Some chemical reactions simply happen when the two reactants come into contact. For example, you may be familiar with the bubbly “volcano” that forms when baking soda and vinegar are placed together in a glass. This reaction is spontaneous because it does not require outside energy to force it to occur.

Most reactions, however, require energy. For example, the chemical reactions that produce a cake do not take place when baking soda, flour, and the other ingredients of a cake are simply left in a pan on the kitchen counter. Heat is required to break the existing chemical bonds in the ingredients so that they can undergo chemical reactions and combine with each other in new ways.

In the laboratory, chemists use heat to create the activation energy needed to get nonspontaneous reactions started. Animals, however, can’t rely on internal Bunsen burners to get their chemical reactions cooking. In order to perform chemical reactions at low temperatures, the body uses special proteins called enzymes, which lower the activation energy necessary for chemical reactions to achievable levels. Enzymes lower the activation energy by interacting with the substrates, the primary molecules or compounds involved in the reaction. If you think of the activation energy needed for a chemical reaction as a mountain that the reactants have to climb, think of an enzyme as opening up a tunnel through the mountain. Less energy is required to go through the tunnel than to climb all the way up the mountain.

Enzymes are not themselves altered when they help reactions along. Consequently, a single enzyme can be used repeatedly in many reactions. Because enzymes can be used over and over again and because they can act very quickly, a relatively small amount of enzyme is needed to facilitate reactions involving relatively large amounts of material.

Each enzyme is designed to fit only the substrates in the reaction that the enzyme is meant to control. The one-to-one correspondence between enzyme and substrate is referred to as specificity. An analogy to a lock and key is useful for understanding the specificity of enzymes. Each enzyme can be thought of as a lock that can interact only with the appropriate key, or substrate. The region of the enzyme that interacts with the substrate is known as the active site.

Enzymes help form bonds by holding two substrates near each other in the active site. Compounds can form bonds with each other more easily when they are adjacent than when they are floating around the cell randomly.

Often, enzymes are named for their substrate. The name of the enzyme is the name of the starting material followed by the “-ase.” For example, maltase is an enzyme that breaks down maltose, a common sugar. (Be careful not to confuse sugars, which end in “-ose,” with enzymes, which end in “-ase.”)

Like all proteins, enzymes have a unique three-dimensional structure that changes under unusual environmental conditions. Enzymes do not function well when their structure is altered.

Depending on where it is normally located in the body, an enzyme will have different temperature and pH values at which its structure is most stable. As conditions deviate from this point, the enzyme’s ability to help along reactions decreases.

Most enzymes work best near a pH of 7, but some enzymes operate most effectively in a particularly acidic environment, such as the stomach; a neutral environment impairs their function. Likewise, the enzymes of creatures that live at high temperatures, such as bacteria that live in hot springs, do not function properly at human body temperature.

In order to control enzyme activity more precisely, the body has developed a number of compounds that turn enzymes on or off and make them work faster or slower. Sometimes these compounds attach to the active site along with the substrate, and sometimes they bind to another site on the enzyme. Activators of enzymes are known as cofactors or coenzymes. Many vitamins are coenzymes. Molecules that prevent enzymes from functioning properly are known as inhibitors.