Thousands of chemical reactions take place within living cells of organisms. Many of these reactions can also be carried out in the laboratory, but outside the cell, they are often much slower. To speed up these reactions so they would take place as fast as they do in cells would require temperatures or pHs that are inconsistent with life. How then is it possible for reactions to occur in cells at rates fast enough to meet the needs of the body? The answer is that cells produce special proteins known as enzymes that catalyze biological reactions, markedly increasing their rates.
The primary species acted on by an enzyme is called the substrate of that enzyme. Most enzymes are named after the substrate on which they act by simply adding –ase to the root name of the substrate. A lipase then, would be an enzyme that acts on lipids, sucrase on sucrose, and so forth. Many enzymes, such as chymotrypsin, trypsin and lysozyme, have older names that do not end in –ase.
Some enzymes owe their specific reactivity only to their specific protein structure. Others, however, are conjugated proteins that require the presence of a unique nonprotein unit to become an active enzyme. The protein portion of a conjugated enzyme is known as an apoenzyme, and its nonprotein portion is termed a cofactor. There are two kinds of cofactors. If the cofactor is an organic unit, it is commonly called a coenzyme. If the cofactor is a metal ion, it is called a metal-ion activator. Some enzymes require both types of cofactors. Na+, K+, Mg2+, Ca2+, Mn2+, Co2+, and Zn2+ are examples of metal-ion activators. Many trace metal ions found in the body are important in enzyme reactions. Some vitamins or their derivatives are coenzymes. Since vitamins and metal ions (often referred to as minerals) are essential for proper enzyme function, it is easy to understand why they are essential components of the diet.
Uses of Enzymes
Enzymes serve many critical functions in the body, catalyzing nearly every reaction that takes place. In recent years, researchers have learned to use enzymes to serve our needs in various ways. Enzymes can be powerful diagnostic tools in medicine, and it is not difficult to measure quickly and accurately the level of a specific enzyme in blood serum or urine. Normally, enzymes appear in blood serum or other extracellular fluids at very low concentrations, but certain disease conditions can markedly increase the level of one or more of them. If disease or injury damages the cell membrane, the enzymes will flow out into the extracellular fluid and eventually enter the bloodstream, where they can be detected easily. Of course, the use of enzymes is not restricted to health care. Commercially, proteolytic enzymes, such as papain, are used to tenderize meat. They catalyze the hydrolysis of connective tissue, reducing the toughness of meat. Food manufacturers use enzymes to partially digest food for infants and others with digestive problems. Certain proteolytic enzymes are used to remove cataracts.
Enzymes are efficient biological catalysts. They increase the rate of a reaction by lowering its activation energy, the energy barrier that must be crossed over as reactants are converted to products. For example, the decomposition of hydrogen peroxide, H2O2, to form oxygen and water has an activation energy in the absence of catalyst of 18 kcal per mole of H2O2. This same reaction occurs in cells, but in the presence of an enzyme called catalase. Catalase reduces the activation energy to around 7 kcal per mole of H2O2, a substantial reduction, which in turn allows the reaction to take place about 100 million times faster. The effect of catalase on the activation energy of this reaction is shown in Figure 1.
The efficiency of an enzyme is given by its turnover number, the number of substrate molecules that one