Enzymes are highly specialized proteins that control the chemical reactions in all living cells. They operate as organic catalysts, lowering the amount of energy needed to power a reaction, thus speeding it up. Many biochemical reactions require large amounts of energy and, in the absence of a catalyzing enzyme, metabolic processes would proceed far too slowly to maintain life. Most enzymes are found within cells, but some are released to catalyze reactions outside cells, such as the digestion of food in the stomach and intestines.

The “lock-and-key” hypothesis is one of the simplest theories of how enzymes operate. The details of this theory are illustrated. According to the theory, enzymes have an active site called the “lock.” Many also have a secondary site (A). The active site is shaped so that only a molecule with the correct complementary shape called the “key” can link to it (B). This forms an enzyme-substrate complex (C). After the reaction has occurred, the products separate from the enzyme (D). The Iock-and-key hypothesis also explains how enzyme action may be inhibited. An alien substrate with a shape similar to that of the true substrate can block the active site (E). Or a different alien substrate can link to the enzyme’s secondary site and distort the active site (F).

Properties of enzymes

Catalysts participate in a reaction by promoting a chemical change, but they themselves remain unaltered. Because each enzyme is usually involved in only a single type of reaction, there are an enormous variety of enzymes. Although enzymes are highly specific, the same enzymes or groups of enzymes are often found in a wide variety of organisms, which accounts for the similarity of the basic metabolic functions in plants, animals, and bacteria. (Metabolic functions are the processes by which food is turned into energy and living tissue.)

Only minute quantities are needed to affect the rate of a reaction because enzymes are extremely fast-acting and can be reused countless times. Catalase, found in the liver, is one of the fastest It can break up 40,000 hydrogen peroxide molecules into water and oxyqen per second.

Most chemical reactions are reversible. In a reversible reaction, the direction of the reaction depends on the physical and chemical conditions at a particular time. An enzyme can break down a substrate A into its products B and C. (A substrate is the molecule that the enzyme acts upon.) The same enzyme is then equally capable of catalyzing the reverse reaction. It does not alter the concentrations in which the three constituents—A, B, and C—are found when the reaction reaches equilibrium (a state of balance); it merely reduces the time needed to reach this state.

Because they are made of protein, enzymes share the properties of proteins. They are sensitive to temperature and acidity. At higher temperatures, enzymes operate more efficiently. But above a certain temperature, the protein becomes damaged; the hydrogen bonds in the molecule start to break and the protein becomes denatured, losing its shape and its effectiveness. Few enzymes can work at temperatures above about 108° F (42° C), though some, such as those in bacteria found in hot springs, can operate at higher temperatures. Enzymes also have an optimum acidity level, often around neutral—neither acidic nor alkaline. Some, such as pepsin (which breaks up protein in the stomach), operate only in acid conditions, while others function best in an alkaline environment.

Controlling enzyme functions

Most enzymes are named for the reaction they regulate; for example, an enzyme that catalyzes the removal of hydrogen from a substrate molecule is called a dehydrogenase. (The suffix ase signifies that the molecule is an enzyme.)

Several theories explain why enzymes are specific and how they operate. All center on the enzyme’s three-dimensional structure. Simplest is the “lock-and-key” hypothesis, postulating that a substrate molecule (the “key”) attaches itself to an active site (the “lock”) of an enzyme molecule, forming a temporary complex. The active site has a specific shape, so only a substrate with the complementary shape can attach itself to this site. Molecules with different shapes cannot attach to the active site, and if the active site is distorted by excessive heat, the enzyme molecule itself will no longer fit.

Some other compounds may be close enough in shape to the substrate to fit into the active site of the enzyme, but these alien substrates are not changed through contact with the enzyme. Rather, they compete with the true substrate for active sites, thus inhibiting the enzyme’s activity.

There are two types of such inhibition. In competitive inhibition, an alien molecule forms only a temporary bond with an enzy me. In noncompetitive inhibition, an alien molecule either permanently blocks an enzyme s active site or affects it by temporarily binding to a site elsewhere on the enzyme. Many inhibitors, particularly of the noncompetitive type, act as poisons. Cyanide, which binds with an enzyme necessary for cellular respiration, is a good example. (Cellular respiration is the process by which cells get oxygen.)

According to more complicated theories of enzyme action, an enzyme has two separate shapes. A nonactive enzyme has one shape: when active, it has another. And some enzymes do not work without the presence of what are called cofactors. Some cofactors participate in the enzyme reaction. Others probably lock into the enzyme away from the active site, holding the enzyme in the correct position to receive the substrate. Water-soluble vitamins are often important because they are cofactors in an enzyme system.

Many species of fungi, such as the pin mold growing on a carrot, obtain nutrients by secreting digestive enzymes into the foodstuff in which they are growing. The enzymes break down the large foodstuff molecules into smaller molecules. These are then absorbed by the fungus. In a similar way, human digestive enzymes break down food into small molecules. These can then be absorbed through the wall of the alimentary canal.

Enzyme systems

Most enzymes work as part of a chain of reactions during metabolism. The product of one enzyme-induced reaction then becomes the substrate for another. The whole reaction sequence is controlled by the slowest step.

Enzymes may consist entirely of protein, or of protein linked to a group that helps to maintain the shape of the molecule. The group may also participate in the reaction. Sometimes, such a group fulfills both these functions.

Trace metal minerals (such as iron and cobalt), often necessary in the diet, are used by these groups.

Sexual reproduction in many animals depends on enzymes. When the head of a sperm hits the outer membrane of an egg, a small sac of special enzymes at the front of the sperm’s head breaks open. The released enzymes break down the egg’s membrane. This allows the sperm to penetrate and fertilize the egg—as shown in the scanning electron micrograph (above). After the egg has been fertilized, its outer membrane undergoes little-understood changes. These changes make the egg resistant to the enzymes of subsequent sperm. This prevents multiple fertilization.