Peptide Bond

Peptide bond: 

The scientist Emil Fischer, known as the father of peptides, discovered the peptide bond. Word “Peptide” is a Greek word meaning “digested.”

Peptides are amides which are formed by the elimination of a water molecule by the condensation of two or more similar or different alpha-amino acids. When two or more than two amino acids undergo condensation, a water molecule is eliminated by which a (-CONH-) peptide bond is formed. In this reaction, due to the reactions between the hydroxyl group of the carboxylic group and the hydrogen atom of the amino group, a water molecule is eliminated. This way, the amide (-CONH-) bond formed by the mutual combination of two amino acids is known as peptide bond and the product formed by mutual condensation of amino acids is known as peptide.

Formation of Peptide bonds:

In the cellular phase, dehydration reactions lead to peptide bonds. Two amino acids normally combine when two oxygen and hydrogen are released from a molecule. In this reaction, the carboxyl group is supplied by them while the hydroxyl group is destroyed. Amino groups from different amino acids lose hydrogen. Next, nitrogen plays a role in the replacement of the hydroxyl group, thereby forming a peptide bond. It is for this reason only that peptide bonds are also called amide substitutes. During peptide bond formation, the amino acids used are called residues after reaction because they lose several atoms and bind together after peptide bond formation.

Oxygen on the side of the carboxyl bond has a negative charge while nitrogen has a positive charge. Both the negative and positive charge charges are small, and it is this interaction that leads to the sharing of nitrogen and carbon by more selectors compared to what they share. The whole process leads to the establishment of an electric dipole.

A number of additional electrons have a double-acting bond that is strong and rotating. A unit comprising a total of 6 molecules is called a peptide set that comes in the form of a flat plane or a ball. Carbohydrates found among amino acids have a total of 4 equal bonds and the ability to rotate freely. Thus, when several amino acids combine together, a series of uncompromising atomic plane atoms is formed around the peptide bond. Flexible carbon bonds properly attach this bond. It is this whole arrangement that allows the peptide chains to bend and rotate, resulting in a highly sophisticated creation capable of reacting.

Classifications of Peptide Bond

On the basis of the number of molecules participating in the condensation process, peptide bond are classified into the following groups:

1. Dipeptide: Dipeptide bond is formed by Condensation between two amino molecules joined by peptide bonds. Amino acids are joined by the dehydration process.

 In this method the removal of water molecules occurs for synthesis of dipeptide bond. Amino acids can be the same or different. The examples of dipeptide bond are  Aspartame (Asparagine-phenylalanine), Carnosine ( β-alanyl-L-histidine), Anserine (β-alanyl-N-methylhistidine) and so on.

2. Tripeptide: In this the condensation takes place between three amino molecules with the elimination of water. The different examples of tripeptide are: Glutathione ( Glutamyl-cystinyl-glycine),  Opthalmic acid (L-γ-Glutamyl-α-L-amino butyrl-glycine)
3. Polypeptide: Condensation takes place between more than three molecules with the elimination of water.

Peptide molecules contain -CO-NH-linkage, which is known as peptide linkage.

H2N-CHR-COOH + H2N-CHR-COOH →  H2N-CHR-CO-NH-CHR-COOH (peptide bond) 

In the structure of the peptide molecule, if there is an amino group at one end and the carboxylic group at the other end, it is the N-terminal end and in the C-terminal end, N-terminal end is towards the left and the C-terminal end is written towards the right.

In polypeptide and protein amino acid is written in brief like -Tripeptide which contains Glycine, Alanine and Serine, and is written as follows:

H2N-CH2-CO-NH-C2H4-CO-NH-C2H4OH-CO-OH 

(Gly-Ala-Ser G-A-S)

Polypeptides are also commonly known as only peptides. For the formation of polypeptide, it is not necessary that only one type of amino acid may take part. Various amino acids mutually combine by peptide bonds to form polypeptide chains. These chains join to form protein. Polypeptides which are made up of more than 100 amino acids and its molecular mass of more than 10,000 are known as proteins. Thus, proteins are polypeptides formed by the condensation of various amino acids which contain more than 100 amino acid units and their molecular mass is extremely high (10,000 u or more)

Proteins can be represented by the following general formula:

Protein = aa₁- aa2 – aa3 – aa4……… where aa1, aa2,aa3,aa4…… etc. represent various amino acids and ‘_’ represent peptide bond. 

Some peptides of biological importance are as follows:

1. Oxytocin: It is a non-peptide. It is a hormone secreted by the posterior lobe. The function of this peptide is to contract the uterus after childbirth and the secretion of hormones in mammals.

Cys- Tyr- Ile -Asn -Cys -Pro-Leu – Glu

2. Vasopressin: It is also a non-peptide. This peptide is also secreted by the posterior lobe of the pituitary gland. Its main function is to help the kidneys to absorb water. 

Cys- Tyr- Phe -Gln -Asn-Cys-Pro- Arg -Gly

3. Angiotensin II: It is an octopeptide. It is found in people with high blood pressure.

Properties of peptide bond:

The amino acid involved in the peptide series loses one H (H2N) and one H(of its COOH), or just one of both in the event of a fatal amino acid. This is called an amino acid “buildup”; assigned by adding “yl” to the word base (examples: tyrosyl, seryl, etc.)

The amino acid end of the N-terminal is shown first and later, following others in their application for progression, all with the postfix “yl”; only the amino acid at the end of the C-terminal is given its unchanged name.

Peptide bond characteristics:

According to Linus Pauling and Robert Corey, peptide bonds are good and organised. The characteristics of a peptide bond are:

• Solid Peptide bonds have double bonds:
• They are not compromised by the warmth or concentration of too much salt.
• They may be broken by exposing them to a solid food or a long base at elevated temperatures.
• Peptide bonds do not change and bonds arranged in these lines balance the protein composition.
• The peptide bond consists of a collection of incomplete positive charge  as well as a partial negative charge collection.

Conclusion

The peptide bond is formed between two organic molecules. Peptide binding is also linked to proteins. When a carboxyl group of one molecule joins an amino group of other molecules a chemical bond is formed between them through the release of water molecules.

 Peptides are the amides and the bonds between these amides are called peptide bonds. These are formed by the dehydration synthesis or eliminations of water molecules. Peptide bond are of three types:-

1. Dipeptide 
2. Tripeptide
3. Polypeptidpe

So the polypeptide part is very important by which protein will from by combination or bonding of many types of bond.