Enzymes, complex proteins produced by all living things. They play an important role in nearly all the chemical reactions that take place in organisms by acting as catalysts. (Catalysts are chemical substances that change the rate of a chemical reaction without themselves being changed by the reaction.) Most often, enzymes allow reactions to begin sooner than they would in their absence, thereby speeding up metabolic processes. Without enzymes, an organism could not function properly because such metabolic processes as respiration, digestion, and photosynthesis would take too much time.

Structure and Functions of Enzymes

Like all proteins, enzymes consist of chains of amino acids linked together. The amino acids within each kind of enzyme have a characteristic arrangement. The bonds between the different amino acids in the chains are weak and may be broken by such conditions as high temperatures or high levels of acids. When the bonds are broken, the enzymes become nonfunctional and disease sometimes occurs.

Thousands of different enzymes are needed to keep the human body functioning normally, each usually acting on only one kind of substrate (the substance that undergoes the reaction), and catalyzing only one kind of reaction. Enzymes are classified according to the type of reaction they catalyze and the type of substrates on which they act. With few exceptions, the names of enzymes end in -ase, as in lipase, protease, and amylase.

Most metabolic processes involve a series of many different chemical changes. In digestion, for example, separate chemical reactions take place in the mouth, stomach, and intestine. Certain enzymes break down the protein, carbohydrate, and fat molecules of food into smaller molecules. Other enzymes assist in passing these smaller molecules into the bloodstream.

The surface of each type of enzyme molecule has a distinctively shaped area of grooves. This area is called an active site. An active site is shaped in such a way that only a certain kind of substrate can bind to it. Sometimes enzymes and substrates bond as a result of random molecular collisions, but frequently the assistance of other chemical substances is required. These chemical substances are electrically charged metal atoms known as cofactors and organic molecules (usually vitamins) known as coenzymes. Among the ways that cofactors work is by neutralizing the similar electrical charges that enzymes and substrates may have, so that they do not repel each other. Among the ways that coenzymes work is by carrying molecules of substrate to enzymes.

When substrate molecules bind to the enzyme molecule, amino acids from the enzyme react with the substrate. The reaction changes the substrate (usually by breaking certain bonds) into products that can be used by the organism. When the reaction is complete, the enzyme molecule releases the products. The enzyme is unchanged by the reaction and can move on to combine with more substrate molecules.

Different cells contain different enzymes. Some enzymes are found in all cells, others only in specific cells. Some enzymes are always present in a cell, and others are produced by a cell only when needed.

Uses of Enzymes

In addition to their natural functions within organisms, enzymes have valuable medical and industrial uses. In medicine, enzymes are used to treat conditions caused by enzyme deficiencies, such as digestive problems related to pancreatic disease. Enzymes are also used to help heal wounds and dissolve blood clots. In industry, enzymes are used in such processes as the brewing of beer, the curdling of cheese, and the leavening of bread.

Enzymes can be extracted from plants, bacteria, fungi, and animal organs. They are difficult to synthesize in the laboratory because of the complexity of their chains of amino acids. In 1968, ribonuclease became the first enzyme to be synthesized.

The Discovery of Enzymes

Enzymes were originally called ferments, because enzyme action was first observed in the fermentation process. In the middle of the 19th century, the French chemist Louis Pasteur suggested that microbe activity caused grape juice to ferment into wine. Later studies proved that fermentation is due to a protein material produced by a microorganism. This protein is now known to be an enzyme.