The lock and key hypothesis is a scientific analogy that states only the correctly sized key fits in the lock, explains an Elmhurst College website. In this analogy, the key refers to a substrate and the lock refers to an enzyme.
For enzymes to catalyze a chemical reaction, they must bind to a specific substrate. The substrate must be the perfect size and shape to fit into the active site of its corresponding enzyme. This basic mechanism causes changes in the chemical bonds of the substrate by altering how the electrons are distributed. This reaction leads to the formation of products released to the surface of the enzyme. The products act to regenerate and prepare the enzyme for another reaction cycle. Since substrates have a complementary shape to the active site of a specific enzyme, reactions can only occur with one or a few similar compounds. Enzyme-protein interactions, antigen-antibody interactions and hormone-receptor interactions are a few examples of the lock and key hypothesis. In 1899, Emil Fischer, a German chemist, proposed this hypothesis to explain the specificity of enzymes. The lock and key hypothesis helps explain why high temperatures denature enzymes, how reactions can be inhibited by changing the shape of the active site, and why some substances can inhibit enzymes.