A peptide bond is the link that joins one amino acid to the next. It forms between the carboxyl group of the first amino acid and the amino group of the second.
The reaction is called a condensation. The carboxyl group loses an OH and the amino group loses an H. Together these leave as one molecule of water, and a new carbon to nitrogen bond is made. Each new link in a chain releases one water molecule.
The peptide bond has an unusual feature. Electrons are shared across the carbonyl oxygen, the carbon and the nitrogen. Chemists describe this as resonance. As a result, the bond behaves partly like a double bond. It cannot rotate freely, and the six atoms around it lie flat in one plane. In almost all peptide bonds in proteins, the neighbouring alpha carbons sit on opposite sides, called the trans arrangement. Bonds just before a proline are the most common exception.
Because each peptide bond is flat, a chain can only bend at the alpha carbons between them. This limits the shapes a chain can take and helps explain regular structures such as the alpha helix.
The reverse reaction is hydrolysis. Adding water back breaks the bond. In water, the balance actually favours hydrolysis, but without a catalyst the reaction is extremely slow. Strong acid, heat or enzymes called proteases speed it up. This mix of stability and breakability suits a molecule that must last but also be recycled.
References
- Berg JM, Tymoczko JL, Stryer L. Biochemistry, 5th ed., Section 3.2: Primary Structure: Amino Acids Are Linked by Peptide Bonds (2002)
- Alberts B et al. Molecular Biology of the Cell, 4th ed., From RNA to Protein (2002)
Educational content about laboratory research. Not medical advice. All Orion compounds are for laboratory research use only.
