What Is Disulfide Bond: Formation, Types, Functions

Disulfide bond is mainly a covalent linkage between the side chain residues in same protein or may be different protein.

In addition of peptide bond Disulfide bond is a different type of covalent bond, is present in protein molecule. This bond is formed due to oxidation of the sulfyhydryl or thiol group (SH group) come from Cysteine (non essential amino acid) residue. Disulfide bond expressed as R-S-S-R1 and also known as S-S bond.

In this article ” what is a disulfide bond “different facts of Disulfide bond, such as formation procedure, types and functions of disulfide bond are described briefly.

How Disulfide Bonds are Formed?

The formation procedure of disulfide bond is described in this point.

Disulfide linkage is one type of covalent linkage in which two thiol groups (SH group) generated from two Cysteine residue is involved in this bond formation. Sanion coming from one Cysteine acts as a nucleophile and it attacks the other Cysteine side chain residue to from the disulfide bond.

The reaction of formation a disulfide bond is-

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

The formation of disulfide bond involves two electron transfer and this transfer takes place from the reduced sulfyhydryl group (S-H) of Cysteine residue to the Cystine (S-S) the oxidized form.

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

Disulfide Bond Type

Disulfide bond is a chemical covalent bond present in the tertiary structure of protein. It is one of the important type of linkage like peptide linkage, hydrogen bonding salt bridge interaction present in protein.

Disulfide Bond In Protein

With the peptide bond, Disulfide linkage is also an very essential bond in peptides or proteins. It is stronger bond than the other bonds contribute in the tertiary structure of protein.

Disulfide bond is present in almost all types of extracellular protein (used in cell structure systems). This linkage is one of the integral component of secondary and tertiary structure of protein (peptide bond is the building block of primary structure).

The disulfide bond usually consist of two parts, one is polar part or hydrophilic part and another is non polar part or hydrophobic part. Among these two part, the hydrophobic part is oriented towards the inner surface of protein, whereas the hydrophilic part is directed towards the outer surface of the bond. This orientation of polar or hydrophilic part helps to form the linkage between two amino acid residue.

The average bond dissociation energy of disulfide bond is approximately 50 kcal/mol and S-S bond length is nearly 2 angstorm. Disulfide linkage is very much strong and very short range bond.

what is a disulfide bond
Disulfide Bond in Protein.
Image Credit : Wikimedia Commons

To know more please follow: 15 Coordinate Covalent Bond Examples: Detailed Insight And Facts

Disulfide Bond Function

Function of disulfide bond in the determining the structure of protein is widespread.

The main activity of disulfide bond is to provide stabilization to the 3D structure of protein and exhibit physiologically appropriate redox procedure. Disulfide bond is an essential part in protein folding and stability. The tertiary structure of protein get stabilization by disulfide interaction.

The disulfide linkage governs the basic biological process in a living organism. The electron transfer process ( two electrons are transferred from Cysteine to Cystine) is accelerated by enzymes, Thirodoxin. Maximum disulfide is formed intramolecular, in some special cases this bond can be formed between two vicinal Cysteine residues and leads the only natural covalent linkage in the polypeptide formation.

 Cleavage of disulfide bond in protein can cause the collapse of the conformation of various biological process and failure of formation of disulfide bond properly may be the reason of severe disorders  as the protein molecules form aggregates and results the cell death.

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

Can Disulfide Bonds Be Broken By Oxidation

Disulfide bond can be broken by oxidation – reduction process and by adding oxidizing and reducing agent. The most widely used reducing agents to prevent the oxidation is the disulfide bond is β- mercaptoethanol known as BME and dithiothritol (DTT).

The oxidation reduction procedure in protein is proceed through in vitro path and it is an exchange reaction between thiol to disulfide. Disulfide bond is generally formed by the oxidation of thiol group (SH) present in

Disulfide bonds are readily oxidized by a various type of oxidants and the rate constants are quite high (105-107 M-1 S-1). The intermediate which is formed in the reaction medium is thiosulfinates [RSS(=O)R.].  This intermediate undergoes further oxidation and at the end of the oxidation, cleavage of disulfide bond is occurred.

Can Disulfide Bond Be Broken By Heat

Disulfide bond can not be broken by applying heat energy. Heat mainly denatures the protein ( proteins become unfolded from folded structure).

The breaking of disulfide bond is basically an irreversible process. Breaking of disulfide bond causes the denaturation of protein at melting temperature (in which protein denatures). This is proceed through disulfide- thiol exchange reaction.

In presence of MTS (methanethiosulphonate) the heat induced  exchange reaction from disulfide to thiol is hindered and heat resistance power of protein is improved.

Heat energy disrupts the hydrogen bond and the non polar hydrophobic interactions in protein. Applying heat increases the internal energy as well as kinetic energy within the molecules . Thus molecules start vibrating rapidly and the weak bonds present in the group of molecule is broken.

Disulfide bond is formed by the hydrophobic and hydrophilic interaction. Due to absorbing heat hydrogen bonding and hydrophobic  interactions become disrupted and as a result breakage of hydrogen bond occurs.

The bond dissociation energy of hydrogen bond is approximately 12-30 kilojoule / mol and for disulfide bond it is almost 251 KJ/mol. Thus cleavage of disulfide bond does not occur by applying normal heat energy.

Globular proteins basically exist in the equilibrium condition between folded and unfolded states. Under normal condition  folded state is mostly favored. Applying heat energy nearly equals to melting temperature (Tm) , the protein is started unfolding, that is denaturation of protein is occurring.

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

Can Disulfide Bond Be Broken By Water

Very precisely it can be predicted that disulfide bond can not be broken by water.

Disulfide bonds are very much strong chemical covalent bond and its bond dissociation energy is also quite high with comparing to other similar covalent bonds. It gives stabilization to the tertiary structure of protein. It is not possible to break a disulfide bond by adding water.

If any alkaline aqueous solution takes part in the reaction with disulfide bond, the hydroxide ions (OH) attacks the disulfide bond and will form a new bond with one of the sulfur atom from the two sulfur atoms formed disulfide bond. As a result disulfide bond will be cleaved.

The above reaction is known as alkaline hydrolysis of disulfide bond.

Frequently Asked Questions (FAQ)

Some frequently asked questions about the disulfide linkage are answered below.

How formation of disulfide bond can be prevented?

Answer: pH of the sample should be low (at or below pH 3-4). At low pH the thiol groups (SH) are protonated and cannot take part in the bond formation reaction.

How disulfide bonds affect protein stability?

Answer: Disulfide bonds reduce the entropy in the protein molecule in its denatured state.

Do disulfide bonds form spontaneously?

Answer: Yes, disulfide bonds can be formed in a spontaneous process by molecular oxygen.

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