Much as an overly twisted rubber band coils back on

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Much as an overly twisted rubber band coils back on itself, the sheets and coils of a protein can fold up even more into compact domains. A “domain” is a part of a protein that is organized as a structurally stable unit. Such units con- stitute a protein’s third level of organization, its tertiary structure 3 . Tertiary structure is what makes a protein a working molecule. As an example, the barrel- shaped domains of some proteins function as tunnels through which ions cross cell membranes. Many proteins also have a fourth level of organization, or quaternary struc- ture: They consist of two or more polypeptide chains bonded together or in close association 4 . Most enzymes and many other proteins are globular, with several polypeptide chains folded into shapes that are roughly spherical. Some proteins aggregate by many thousands into much larger structures, with their polypeptide chains organized into strands or sheets. The keratin in your hair is an example 5 . Some fibrous proteins contribute to the structure and organization of cells and tissues. Others, such as the actin and myosin that form filaments in muscle cells, are part of the mechanisms that help cells and cell parts move. The shape of each protein defines its biological activity. s OH s OH H s methionine CH 2 CH 2 CH 2 H s s OH CH 3 S OH serine CH 2 OH + s methionine methionine serine arginine glutamine CH 2 CH 2 H s CH 3 S serine A B FIGURE 2.17 Animated! Polypeptide forma- tion. Chapter 7 returns to protein synthesis. A Two amino acids (here, methionine and serine) are joined by condensation. A peptide bond forms between the carboxyl group of the methionine and the amine group of the serine. B Peptide bonds join additional amino acids to the carboxyl end of the chain. The resulting polypeptide can be thousands of amino acids long. 61576_02_c02_p018-039.indd 34 61576_02_c02_p018-039.indd 34 11/13/08 9:45:57 AM 11/13/08 9:45:57 AM
Chapter 2 Molecules of Life 35 The Importance of Protein Structure An enzyme speeds up a process, a receptor protein receives an energy signal, hemoglobin molecules in your blood carry oxygen—you and all other organisms consist of and depend on protein function. However, a protein only functions properly if it stays coiled, folded, and packed in a particular way, because the shape of each protein defines its biological activity. That shape depends on many hydrogen bonds and other interactions that heat, shifts in pH, or detergents can disrupt. At such times, proteins denature , which means they unwind and otherwise lose their three- dimensional shape. Once a protein’s shape unravels, so does its function. You can see denaturation in action when you cook an egg. A protein called albumin is a major component of egg white. Heat does not disrupt the covalent bonds of albumin’s primary structure, but it does destroy the weaker hydrogen bonds that maintain the protein’s shape. When a translucent egg white turns opaque, the albumin has been denatured. For very few proteins, denaturation is

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