21.8
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 co…
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.
View the full transcript and gain access to JoVE Core videos
Q1: How does a peptide bond form between amino acids?
A peptide bond forms through a condensation reaction where the carboxyl group of one amino acid links to the amino group of another, releasing a water molecule. This covalent bond creates an amide group, joining the two amino acids together. The reaction is reversible through hydrolysis, which adds water back to break the bond.
Q2: What is the structural difference between cis and trans peptide bonds?
In cis conformation, the alpha carbons of both amino acids are on the same side of the peptide bond, while in trans conformation they are on opposite sides. Peptide bonds predominantly occur in the trans conformation, except when proline contributes its amino group to bond formation, where cis bonds are more common.
Q3: Why does a peptide bond have a rigid planar structure?
The peptide bond exhibits rigidity due to resonance between the carbonyl group's double bond and the carbon-nitrogen single bond. This electron sharing decreases the single bond length and increases the double bond length, preventing rotation around the peptide bond and creating its characteristic planar structure.
Q4: What are N-terminal and C-terminal ends in a peptide chain?
The N-terminal is the end of a peptide chain with a free amino group not involved in peptide bond formation, while the C-terminal has a free carboxyl group. Amino acid sequences always begin at the N-terminal and end at the C-terminal, establishing the directional orientation of the polypeptide chain.
Q5: How are peptides classified based on the number of amino acids?
Peptides are classified by chain length: dipeptides contain two amino acids, tripeptides contain three, and tetrapeptides contain four. When several amino acids link through peptide bonds, they form polypeptides. Polypeptides that fold into specific three-dimensional structures with distinct functions are called proteins.
Q6: How can peptide bonds be broken down?
Peptide bonds break through hydrolysis, which adds a water molecule to cleave the bond. This can occur rapidly using chemical catalysts like acids or enzymes called proteases, or slowly through spontaneous hydrolysis in the presence of water alone. Both mechanisms reverse the condensation reaction.
Q7: What role does resonance play in peptide bond chemistry?
Resonance in peptide bonds involves electron sharing between the carbonyl group's double bond and the carbon-nitrogen single bond. This resonance stabilizes the bond structure, reduces rotation around the peptide linkage, and contributes to the rigid planar geometry essential for protein folding and function.