Peptide Bond vs Disulfide Bond: Comparative Analysis and Facts

In this article “ Peptide Bond vs Disulfide Bond”, the difference and comparison between these two bonds are discussed briefly.

SubjectPeptide BondDisulfide Bond
Connection betweenPeptide bonds connect any two amino acids.Disulfide bonds are the linkage between the thiol groups of cysteine residue in proteins.
FunctionPeptide bonds form the primary structure of protein molecules.Function of this disulfide bond is to stabilize the secondary and tertiary structure of proteins.
Participating atomsNitrogen atom from the amine group of one amino acid forms the peptide linkage with the carbon atom of carboxylic acid group coming from another amino acid.Cysteine, having a -SH group in its side chain participates in the disulfide bond formation.
Formation ReactionPeptide bonds are formed through condensation reaction.Disulfide Bonds are formed through oxidative folding process.
  

The formation of disulfide linkage involves a reaction between the thiol groups come from two cysteine residue, in which S anion can act as nucleophile and attacks the side chain of another cysteine residue to form the disulfide linkage.

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Definition of Disulfide Linkage And Peptide Linkage

Disulfide bonds are formed between the side chain sulfhydryl (SH) groups of cysteine residues through the oxidative folding process. This sulfide linkage is key component of secondary and tertiary structure of protein molecules. This is basically a chemical covalent bond, plays an important role in the heat induced gelation of globular proteins. Disulfide linkage (S-S linkage) helps to bind two peptide chain or different parts of any other peptide chain and becomes the structural determinant of protein.

Peptide bond is a chemical covalent bond which is also known as amide (CONH2) linkage present in protein primary structure. This bond is formed by linking two consecutive alpha (α)amino acid. In this peptide bond formation, amine group (NH2) from one alpha amino acid reacts with the carboxylic acid group (COOH) of another alpha amino acid through the condensation process (elimination of one water molecule).

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Frequently Asked Questions (FAQ)

Can Peptides have Disulfide Bonds?

These disulfide linkages are found in almost all types of extracellular peptides or proteins and it is an integral component of the tertiary structure of protein.

In protein disulphide bridges , that are formed by the coupling of two thiol groups (SH groups) are the backbone of secondary and tertiary structure of protein. This linkage is expressed as “R-S-S-R1”,also familiar as S-S bond.

This linkage usually consist of polar or hydrophilic part and non polar or hydrophilic part. The hydrophilic side chains are often oriented towards the surface, while the non polar or hydrophilic part are mostly pushed inside the protein structure. With the polar groups, accessible on the surface, the proteins are able to link with the other protein or non protein molecules.

Thus disulfide linkage stabilizes the 3D structure of protein and exhibit physiologically appropriate redox activity. This linkages also help to decrease entropic choices that facilitate folding progression towards the improperly folded configuration.

 This bond can govern the basic biological processes in living organisms. The formation of the disulfide bond by the linking between the two side chain consisting S atoms leads to two electron transfer process from a reduced sulfyhydryl groups of cysteins (S-H) to the oxidized cystine group (S-S).

The above reaction is often accelerated by enzyme catalyst, thioredoxin or protein disulfide isomerases.

Disulfide linkages are basically formed intramolecular, but in some cases it may be formed between two visinal cysteines.

Disulfide Bond in Protein.
Image Credit: Wikimedia Commons

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What Peptides Can Form Disulfide Bonds?

The peptides having no sulfyhydryl groups (SH groups) can not participate in the disulfide bond formation.

There are almost 500 amino acids known to the scientists. Among them only twenty amino acids are there which are the building component of any living organisms. In peptide bond formation almost all these twenty amino acids can participate ,but they all can not take part in disulfide bond formation.

The amino acids only having thiol or sulfuhydryl groups can form the disulfide linkage. In this group cysteine is the only amino acid (non essestial amino acid) which has the capability of this bond formation.

 Cysteine is slightly different from the other known amino acids due to its reactive SH group. Thus two cysteine residue can form the disulfide linkage between different parts of the same protein molecules through their oxidisable thiol groups . Even the residues can form this linkage between two separate polypeptide chains also.

Cysteine is very important for collagen and main protein for skin, nails and hair. Cysteine can be mostly found in extracellular domains of membrane proteins and in secretary proteins also.

There is another sulfur containing amino acid , Methionine, can not participate in disulfide linkage formation due to absence of thiol groups .It has methyl group (CH3) attached with sulfur. This attachment makes methionine more hydrophobic, sterically hindered and much less reactive with comparing to cysteine.

 

Structure of Methionine.
Image Credit: Wikimedia Commons

The advantage of cysteine over other sulfur containing amino acid in formation of disulfide linkage is that cysteine can be easily oxidized and form the dimer that contains disulfide linkage between two cysteine residue in a poly peptide chain.

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