21.7: Peptide Bonds
A peptide bond covalently attaches amino acids through a dehydration reaction. One amino acid's carboxyl group and another amino acid's amino group combine, releasing a water molecule. The resulting bond is the peptide bond. The products that such linkages form are peptides. As more amino acids join this growing chain, the resulting chain is a polypeptide. Each polypeptide has a free amino group at one end. This end has the N-terminal, or the amino-terminal, and the other end has a free carboxyl group—the C- or carboxyl-terminal. While the terms polypeptide and protein are sometimes used interchangeably, a polypeptide is technically a polymer of amino acids, whereas the term protein is used for a polypeptide or polypeptides that have combined together and often have bound non-peptide prosthetic groups, a distinct shape, and a unique function.
Chemistry of Peptide Bonds
A peptide bond has a rigid planar structure due to resonance. This resonance involves the sharing of electrons between the double bonds present in the carbonyl group and the peptide bond between carbon and nitrogen, which is a single bond. This results in the length of the double bond increasing and the length of the single bond decreasing from the expected length in the absence of resonance.
Peptide bonds can occur in two possible conformations: cis and trans. In the cis configuration, the alpha carbons of both the amino acids connected by the peptide bond are on the same side of the bond; and in the trans configuration, the alpha carbons of the two amino acids connected by the peptide bond are on the opposite sides of the bond. 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.
This text has been adapted from Openstax, Biology 2e, Chapter 3.4: Proteins.