8885ac05157-1898/12/038:55 AM Page157mac78mac78:385p chapter PROTEIN FUNCTION 5.1 Reversible Binding of a Protein to a Ligand: proteins interact with other molecules and how their in- Oxygen-Binding Proteins 158 teractions are related to dynamic protein structure. The 5.2 Complementary Interactions between Proteins importance of molecular interactions to a proteins func and Ligands: The Immune System and tion can hardly be overemphasized. In Chapter 4, we saw Immunoglobulins 174 that the function of fibrous proteins as structural ele- 5.3 Protein Interactions Modulated by Chemical Energy ments of cells and tissues depends on stable, long-term Actin, Myosin, and Molecular Motors 182 quaternary interactions between identical polypeptide chains. As we shall see in this chapter, the functions of many other proteins involve interactions with a variety have occasionally seen in almost dried blood, placed of different molecules. Most of these interactions are between glass plates in a desiccator, rectangula fleeting, though they may be the basis of complex phys- iological processes such as oxygen transport, immune crystalline structures, which under the microscope had function, and muscle contraction-the topics we exam- sharp edges and were bright red ine in detail in this chapter. The proteins that carry out -Friedrich Ludwig Hunefeld, Der Chemismus in these processes illustrate the following key principles of der thierischen Organisation, 1840 protein function, some of which will be familiar from the (one of the first observations of hemoglobin) chapte The functions of many proteins involve the Since the proteins participate in one way or another in all reversible binding of other molecules. A molecule chemical processes in the living organism, one may bound reversibly by a protein is called a ligand expect highly significant information for biological A ligand may be any kind of molecule, including chemistry from the elucidation of their structure and their another protein. The transient nature of protein- transformations ligand interactions is critical to life, allowing an -Emil Fischer article in berichte der deutschen organism to respond rapidly and reversibly to changing environmental and metabolic chemischen gesellschaft zu berlin, 1906 A ligand binds at a site on the protein called the binding site, which is complementary to the nowing the three-dimensional structure of a protein ligand in size, shape, charge, and hydrophobic or an important part of understanding how the pro- hydrophilic character. Furthermore, the interaction tein functions However the structure shown in two di is specific: the protein can discriminate among the mensions on a page is deceptively static. Proteins are thousands of different molecules in its environment dynamic molecules whose functions almost invariably and selectively bind only one or a few. a given depend on interactions with other molecules, and these protein may have separate binding sites for several interactions are affected in physiologically important different ligands. These specific molecular ways by sometimes subtle, sometimes striking changes interactions are crucial in maintaining the high in protein conformation. In this chapter, we explore how degree of order in a living system. (This discussionchapter PROTEIN FUNCTION 5.1 Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins 158 5.2 Complementary Interactions between Proteins and Ligands: The Immune System and Immunoglobulins 174 5.3 Protein Interactions Modulated by Chemical Energy: Actin, Myosin, and Molecular Motors 182 I have occasionally seen in almost dried blood, placed between glass plates in a desiccator, rectangular crystalline structures, which under the microscope had sharp edges and were bright red. —Friedrich Ludwig Hünefeld, Der Chemismus in der thierischen Organisation, 1840 (one of the first observations of hemoglobin) Since the proteins participate in one way or another in all chemical processes in the living organism, one may expect highly significant information for biological chemistry from the elucidation of their structure and their transformations. —Emil Fischer, article in Berichte der deutschen chemischen Gesellschaft zu Berlin, 1906 5 Knowing the three-dimensional structure of a protein is an important part of understanding how the pro￾tein functions. However, the structure shown in two di￾mensions on a page is deceptively static. Proteins are dynamic molecules whose functions almost invariably depend on interactions with other molecules, and these interactions are affected in physiologically important ways by sometimes subtle, sometimes striking changes in protein conformation. In this chapter, we explore how proteins interact with other molecules and how their in￾teractions are related to dynamic protein structure. The importance of molecular interactions to a protein’s func￾tion can hardly be overemphasized. In Chapter 4, we saw that the function of fibrous proteins as structural ele￾ments of cells and tissues depends on stable, long-term quaternary interactions between identical polypeptide chains. As we shall see in this chapter, the functions of many other proteins involve interactions with a variety of different molecules. Most of these interactions are fleeting, though they may be the basis of complex phys￾iological processes such as oxygen transport, immune function, and muscle contraction—the topics we exam￾ine in detail in this chapter. The proteins that carry out these processes illustrate the following key principles of protein function, some of which will be familiar from the previous chapter: The functions of many proteins involve the reversible binding of other molecules. A molecule bound reversibly by a protein is called a ligand. A ligand may be any kind of molecule, including another protein. The transient nature of protein￾ligand interactions is critical to life, allowing an organism to respond rapidly and reversibly to changing environmental and metabolic circumstances. A ligand binds at a site on the protein called the binding site, which is complementary to the ligand in size, shape, charge, and hydrophobic or hydrophilic character. Furthermore, the interaction is specific: the protein can discriminate among the thousands of different molecules in its environment and selectively bind only one or a few. A given protein may have separate binding sites for several different ligands. These specific molecular interactions are crucial in maintaining the high degree of order in a living system. (This discussion 157 8885d_c05_157-189 8/12/03 8:55 AM Page 157 mac78 mac78:385_REB: