Principles of Biochemistry Volume 1 - A. Lehninger 1985

Biomolecules
Water
Peptides are chains of amino acids

Two molecules of the same or different Amino Acids can covalently bind to each other via a substituted amide linkage (see Table 3-4), known as a peptide bond, to form a dipeptide molecule. A peptide bond is formed by the elimination of the elements of Water from the carboxyl group of one Amino Acid and the α-amino group of another under the action of strong condensing agents (Fig. 5-18). Three amino acids can join in a similar fashion via two peptide bonds to form a tripeptide; in the exact same manner, tetrapeptides and pentapeptides can be produced. Linking A large number of amino acids in this way gives rise to a Structure called a polypeptide. Peptides of varying lengths are produced by the partial Hydrolysis of very long polypeptide chains of Proteins, which may contain hundreds of amino acid residues.

Figure 5-19 illustrates The structure of a pentapeptide. The amino acid units that make up a peptide are commonly referred to as residues (since they are no longer complete amino acids, having lost one hydrogen atom from each amino group and both oxygen and hydrogen atoms from each carboxyl group). The amino acid residue at the end of the peptide with a free α-amino group is called the amino-terminal (or N-terminal) residue, whereas the residue at the opposite end bearing a free carboxyl group is called the carboxy-terminal, or C-terminal, residue. Peptide names are derived from the constituent amino acid residues in the order of their sequence, starting from the N-terminal residue, as shown in Fig. 5-19.

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Fig. 5-18. Formation of a dipeptide.

Fig. 5-19. STRUCTURE OF THE pentapeptide serylglycyltyrosylalanylleucine. Peptide names are derived from the amino acid names, starting with the amino-terminal residue. Peptide bonds are shaded, and R groups are highlighted in red.