Uuy

COO-

Amino-terminal end

Carboxyl-terminal end

FIGURE 3-14 The pentapeptide serylglycyltyrosylalanylleucine, or Ser-Gly-Tyr-Ala-Leu. Peptides are named beginning with the amino-terminal residue, which by convention is placed at the left. The peptide bonds are shaded in yellow; the R groups are in red.

at the other end, which has a free carboxyl group, is the carboxyl-terminal (C-terminal) residue.

Although hydrolysis of a peptide bond is an exer-gonic reaction, it occurs slowly because of its high activation energy. As a result, the peptide bonds in proteins are quite stable, with an average half-life (i1/2) of about 7 years under most intracellular conditions.

Peptides Can Be Distinguished by Their Ionization Behavior

Peptides contain only one free a-amino group and one free a-carboxyl group, at opposite ends of the chain (Fig. 3-15). These groups ionize as they do in free amino acids, although the ionization constants are different because an oppositely charged group is no longer linked to the a carbon. The a-amino and a-carboxyl groups of all nonterminal amino acids are covalently joined in the peptide bonds, which do not ionize and thus do not contribute to the total acid-base behavior of peptides. How

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