Enzymes are organic catalysts. A catalyst is any substance that affects the rate of a chemical reaction without itself being changed. Enzymes are crucial to living things because all living systems must have continuously controlled chemical activity. Enzymes regulate metabolism by speeding up certain chemical reactions. They affect the reaction rate by decreasing the activation energy.
Enzymes are proteins, and thus, thousands of different enzymes can conceivably be formed. Many enzymes are conjugated proteins (proteins that consist of amino acids attached to other groups via covalent bonds) and have a nonprotein coenzyme. In these cases, both components must be present for the enzyme to function.
Enzymes are very selective; they may catalyze only one reaction or one specific class of closely related reactions. The molecule upon which an enzyme acts is called the substrate. There is an area on each enzyme to which the substrate binds called the active site. The following characteristics are true for all enzymes:
Most enzyme-catalyzed reactions are reversible. The product synthesized by an enzyme can be decomposed by the same enzyme. An enzyme that synthesizes maltose from glucose can also hydrolyze maltose back to glucose. The two models that follow describe the binding of the enzyme to the substrate.
This theory holds that the spatial structure of an enzyme’s active site is exactly complementary to the spatial structure of its substrate. The two fit together like a lock and key. In other words, receptors are large proteins that contain a recognition site (lock) that is directly linked to transduction systems. When a drug or endogenous substance (key) binds to the receptor, a sequence of events is started. Although this theory has been largely discounted, it is still frequently used as a teaching tool when explaining drug interactions with receptors and enzymes.
This more widely accepted theory describes the active site as having flexibility of shape. When the appropriate substrate comes in contact with the active site, the conformation of the active site changes to fit the substrate.
Enzyme action and the reaction rate depend on several environmental factors including temperature, pH, and the concentration of enzyme and substrate.
In general, as the temperature increases, the rate of enzyme action increases until an optimum temperature is reached (usually around 40°C). Beyond optimal temperature, heat alters the shape of the active site of the enzyme molecule and deactivates it, leading to a rapid drop in rate.
For each enzyme there is an optimal pH; above and below that, enzymatic activity declines. Maximal activity of many human enzymes occurs around pH 7.2, which is the pH of most body fluids. Exceptions include pepsin, which works best in the highly acidic conditions of the stomach (pH = 2), and pancreatic enzymes, which work optimally in the alkaline conditions of the small intestine (pH = 8.5). In most cases the optimal pH matches the conditions under which the enzyme operates.
The concentrations of substrate and enzyme greatly affect the reaction rate. When the concentrations of both enzyme and substrate are low, many of the active sites on the enzyme are unoccupied, and the reaction rate is low. Increasing the substrate concentration will increase the reaction rate until all of the active sites are occupied. After this point, further increase in substrate concentration will not increase the reaction rate, and the reaction is said to have reached the maximum velocity, Vmax.
The active site of an enzyme is specific for a particular substrate or class of substrates. However, it is possible for molecules that are similar to the substrate to bind to the active site of the enzyme. If a similar molecule is present in a concentration comparable to the concentration of the substrate, it will compete with the substrate for binding sites on the enzyme and interfere with enzyme activity. This is known as competitive inhibition because the enzyme is inhibited by the inactive substrate, or competitor. If sufficient quantities of the substrate are introduced, however, the substrate can outcompete the competitor and will still be able to reach the Vmax; however, this will require much higher concentrations of substrate than would be necessary without the competitor.
A noncompetitive inhibitor is a substance that forms strong covalent bonds with an enzyme, making it unable to bind with its substrate, and consequently a noncompetitive inhibitor cannot be displaced by the addition of excess substrate. Therefore, noncompetitive inhibition is irreversible. Because this inhibition is noncompetitive, addition of excess substrate will not influence the rate of the reaction, and the reaction will never reach Vmax. A noncompetitive inhibitor may be bound at, near, or far from the active site. When the inhibition takes place at a site other then the active site, this is called allosteric inhibition. (Allosteric means “other site” or “other structure.”) The interaction of an inhibitor at an allosteric site changes the structure of the enzyme so that the active site is also changed.
Every reaction in the body is regulated by enzymes. Some of the basic reaction types are listed below.
Hydrolysis reactions function to digest large molecules into smaller components. Lactase hydrolyzes lactose to the monosaccharides glucose and galactose. Proteases degrade proteins to amino acids, and lipases break down lipids to fatty acids and glycerol.
In multicellular organisms, digestion can begin outside of the cells in the gut. Other hydrolytic reactions occur within cells.
Synthesis reactions (including dehydrations) can be catalyzed by the same enzymes as hydrolysis reactions, but the directions of the reactions are reversed.
These reactions occur in different parts of the cell. For example, protein synthesis occurs in the ribosomes and involves dehydration reactions between amino acids.
Synthesis is required for growth, repair, regulation, protection, and production of food reserves such as fat and glycogen by the cell. The survival of an organism depends on its ability to ingest substances that it needs but cannot synthesize. Once ingested, these substances are converted into useful products.
Certain vitamin cofactors and essential amino acids cannot be synthesized by humans. If they are not available in the diet, deficiency diseases will occur.
Many enzymes require the incorporation of a nonprotein molecule to become active. These molecules, called cofactors, can be metal cations such as Zn2+ or Fe2+ or small organic groups called coenzymes. Most coenzymes cannot be synthesized by the body and are obtained from the diet as vitamin derivatives. Cofactors that bind to the enzyme by strong covalent bonds are called prosthetic groups.