Disulfide Bond Structure: Detailed Explanations

Disulfide bond plays a very much important role to form the secondary structure of protein. It is basically a covalent type bond.

Disulfide bond is formed by the oxidation of sulfyhydryl or thiol group (S-H group), present only in Cysteine amino acid (non essential amino acid). It is also known as S-S bond . Disulfide bond is denoted by R-S-S-R1. In which “R” is the amino acid residue.

In this article “disulfide bond structure” basically the type and formation with other detailed facts are explained briefly.

Disulfide Bond Structure And Formation

This chemical bond present in the protein structure.

It is one type of covalent linkage formed between two thiol groups (SH group) present mainly in Cysteine residue. One S-1 coming from one sulfyhydryl group acts as a nucleophile (electron rich) and it attacks another cysteine residue to form the disulfide bond.

 The formation reaction of a disulfide bond is-

R-SH + R1-SH + (1/2) O2 ⇌ R-S-S-R1 + H2O

[R denotes the chain residue of a peptide].

This bond forming Cysteine chain residue may come from the same protein or from distinct that is two different protein.

Formation of disulfide bond leads to two electron transfer from the reduced S-H group to oxidized S-S group.

disulfide bond structure
Disulfide Bond Formation.
Image Credit : Wikimedia Commons

To know more please follow: Is HBr Ionic or Covalent : Why? How, Characteristics and Detailed Facts

Type of Disulfide Bond

Like the peptide bond, disulfide bond plays an important role to stabilize the tertiary structure of protein. Disulfide bond is generally a covalent linkage formed between two sulfyhydryl group and stronger than similar covalent linkage.

This bond is consisting of two parts. One is polar part or hydrophilic part that is oriented towards the outer surface of the protein and helps to participate in various reaction and attract the solvate molecules.

Another part is non polar part or hydrophobic part oriented mostly towards the inner surface of the protein. This characteristic orientation of non polar part towards the inner surface has a physical significance. It takes part in the bond formation with different amino acid.

 Disulfide bonds have their bond dissociation energy approximately 50 Kcal/mol and the S-S bond distance is nearly about 200pm. From this data it can be concluded that disulfide bond is relatively strong and short range bond also.

To know more please follow: Peptide bond formation: How, Why ,Where ,Exhaustive Facts around it

Function of Disulfide Bond

One of the most important function of disulfide bond is to determine the secondary and tertiary structure of protein.

The main function of disulfide bond is that it provides the extra stabilization to the 3D structure of protein molecule (during protein synthesis). Thus it is called as the building block of tertiary protein structure (Peptide bonds stabilize the primary structure of protein).

It also helps in the folding of the single polypeptide chain or inter protein.

Protein gains this extra stability as the entropy of the denatured state of protein is diminished through formation of the disulfide bond.

In living organisms variety of biological and physiological process is governed by disulfide bond. Thirodoxin, an enzyme that accelerated the electron transfer process from reduced S-H group to oxidized S-S group. In most of the cases disulfide bond is formed in intramolecular way but in some of the special cases intermolecular disulfide bond can also be formed.

 Cleavage of disulfide bond can cause a lot of negative impact in a living body as the important biological process will be collapsed and conformation of many important structure will be disrupted and as a result cell growth can be affected.

To know more please check: Peptide Bond vs Phosphodiester Bond: Comparative Analysis And Facts

Cleavage of Disulfide Bond

Breaking of a disulfide bond is not an easy task due to its strong covalent linkage.

Disulfide bond can be cleaved by adding reducing agent. Among them the most common reducing agent used are β- mercaptoethanol or BME and dithiothritol or DTT.

The cleavage reaction of disulfide bond can be accelerated by adding some enzyme, Thioredoxin (TRX) and Glutaredoxin(GRX). These enzymes help to protect the newly incorporated Cysteine residue.

Disulfide bond cannot be broken by applying heat because heat can only break the weak linkages present in the protein structure like the hydrogen bond or nonpolar hydrophobic interactions at its melting temperature (Tm) . The main reason of not breaking the disulfide bond that disulfide bond is much more stronger bond than a hydrogen bond( bond dissociation energy of a hydrogen bond and a disulfide bond is 2.8-7.2 kcal/mol and 60 kcal/mol respectively).

Disulfide bond cannot also cleaved by normal hydrolysis (by reacting with water). The cleavage occurs basically at higher pH (alkaline pH). If only alkaline solution is reacted with disulfide bond, then hydroxyl ion (OH-1) attacks the disulfide bond and as a result a new bond is formed with one of the sulfur atoms. In this way disulfide bond is split.

To know more please go through: Peptide Bond vs Disulfide Bond: Comparative Analysis and Facts

Some frequently asked questions about disulfide bond are answered below.

Frequently Asked Questions (FAQ)

Do disulfide bonds form spontaneously?

Answer: Disulfide bond can be formed spontaneously in presence of molecular oxygen (O2).

Are disulfide bonds polar?

Answer: The polarity of disulfide bond is not so large. It has less polarity with comparing to two Cysteine residues.

Which peptides can form disulfide bond?

Answer: The amino acids having no sulfyhydryl group (SH group) cannot participate in disulfide bond formation. Thus Cysteine is the only amino acid can form disulfide bond.

Structure of Cysteine.
Image Credit: Wikimedia Commons

Read more about following Structure & Characteristics

Peptide Bond
Hyaluronic Acid
Alanine Amino Acid
Glycolic Acid
Glutamic Acid
Hexanoic Acid
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