Unit two. The Living Cell
5. Energy and Life
5.4. How Enzymes Work
Enzymes, which can be made of proteins or nucleic acids, are the catalysts used by cells to touch off particular chemical reactions. By controlling which enzymes are present, and when they are active, cells are able to control what happens within themselves, just as a conductor controls the music an orchestra produces by dictating which instruments play when.
An enzyme works by binding to a specific molecule and stressing the bonds of that molecule in such a way as to make a particular reaction more likely. The key to this activity is the shape of the enzyme. An enzyme is specific for a particular reactant, or substrate, because the enzyme surface provides a mold that very closely fits the shape of the desired reactant. For example, the blue-colored lysozyme enzyme in figure 5.5 is contoured to fit a specific sugar molecule (the yellow reactant). Other molecules that fit less perfectly simply don’t adhere to the enzyme’s surface. The site on the enzyme surface where the reactant fits is called the active site (panel 1 below). The site on the reactant that binds to an enzyme is called the binding site. Enzymes are not rigid. The binding of the reactant induces the enzyme to change its shape slightly. In figure 5.5b and in panel 2 of the Key Biological Process illustration below, the edges of the enzyme now hug the reactant(s), leading to an “induced fit” between the enzyme and its reactant, like a hand wrapping around a baseball.
Figure 5.5. Enzyme shape determines its activity.
(a) A groove runs through the lysozyme enzyme (blue in this diagram) that fits the shape of the reactant (in this case, a chain of sugars). (b) When such a chain of sugars, indicated in yellow, slides into the groove, it induces the protein to change its shape slightly and embrace the substrate more intimately. This induced fit causes a chemical bond between two sugar molecules within the chain to break.
An enzyme lowers the activation energy of a particular reaction. In the case of lysozyme, an enzyme found in human tears, the enzyme has an antibacterial function, encouraging the breaking of a particular chemical bond in molecules that make up the cell wall of bacteria (figure 5.5). The enzyme weakens the bond by drawing away some of its electrons. Alternatively, an enzyme may encourage the formation of a link between two reactants, like the blue and red colored molecules in panel 2 below by holding them near each other. Regardless of the type of reaction, the enzyme is not affected by the chemical reaction and is available to be used again.
Every organism contains thousands of different kinds of enzymes that together catalyze a bewildering variety of reactions. Often several of these reactions occur in a fixed sequence called a biochemical pathway, the product of one reaction becoming the substrate for the next. You can see in the biochemical pathway shown in figure 5.6 how the initial substrate is altered by enzyme 1 so that it now fits into the active site of another enzyme, becoming the substrate for enzyme 2, and so on until the final product is produced. Because these reactions occur in sequence, the enzymes involved are often positioned near each other in the cell. For example, the enzymes involved in this biochemical pathway are all embedded in a membrane near each other. Many biochemical pathways occur in membranes, although enzyme assemblies also occur in organelles and within the cytoplasm. The close proximity of the enzymes allows the reactions of the biochemical pathway to proceed faster. Biochemical pathways are the organizational units of metabolism. We will discuss them more in chapters 6 and 7.
Figure 5.6. A biochemical pathway.
The original substrate is acted on by enzyme 1, changing the substrate to a new form recognized by enzyme 2. Each enzyme in the pathway acts on the product of the previous stage.
Factors Affecting Enzyme Activity
Temperature and pH can have a major influence on the action of enzymes. Enzyme activity is affected by any change in condition that alters the enzyme’s three-dimensional shape.
Temperature. When the temperature increases, the bonds that determine enzyme shape are too weak to hold the enzyme’s peptide chains in the proper position, and the enzyme denatures. As a result, enzymes function best within an optimum temperature range, which is relatively narrow for most human enzymes. In the human body, enzymes work best at temperatures near the normal body temperature of 37°C, as shown by the brown curve in figure 5.7a. Also notice that the rates of enzyme reactions tend to drop quickly at higher temperatures, when the enzyme begins to unfold. This is why an extremely high fever in humans can be fatal. However, the shapes of the enzymes found in hotsprings bacteria (the red curve) are more stable, allowing the enzymes to function at much higher temperatures. This allows the bacteria to live in water that is near 70°C.
pH. In addition, most enzymes also function within an optimal pH range, because the shape-determining polar interactions of enzymes are quite sensitive to hydrogen ion (H+) concentration. Most human enzymes, such as the protein-degrading enzyme trypsin (the dark blue curve in figure 5.7b) work best within the range of pH 6 to 8. Blood has a pH of 7.4. However, some enzymes, such as the digestive enzyme pepsin (the light blue curve) are able to function in very acidic environments such as the stomach, but can’t function at higher pHs.
Figure 5.7. Enzymes are sensitive to their environment.
The activity of an enzyme is influenced by both (a) temperature and (b) pH. Most human enzymes work best at temperatures of about 40°C and within a pH range of 6 to 8.
Key Learning Outcome 5.4. Enzymes catalyze chemical reactions within cells and can be organized into biochemical pathways. Enzymes are sensitive to temperature and pH because both of these variables influence enzyme shape.