Definition of Lock and Key Model

Enzymes are complex proteins produced by living cells that catalyze biochemical reactions at different body temperatures. They catalyze chemical reactions by binding substrates to the active site on the enzyme. When the substrate binds to the enzyme, it changes the distribution of electrons in the substrate's chemical bonds and causes reactions that lead to the formation of products. The enzyme releases these products from its surface so it can regenerate for another reaction cycle.

Specificity

• Although enzymes cause molecules to change by acting as catalysts that break down larger molecules into smaller ones, they remain unchanged. Enzymes are therefore very highly specific. They function best within a narrow temperature and pH range. Each chemical reaction necessary to sustain life requires its own specific enzyme.

Shape

• The enzyme's active site has a shape that corresponds to the molecule part with which it reacts, otherwise known as the substrate. The enzyme and substrate fit into each other, which allows a reaction to take place. The manner in which the shapes allow the enzyme and substrate to fit into each other led to the lock-and-key theory.

Fischer's Theory

• In 1894, Emil Fischer made an analogy between a lock-and-key model and the specific action of the binding between an enzyme and a single substrate. According to the analogy, the enzyme is the lock and the substrate is the key. Only the correct key, or substrate, fits into the existing keyhole, or the active site, of the lock, or the enzyme.

Problematic Loophole

• Once the substrate unlocks the enzyme, it releases the chemical reaction. The lock-and-key theory definitely explains enzymes' high specificity, but it does not take into account enzymes' stabilization periods during the transition state. Moreover, the theory fails to address enzyme flexibility and is itself inflexible.

Modification

• In 1958, Daniel Koshland modified the lock-and-key theory to address the model's rigidity and the enzyme's stabilization period. Koshland's modification is called the induced-fit theory. It suggests that substrates help determine the enzyme's final shape and that the enzyme is actually more flexible than previously realized. This modified lock-and-key model addresses the reason some compounds can bind to the enzyme but fail to spark a reaction if the enzyme has been distorted too many times.