Mechanism of Enzyme Action

1. Substrate Binding: The enzyme has a specific region called the Active Site — a three-dimensional pocket or groove on its surface with a shape and chemical environment complementary to the specific substrate molecule.

The substrate molecule binds to the active site, forming an Enzyme-Substrate (ES) Complex. This binding is highly specific.

2. Catalysis: Once the ES complex forms, the enzyme either:

• Brings two substrates into close proximity
• Strains/distorts the substrate molecule to weaken specific bonds
• Creates a favourable chemical microenvironment (pH, charge)

This lowers the Activation Energy required for the reaction, dramatically speeding it up.

3. Product Release: The reaction occurs, substrates are converted to products, and the products are released from the active site. The enzyme is unchanged and free to catalyze the next reaction.

1. Lock and Key Model (Fischer, 1894): The enzyme (lock) and substrate (key) have perfectly complementary, rigid shapes. The substrate fits exactly into the active site.

2. Induced Fit Model (Koshland, 1958) — More Accepted: The active site is NOT perfectly rigid. When the substrate approaches, the enzyme's active site changes shape slightly to fit the substrate better — like a hand fitting into a glove. This model better explains enzyme behavior.