3.1.2—Enzyme mode of action

An enzyme’s active site is a three-dimensional region whose shape and chemical groups are complementary to a particular substrate. Specific binding creates a temporary enzyme–substrate complex and positions the reactants for catalysis.

1. Collision: The substrate collides with the active site at a suitable orientation and speed.
2. Binding: Complementary shape and chemical interactions hold the substrate in the active site, forming a temporary enzyme–substrate complex.
3. Catalysis: The active site helps destabilise relevant substrate bonds and provides an alternative pathway with lower activation energy, so products form more readily.
4. Release: Products leave the active site and the unchanged enzyme is ready to bind another substrate.

The protein’s tertiary structure determines the active-site shape, so a change in amino-acid sequence can change the site and enzyme specificity. Complementary binding is therefore the gate for the reaction; a collision with the wrong shape need not produce a complex or products.

The active site does not permanently lock the substrate in place, and the enzyme is not used up. Lower activation energy increases reaction rate through an alternative route; it does not change the overall energy difference or move the equilibrium. The small binding adjustment described by induced fit is developed in the next card.