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Importance of Peptide Bonding

Peptide bonding (or amide bonding) is one of the most important reactions in biochemistry, as it is the bond used by amino acids to form proteins. Amino acids form peptide bonds with other amino acids when the amino group of the first amino acid bonds with the carboxyl group of the second amino acid. A chain of amino acids connected by peptide bonds is called a polypeptide chain, while each individual amino acid is referred to as a residue. These chains then fold due to various internal and external forces in order to become proteins.

Partial-Double Bond of Peptide Bond

The stability of the peptide bond is due to the resonance of amides. The double bond resonance form of the peptide bond helps to increase stability and decrease rotation about that bond. The partial double bond character is either strengthened or weakened depending upon the environment that it is in. An example would be a hydrophobic environment where the double-bond form would be highly discouraged since the double-bond form has a positive charge on the nitrogen and a negative charge on the oxygen.

The partial double bond results in the amide group being planar thus causing them to take either the cis or trans conformation. While the proteins exist in their unfolded state, the peptide groups can isomerize at free will and thus often take the form of both conformations. This however, is not true in the folded state where with very rare exceptions, only one conformation is taken up at each position. In peptide bonds, the vast majority take the trans conformation with a cis:trans ratio of around 1:1000.

Amide Bond Resonance

Overall Reaction

The overall reaction involves the reaction of two amino acidswhich is catalyzed by enzymes enzymein the body to produce the two amino acids combined and a water molecule.

Peptide Bond1.jpg

Reaction Mechanism

This reaction to form peptide bonds involves reacting the amine group of one amino acid (the N-terminal) to the carboxylgroup of another amino acid (the C-terminal). A peptide bond is a dehydration reaction, or condensation reaction, meaning it releases a molecule of water through the course of the reaction. The molecule formed by a peptide bond is called an amide.

Peptide Bond Mechanism.jpg

In the presence of water, the peptide bond will break spontaneously; this is called amide hydrolysis. This occurs because the peptide reaction possesses an equilibrium that pushes the reaction towards hydrolysis (heading in the reverse direction), which means the reaction is endothermic, and requires energy to proceed. Although this reaction requires an input of energy, peptide bonds are still stable bonds as the rate of hydrolysis is incredibly slow. Enzymes facilitate the hydrolysis reaction of peptides to form proteins in living organisms.

The hydrolysis reaction is very slow because the bond between the amino group and carboxyl group is stable due to resonance. The lone pair from the nitrogen donates electrons to the carbonyl. The resonance decreases the electrophilicity, and stabilizes the carbonyl forming the peptide bond. Even though peptide bonds are stable, they can still react. The reactions of peptide bonds involve attack at the carbonyl carbon and the formation of a tetrahedral intermediate.


1. Berg, Jeremy M. (2007). Biochemistry, 6th Ed., Sara Tenney. ISBN0-7167-8724-5.



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