It is a covalent bond formed between two amino acids.
Living organisms use peptide bonds to form long chains of amino acids , known as proteins.
Proteins are used in many roles, including structural support, catalyzing important reactions, and recognizing molecules in the environment.
Therefore, a peptide bond is the basis of most biological reactions. The formation of peptide bonds is a requirement for all life, and the process is very similar in all forms of life.
Formation of peptide bonds
At the molecular level, a peptide bond is formed through a dehydration reaction . Two amino acids can bond with each other when two hydrogens and one oxygen are removed from the molecules.
An amino acid has a carboxyl group for the reaction, and loses a hydroxyl group in the reaction. The amino group of the other amino acid loses a hydrogen.
Nitrogen then substitutes in place of the hydroxyl group, forming a peptide bond. This is the reason why peptide bonds are also known as substituted amide bonds.
The two amino acids are now known as residues, since they have lost several atoms and are now covalently bonded to each other.
The carbon-nitrogen bond formed in a peptide bond is different from the carbon-nitrogen bonds in other parts of the molecule.
The oxygen on the carboxyl side of the bond is slightly negative in charge. Nitrogen retains a slightly positive charge.
This interaction causes carbon and nitrogen to share more constituents than they normally would, and an electric dipole is established.
The extra electrons make the bond act like a double bond, which is rigid and cannot rotate.
This 6-molecule unit is known as the peptide group and is often represented as a ball or plane. The carbons in the center of each amino acid have 4 equal bonds, and can rotate freely.
Therefore, when many amino acids join together, they form chains of rigid planes of atoms around the peptide bond, connected by flexible carbon bonds.
This allows a peptide chain to twist and bend, leading to advanced formations that can catalyze reactions.
While scientists have discovered how to connect a chain of several amino acids, a typical protein has thousands of residues connected in series.
Furthermore, the reaction favors individual amino acids and requires considerable activation energy. Therefore, creating protein without enzymes is not easy.
To do this efficiently, cells have developed a mechanism to build new proteins.
In the genome of each organism, there are codons that describe different amino acids. The genome carries the exact sequence of these amino acids, which together will produce a functional protein.
First, the information must be copied into a messenger RNA (mRNA) molecule. The transfer RNAs (tRNAs) are then bound to specific amino acids.
These tRNAs correspond to different codons of mRNA, which in turn correspond to different codons of DNA.
The actual peptide bond is formed in a special protein macrostructure known as the ribosome.
The ribosome is a very large and complex cellular structure that consists of proteins, RNA, and various other components that help catalyze the formation of a peptide bond.
This is known as the elongation stage of protein synthesis. The ribosome helps bind tRNA to the corresponding mRNA.
In turn, RNA changes shape slightly, catalyzing the reaction between two amino acids and expelling a molecule of water. The chain exits the ribosome.
The ribosome, being a large protein itself, changes shape after the reaction occurs, moving further down the mRNA chain, starting the process again.
Finally, a codon is found that signals the end of the protein, allowing the ribosome to know that the entire protein has been created. At this point, the mRNA and the new protein will be expelled, and a new mRNA will be taken up, creating a completely different protein.
All life is based on bonds between approximately 20 different amino acids, which all organisms use and modify for their own purposes.
The number of different combinations is unlimited, whereas peptide groups in proteins form peptide backbones in all proteins.
The different groups attached to each amino acid cause the molecule to fold and bend into complicated structures, due to weak interactions between the molecules of different groups.
Therefore, across the many millions of proteins created by different species, there are several very similar structures that correspond to similar amino acid sequences.
Because amino acids are connected in a series in a similar direction, scientists typically draw and identify proteins starting from the amino or nitrogen side, and traversing the carboxyl terminal as the end point.