Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 167

Unlike DNA and RNA, which consist of a chain of nucleotides held together by phosphodiester bonds between the sugars and phosphates, proteins consist of chains of 20 different amino acids held together by peptide bonds (see Figure 2.19). The peptide bond is formed by joining the amino group (NH₂) of one amino acid to the carboxyl group (COOH) of the previous amino acid. These amino acids in turn are attached to other amino acids by the same type of bond, making a chain. A short chain of amino acids is called an oligopeptide, and a long chain is called a polypeptide.

Figure 2.19 Two amino acids joined by a peptide bond. The bond connects the amino group on the second amino acid to the carboxyl group on the preceding amino acid. R is the side group of the amino acid that differs in each type of amino acid.

Like RNA and DNA, polypeptide chains have directionality and a way to distinguish the ends of the chain from each other. In polypeptides, the direction is defined by their amino and carboxyl groups. One end of the chain, the amino terminus, or N terminus, has an unattached amino group. The amino acid at this end is called the N-terminal amino acid. On the other end of the polypeptide, the final carboxyl group is called the carboxy terminus, or C terminus, and the amino acid is called the C-terminal amino acid. As we shall see, proteins are synthesized from the N terminus to the C terminus.

Protein structure terminology is the same as that for RNA structures. Proteins have primary, secondary, and tertiary structures, as well as quaternary structures. All of these are shown in Figure 2.20.