In contrast with antibodies, which bind and simply present their ligands to other components of the immune system, enzymes promote the chemical alteration of their ligands, called substrates. Almost every chemical reaction in the cell is catalyzed by a specific enzyme. Like all catalysts, enzymes do not affect the extent of a reaction, which is determined by the change in free energy AG between reactants and products (Chapter 2). For reactions that are energetically favorable (—AG), enzymes increase the reaction rate by lowering the activation energy (Figure 3-16). In the test tube, catalysts such as charcoal and platinum facilitate reactions but usually only at high temperatures or pressures, at extremes of high or low pH, or in organic solvents. As the cell's protein catalysts, however, enzymes must function effectively in aqueous environment at 37°C, 1 atmosphere pressure, and pH 6.5-7.5.
Two striking properties of enzymes enable them to function as catalysts under the mild conditions present in cells: their enormous catalytic power and their high degree of specificity. The immense catalytic power of enzymes causes the rates of enzymatically catalyzed reactions to be 106-1012 times that of the corresponding uncatalyzed reactions under otherwise similar conditions. The exquisite specificity of enzymes—their ability to act selectively on one substrate or a small number of chemically similar substrates—is exemplified by the enzymes that act on amino acids. As noted in Chapter 2, amino acids can exist as two stereoisomers, designated l and d, although only l isomers are normally found in biological systems. Not surprisingly, enzyme-catalyzed reactions of l-amino acids take place much more rapidly than do those of d-amino acids, even though both stereoisomers of a given amino acid are the same size and possess the same R groups (see Figure 2-12).
Approximately 3700 different types of enzymes, each of which catalyzes a single chemical reaction or set of closely related reactions, have been classified in the enzyme database. Certain enzymes are found in the majority of cells because they catalyze the synthesis of common cellular products (e.g., proteins, nucleic acids, and phospholipids) or take part in the
▲ FIGURE 3-16 Effect of a catalyst on the activation energy of a chemical reaction. This hypothetical reaction pathway depicts the changes in free energy G as a reaction proceeds. A reaction will take place spontaneously only if the total G of the products is less than that of the reactants (-AG). However, all chemical reactions proceed through one or more high-energy transition states, and the rate of a reaction is inversely proportional to the activation energy (AG*), which is the difference in free energy between the reactants and the highest point along the pathway. Enzymes and other catalysts accelerate the rate of a reaction by reducing the free energy of the transition state and thus AG*.
production of energy by the conversion of glucose and oxygen into carbon dioxide and water. Other enzymes are present only in a particular type of cell because they catalyze chemical reactions unique to that cell type (e.g., the enzymes that convert tyrosine into dopamine, a neurotransmitter, in nerve cells). Although most enzymes are located within cells, some are secreted and function in extracellular sites such as the blood, the lumen of the digestive tract, or even outside the organism.
The catalytic activity of some enzymes is critical to cellular processes other than the synthesis or degradation of molecules. For instance, many regulatory proteins and intracellular signaling proteins catalyze the phosphorylation of proteins, and some transport proteins catalyze the hydrolysis of ATP coupled to the movement of molecules across membranes.
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