Molecular Biology: Protein Structure and Functions - Stepanov V.M. 2005

Peptides

When the carboxyl group of one α-amino acid acylates the amino group of another, the resulting amide bond is referred to as a peptide bond, and the compound itself is called a peptide:

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Thus, Peptides are compounds constructed from α-amino acid residues linked together by peptide bonds. For relatively long peptides, the term Polypeptides is sometimes used. It should be noted that other amide bonds can also form involving Amino Acids—for instance, between the γ-carboxyl group of a glutamic acid residue and the ε-amino group of Lysine during fibrin chain cross-linking as a Blood clot ages. Although chemically similar to peptide bonds, such bonds are not classified as true peptide bonds and are sometimes referred to as isopeptide bonds.

The formation of peptide bonds in an aqueous environment is thermodynamically unfavorable, which necessitates the preliminary activation of the interacting groups (most commonly the carboxyl group) during both chemical Peptide Synthesis and biological Biosynthesis. However, kinetically, the peptide bond is quite stable, and its hydrolytic Cleavage occurs only in the presence of chemical catalysts (acids or bases) or through catalysis by specific Enzymes (peptidases).

Naturally, a dipeptide formed by the Condensation of two amino acid residues retains both a carboxyl and an amino group. Consequently, the attachment of further amino acid residues can proceed from both ends, enabling the formation of very long peptide chains consisting of several hundred or even thousands of amino acid residues. Within a peptide chain, a distinction is made between the N-terminal (or amino-terminal) residue, which possesses a free amino group, and the C-terminal residue, which bears a free α-carboxyl group.