Disulfide bridges – all about the topic

In the following article you will learn, among other things, what a disulfide bridge is, how it is formed and what the function of such a bond is. So no problem if you’re not sure what the disulfide bridges are all about. If you stick with it to the end, you will have a good overview of this important topic, which is one of the amino acids in chemistry.

Have fun with your studying!

What are disulfide bridges?

In chemistry, a disulfide bridge, also known as a disulfide bond or disulfide bridge bond, is a covalent bond between two sulfur atoms whose only free valence is saturated with an organyl radical. In biochemistry, on the other hand, the disulfide bond is the covalent bond, i.e. an atomic bond, between the sulfur atoms of two cysteine ​​molecules that occur in the amino acid side chain of a protein. Two cysteine ​​residues in proteins linked by a disulfide bond are also referred to as a cystine bridge.

The picture shows a schematic representation of four disulfide bridges within a peptide chain of a protein. The bond is intrachenar, i.e. localized within a polymer chain.

Formation of disulfide bridges

Disulfide bridges are (in eukaryotes) either inserted into the proteins during translation if parts of them are already in the endoplasmic reticulum (ER) during their synthesis, or afterwards if they are completely in the ER or another membrane-enclosed cell organelle . The second case represents a so-called post-translational modification. In the case of prokaryotes, this applies analogously to translation into the periplasm.

via wikipedia.org

The functional groups involved in the formation of a disulfide bond are called thiol groups or mercapto groups. The formation of such a sulfur-sulfur bond can be described as oxidation, i.e. the release of hydrogen or electrons.

The oxidation reaction is: R-SH + HS-R’ → RSSR’ + 2 H+ + 2e−

And the reduction: 2 Fe3+ + 2 e− → 2 Fe2+

R and R’ denote the cysteines on the peptide or protein. The two excess hydrogen atoms are bound by a hydrogen acceptor. They can ultimately be transferred to oxygen, for example:

4H + O2 → 2H2O

Since the formation of a disulfide bridge between two mercapto groups can be described as oxidation, the reaction can only take place in an oxidative environment. For this reason, cytoplasmic proteins usually do not contain any disulfide bridges, since the cytoplasm is a reducing environment.

The formation of disulfide bridges is not a spontaneous process. It requires a redox reaction, which requires an appropriate reaction partner for electron transfer. The training is also catalyzed by enzymes. If a protein has more than just two cysteines, it is possible that linking the “wrong” cysteines will result in disulfide bridges that do not correspond to the native state of the protein. A reconnection of false disulfide bridges must therefore take place.

Involved in the formation of disulfide bonds is the tripeptide glutathione (GSH), an isopeptide present in the cytoplasm of both prokaryotes and eukaryotes.

Function of the disulfide bridges

Disulfide bridges are primarily used to shape and stabilize the three-dimensional protein structure by forming loops within the amino acid chains or linking several amino acid chains to form a functional protein. As covalent bonds, the disulfide bridges have a much more fixing effect than, for example, the hydrogen bridge bonds that also occur in the molecule. The formation of disulfide bonds can also be a separate step in the folding of a protein without delaying subsequent folding steps.

Importance of disulfide bridges for recombinant protein expression

Disulfide bridges in proteins limit their ability to be expressed recombinantly, i.e. their biotechnological production. Without disulfide bridges, for example, the folding of the protein is disturbed. In addition to proteolytic degradation, excessive protein production can lead to the formation of inclusion bodies. The interior of the inclusion body is protected from reduction and therefore randomly forms disulfide bridges to other proteins. Both misfolding and inclusion body formation necessitate purification of the protein and often result in a partially functioning protein.

By using certain reducing agents, the randomly formed disulfide bonds in the inclusion bodies can be broken again. This happens through reduction, the individual proteins are separated from each other and dissolve. They now contain only reduced disulfide bonds.

In order to obtain a functional, correctly folded protein, the disulfide bonds must be re-formed in a controlled manner so that only the «desired» cysteine ​​pairs form a bond with one another. To achieve this, the proteins are mixed with glutathione (GSH). Re-oxidation of the disulfide bonds to the protein’s native state occurs.

Everything you need to know about disulfide bridges at a glance!

Well, understood everything? Last but not least, here is a summary of the most important aspects of disulfide bridges so that you are well prepared for your next exam.

  • In biochemistry, a disulfide bridge or disulfide bond is the covalent bond (atom bond) between the sulfur atoms of two cysteine ​​molecules that occur in the amino acid side chain of a protein.
  • In eukaryotes, disulfide bridges are either inserted into the proteins during translation or afterwards.
  • The functional groups involved in the formation of a disulfide bond are called thiol groups or mercapto groups. The formation of such a sulfur-sulfur bond can be described as oxidation.
  • Since the formation of a disulfide bridge can be described as oxidation, the reaction can only occur in an oxidative environment.
  • The tripeptide glutathione (GSH) is involved in the formation of disulfide bridges.
  • Disulfide bridges are primarily used to shape and stabilize the three-dimensional protein structure by forming loops within the amino acid chains or linking several amino acid chains to form a functional protein.