Proteins, the workhorses of the cell, are the most abundant and functionally versatile of the cellular macromolecules. Cells string together 20 different amino acids in linear chains, each with a defined sequence, to form proteins (see Figure 2-14), which commonly range in length from 100 to 1000 amino acids. During or just after its polymerization, a linear chain of amino acids folds into a complex shape, conferring a distinctive three-dimensional structure and function on the protein (Figure 1-7). Humans obtain amino acids either by synthesizing them from other molecules or by breaking down proteins that we eat.
FIGURE 1-7 Models of some representative proteins drawn to a common scale and compared with a small portion of a lipid bilayer, a DNA molecule, and an RNA molecule. Each protein has a defined three-dimensional shape held together by numerous chemical bonds. The illustrated proteins include enzymes (glutamine synthetase and adenylate kinase), an antibody (immunoglobulin), a hormone (insulin), and the blood’s oxygen carrier (hemoglobin).
[Glutamine synthetase data from H. S. Gill and D. Eisenberg, 2001, Biochemistry 40:1903–1912, PDB ID 1fpy. Insulin data from E. N. Baker et al., 1988, Phil. Trans. R. Soc. Lond. B Biol. Sci. 319:369–456, PDB ID 4ins. Hemoglobin data from G. Fermi et al., 1984, J. Mol. Biol. 175:159–174, PDB ID 2hhb. Immunoglobulin data from L. J. Harris et al., 1998, J. Mol. Biol. 275:861–872, PDB ID 1igy. Adenylate kinase data from G. Bunkoczi et al., PDB ID 2c9y.]
Proteins have a variety of functions in the cell. Many proteins are enzymes, which accelerate (catalyze) chemical reactions involving small molecules or macromolecules (see Chapter 3). Certain proteins catalyze steps in the synthesis of all proteins; others catalyze synthesis of macromolecules such as DNA and RNA. Cytoskeletal proteins serve as structural components of a cell; for example, by forming an internal skeleton. Other proteins associated with the cytoskeleton power the movement of subcellular structures such as chromosomes, and even of whole cells, by using energy stored in the chemical bonds of ATP (see Chapters 17 and 18). Still other proteins bind adjacent cells together or form parts of the extracellular matrix (see Figure 1-4). Proteins can be sensors that change shape as temperature, ion concentrations, or other properties of the cell change. Many proteins that are embedded in the cell-surface (plasma) membrane import and export a variety of small molecules and ions (see Chapter 11). Some proteins, such as insulin, are hormones; others are hormone receptors that bind their target protein or small molecule and then generate a signal that regulates a specific aspect of cell function. Other important classes of proteins bind to specific segments of DNA, turning genes on or off (see Chapter 9). In fact, much of molecular cell biology consists of studying the function of specific proteins in specific cell types.