Disulfide bond

Main chemical bonds connecting within or between peptide chains

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Disulfide bond is connected differently Peptide chain Or in the same peptide chain, two are not Homocysteine Residue Of Mercapto group Of Chemical bond 。 The disulfide bond is relatively stable covalent bond , on protein In the molecule, it plays a stable peptide chain space structure Role of. The more disulfide bonds, the greater the stability of protein molecules against the influence of external factors.

In chemistry, disulfide bond refers to functional group 。 The disulfide bond usually consists of two mercaptan Group coupling. In biology, two Cysteine The disulfide bond formed between mercaptan groups in the residue is Protein secondary structure and Tertiary structure An important part of. This bond exists in the protein Steric structure of molecule It plays an important role in the formation.

Chinese name: Disulfide bond
Foreign name: disulfide bond/disulfide bridge/S-S bond
Nature: Chemical bond
Reaction: 2RSH ⇌ RS−SR’ + 2H+ + 2e−

Interpretation: Chemical bond connecting sulfhydryl groups of two different cysteine residues in different peptide chains or the same peptide chain
Features: Relatively stable
Length: About 2.05A

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nature

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The disulfide bond bonding ability is strong, and the typical disulfide bond dissociation energy is 60 kcal/mol (251 kJ/mol). Because disulfide bond is about 40% weaker than C-C bond and C-H bond, disulfide bond is often "weak bond" in many molecules. In addition, the S-S bond reflects the polarization characteristics of divalent sulfur, which is easily affected by polar reagents (including Electrophilic reagent and Nucleophile , especially nucleophilic reagents)^[1] 。

The oxidation of two cysteine molecules is reversible, forming disulfide bonds.^[5]

The length of disulfide bond is about 2.05 A, which is more than C-C Key length About 0.5 A. Rotating around the S-S axis barrier Lower. Disulfide For those close to 90 ° Dihedral angle There are obvious preferences. When the angle is close to 0 ° or 180 °, disulfide is a better oxidant. Two disulfides with the same R group are called symmetrical disulfide, such as diphenyl disulfide and Dimethyl disulfide 。 When two R groups are not identical, the compound is called asymmetric or mixed disulfide^[2] 。

function

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Disulfide bond and protein high structure biological activity Related to, and also related to Protein renaturation There are also connections. as Ribonuclease A through Mercaptoethanol （ reducing agent ）And urea (Protein Denaturant ）After processing Denaturation , 4 pairs of disulfide bonds broken, Polypeptide chain When stretched out, the advanced structure changes and loses biological activity. If using Dialysis A large amount of reducing agent and denaturant are removed. In the presence of a small amount of reducing agent, four pairs of disulfide bonds are re formed at the original position, and the polypeptide is stretched Peptide chain It will spontaneously fold into a natural conformation, and the biological activity will be restored. This experiment also proves that the information of protein high-level structure exists in Primary structure in^[3] 。

Reduction reaction

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One of the most important characteristics of disulfide bond is that it reducing agent Active splitting 。 There are more reducing agents for disulfide bond cleavage. stay biochemistry The common reducing agents are mercaptan as β - mercaptoethanol （β-mercaptoethanol， β-ME ）Or Dithiothreitol （DTT）。 Usually, excessive mercaptan reagent is used to ensure the complete cleavage of disulfide bond. Other reducing agents include tris (2-carboxyethyl) phosphine, TCEP ], different from β - ME and DTT, TCEP is tasteless, selective, and can Acidic environment Working under (unlike DTT) Hydrophilicity , resistant to oxidation. In addition, TCEP usually does not need to be removed before protein mercaptan modification^[4] 。

The oxidation and reduction exchange between mercaptans and disulfide bonds is the main reaction for the formation and rearrangement of disulfide bonds in proteins. The rearrangement of disulfide bonds in proteins is usually carried out through mercaptan and disulfide bond exchange reactions in proteins; Cysteine Group of residues sulfide It will attack a disulfide bond of its own protein. This disulfide rearrangement process (called disulfide shuffle) does not change the number of disulfide bonds in proteins, but only their positions.

The oxidation and reduction of disulfide bonds formed through mercaptan and disulfide bond exchange in organisms are subject to a Thioredoxin The promotion of. This small protein is necessary in all known organisms, and it contains two cysteine Amino acid residue , arranged in a contiguous manner (one by one). This allows it to form an internal disulfide bond or a non internal disulfide bond with other proteins. Therefore, it can be used as a reduced or oxidized disulfide bond memory pool 。

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