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.

Amino acids covalently bond to each other through peptide bonds. A peptide bond links the carboxyl group of one amino acid to the amino group of the other. Peptides are small chains of amino acids ranging from two to fifty monomers.

A peptide bond forms through a condensation reaction that releases a water molecule. When an amino group and a carboxyl group link together, it results in the formation of an amide group. The peptide bond has a rigid planar structure and shows some characteristics of a double bond. This is because the double bond on the carbonyl can act as a resonance structure with the carbon-nitrogen peptide bond. 

This resonance leads to the decrease in the length of the single bond, an increase in the length of the double bond, and inhibits rotation of the groups around the peptide bond.

Peptide bonds can exist in cis and trans conformations. In the cis conformation, the alpha carbons are on the same side of the peptide bond, and in the trans conformation, they are on opposite sides of the bond. Peptide bonds mostly occur in the trans conformation except when proline contributes its amino group to bond formation.

Amino acids linked through peptide bonds have an N-terminal and a C-terminal. The N-terminal is the end where the amino group is not involved in the formation of a peptide bond, and the C-terminal is the end where the carboxyl group is not involved in the formation of a peptide bond.

The sequence of amino acids in a peptide or protein always starts with the N-terminal and ends with the C-terminal. 

When two amino acids link together through a peptide bond, they are known as dipeptides.

When three and four amino acids link together through peptide bonds, they are known as tripeptides and tetrapeptides, respectively. And when several amino acids link together through peptide bonds, they are known as polypeptides.

Peptide bonds can be broken down rapidly through hydrolysis using chemical catalysts, such as acids or enzymes known as proteases. Breaking of peptide bonds involves the addition of a water molecule.

Alternatively, these bonds can break spontaneously through a slow process in the presence of water.