: The substrate is chemically and structurally compatible enough to interact with the active site; this preserves specificity.
• Small adjustment: As binding proceeds, the enzyme and active site can change shape slightly rather than behaving as perfectly rigid structures.
• Improved fit: The adjustment brings reactive groups into a more effective arrangement and helps the enzyme stabilise the reaction pathway.
• Catalytic consequence: A better enzyme–substrate arrangement increases the ability of the enzyme to catalyse the reaction; the adjustment is part of binding, not a permanent change to the whole protein.

Lock-and-key emphasises a pre-existing rigid complementarity. Induced fit keeps the idea of selective binding but explains why the active site can adjust around a suitable substrate to improve catalysis.

Induced fit is not permanent deformation or denaturation, and it does not make every molecule a suitable substrate. Card 4542 covers the general collision–complex–product process; this card explains the binding model. Rate measurement is handled separately in card 4544.