Enzymes either make certain reactions possible or speed up a reaction that would take too long otherwise. An enzyme works using the lock-and-key method. The substrate is the substance acted on by the enzyme. A substrate acts as the key; its shape must fit exactly into the enzyme for the enzyme to work.
Reactions need a bit of energy to start; enzymes lower this activation energy for the reaction to occur. An enzyme has a region called the active site, which has a unique shape into which only a specific substrate fits.
In 1894, Emil Fischer presented the lock-and-key analogy of enzyme action. This analogy is not altogether correct. Another theory, the induced-fit theory, states that when the substrate inserts itself into the enzyme, it forces the enzyme to change its shape. Chemical bonds, such as hydrogen bond and covalent bonding, cause the substrate to fit into the enzyme and allow them to "recognize" each other.
Enzymes cease their actions through a variety of different factors. Many enzymes stop functioning through negative feedback. The end product of the reaction fits into another site of the enzyme, rendering it inactive. Temperatures and pH levels must fall within a small range for the enzymes to act ideally. When these factors are not ideal, the enzyme, which is a protein, becomes denatured, and its structure changes. Once an enzyme is denatured, it is no longer effective.