Proteins are central to all living processes (Figure 15.4). Many proteins are enzymes, the biological catalysts that drive the chemical reactions of the cell; others are structural components, providing scaffolding and support for membranes, filaments, bone, and hair. Some proteins help transport substances; others have a regulatory, communication, or defense function.
All proteins are composed of amino acids, linked end to end. There are 20 common amino acids found in proteins; these amino acids are shown in < Figure 15.5 with both their three- and one-letter abbreviations. (Other amino acids that are sometimes found in proteins are modified forms of the common amino acids.) The 20 common amino acids are similar in structure, differing only in the structures of the R (radical) groups. The amino acids in proteins are joined by peptide bonds (Figure 15.6) to form polypeptide chains, and a protein consists of one or more polypeptide chains. Like nucleic acids, polypeptides have polarity with one end having a free amino group (NH2) and the other end possessing a free carboxyl group (CO2H). Some proteins consist of only a few amino acids, whereas others may have thousands.
Like that of nucleic acids, the molecular structure of proteins has several levels of organization. The primary structure of a protein is its sequence of amino acids (Figure 15.7a). Through interactions between neighboring amino acids, a polypeptide chain folds and twists into a secondary structure (Figure 15.7b); two common secondary structures found in proteins are the beta (p) pleated sheet and the alpha (a) helix. Secondary structures interact and fold further to form a tertiary structure (Figure 15.7c), which is the overall, three-dimensional shape of the protein. The secondary and tertiary structures of a protein are ultimately determined by the primary structure—the amino acid sequence—of the protein. Finally, some proteins consist of two or more polypeptide chains that associate to produce a quaternary structure ( < Figure 15.7d).
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