Peptide Bond
The amino acids are held together in a protein by covalent peptide bonds or linkages. These bonds are rather strong and serve as the cementing material between the individual amino acids (considered as bricks). Some examples of Peptide bonds are Glutathione, Insulin, Hemoglobin, Glycine-alanine (Gly-Ala), Isoleucine-aspartic acid (Ile-Asp), Carnosine (Ala-His).
Formation of a peptide bond
When the amino group of an amino acid combines with the carboxyl group of another amino acid, a peptide bond is formed. Note that a dipeptide will have two amino acids and one peptide (not two) bond. Peptides containing more than 10 amino acids (decapeptide) are referred to as polypeptides.
Different Forms of Peptide Bond
Dipeptide = contains 2 amino acid units.
Tripeptide = contains 3 amino acid units.
Tetrapeptide = contains 4 amino acid units.
Oligopeptide = contains not more than 10 amino acid units.
Polypeptide = contains more than 10 amino acid units, up to 100 residues.
Macropeptides = made up of more than 100 amino acids.
Characteristics of peptide bonds
The peptide bond is rigid and planar with partial double bond in character. It generally exists in trans configuration. Both >C=O and -NH groups of peptide bonds are polar and are involved in hydrogen bond formation. The cis configuration usually occurs when proline contributes its amino group to the formation of the bond; however, only around 10% of prolines are preceded by cis bonds.
Rotation is not possible around peptide bonds due to his rigid structure. However, rotation can occur around the bonds that link the alpha carbon to the nitrogen and carbon atoms, respectively.
Writing of peptide structures
Conventionally, the peptide chains are written with the free amino end (N-terminal residue) at the left, and the free carboxyl end (C-terminal residue) at the right. The amino acid sequence is read from N-terminal end to C-terminal end. Incidentally, the protein biosynthesis also starts from the N-terminal amino acid.
Shorthand to read peptides
The amino acids in a peptide or protein are represented by the 3-letter or one letter abbreviation. This is the chemical shorthand to write proteins.
Naming of peptides
For naming peptides, the amino acid suffixes -ine (glycine), -an (tryptophan), -ate (glutamate) are changed to -yl with the exception of C-terminal amino acid. Thus a tripeptide composed of an Nterminal glutamate, a cysteine and a C-terminal glycine is called glutamyl-cysteinyl-glycine.
Degradation of Peptide Bond
The degradation of a peptide bond involves the breaking of the covalent bond that links two amino acids within a protein or peptide molecule. There are two main processes by which peptide bonds can be degraded:
- Hydrolysis
- Chemical Methods
Source: Biochemistry by Dr. U. Satyanarayana and Dr. U. Chakrapani
