1. Overview of Disulfide Bonds
A disulfide bond is a covalent chemical bond formed by the oxidation reaction between sulfhydryl groups of two cysteine residues, predominantly existing in the three-dimensional structure of proteins. Its formation stabilizes protein folding and sustains biological functions. Especially under harsh external conditions such as high temperature and extreme pH values, disulfide bonds link different polypeptide chains or distinct segments of a single polypeptide chain, consolidating the spatial conformation of proteins and exerting profound effects on various structurally-dependent proteins including enzymes, hormones and antibodies.
The endogenous formation mechanisms of disulfide bonds in prokaryotic and eukaryotic expression systems have been well elucidated. In essence, disulfide bond generation originates from nucleophilic attack between thiol groups and oxidized disulfides, followed by a series of enzyme-catalyzed thiol-disulfide exchange reactions, covering oxidation, reduction and isomerization reactions. In oxidation reactions, two thiol groups undergo dehydrogenation catalyzed by oxidases to form new disulfide bonds, which is indispensable for correct protein folding. Reductases cleave intramolecular disulfide bonds of target proteins to release free thiol groups, facilitating the correction of mispaired disulfide bonds. Disulfide bond isomerization achieves structural rearrangement via two pathways: the two-step mechanism involves sequential reduction cleavage and oxidative regeneration; the one-step mechanism relies on isomerases to catalyze the formation of mixed disulfide intermediates, which further react with adjacent cysteines to reconstruct brand-new disulfide bonds without generating free thiols, realizing efficient rearrangement through dynamic interactions between enzyme active sites and substrates.
2. Effects of Disulfide Bonds on Bioactivity and Stability of Proteins
Disulfide bonds are core determinants maintaining the biological activity of proteins. Firstly, they stabilize the native three-dimensional conformation by cross-linking polypeptide segments, and correct spatial structure is the prerequisite for proteins to bind specific ligands and substrates and execute physiological functions. Secondly, disulfide bond formation facilitates orderly protein folding during biosynthesis, suppresses misfolding and protein aggregation, and preserves complete biological activity. Misfolded proteins not only lose physiological functions but also form cytotoxic aggregates. In addition, the dynamic formation and cleavage of disulfide bonds can regulate protein functions; for instance, disulfide bonds located in the active centers of certain enzymes serve as functional switches to modulate enzymatic activity under varying physiological conditions.
Disulfide bonds also significantly enhance protein stability. As extra covalent cross-links, they reinforce thermal tolerance and prevent protein denaturation under high temperature, which easily disrupts non-covalent interactions including hydrogen bonds and hydrophobic forces. They also improve protein resistance against chemical denaturants such as urea, high salt and extreme pH environments, maintaining structural and functional integrity. Furthermore, disulfide bonds effectively inhibit abnormal protein aggregation, elevating protein solubility and long-term storage stability.
3. Key Influencing Factors of Disulfide Bond Formation
The formation efficiency and pairing accuracy of disulfide bonds in recombinant proteins are mainly governed by four categories of factors: chemical environment, physical conditions, intrinsic protein structure and biological regulation.
In terms of chemical environment, thiol oxidation preferentially proceeds under weakly alkaline conditions (pH 8-9), while acidic environments severely inhibit the reaction, and extreme pH values directly destroy mature disulfide bonds. Reducing agents such as DTT and glutathione break disulfide bonds, whereas oxidants including hydrogen peroxide and dissolved oxygen promote disulfide bond synthesis; heavy metal ions like Cu²⁺ and Fe²⁺ can catalyze spontaneous oxidation of sulfhydryl groups.
For physical conditions, excessive temperature triggers protein thermal denaturation and disulfide bond rupture; high shear force in production processes also easily breaks established disulfide bonds.
From the perspective of protein structure, the spatial proximity of paired cysteine residues is a fundamental prerequisite for effective disulfide bond formation.
In biological regulation, subcellular localization plays a decisive role: secretory proteins located in the oxidative endoplasmic reticulum environment are prone to form correct disulfide bonds, while cytoplasmic proteins in reducing environments face great obstacles in disulfide bond assembly. Moreover, post-translational modifications such as S-nitrosylation of cysteine residues can interfere with normal disulfide bond pairing.
4. Strategies for Correct Disulfide Bond Formation and Detection Methods
4.1 Optimization Strategies for Accurate Disulfide Bond Formation
Based on the influencing factors summarized above, systematic optimization strategies are proposed to achieve standardized disulfide bond assembly:
Selection of Optimal Expression Hosts
For Escherichia coli prokaryotic expression systems, oxidative cytoplasmic mutant strains represented by SHuffle strains are preferred to construct intracellular oxidative microenvironments. Periplasmic expression is also adopted to utilize the endogenous Dsb redox system to boost disulfide bond biosynthesis.
Optimization of Fermentation and Induction Conditions
Low-temperature induction at 16–25 °C slows down protein folding kinetics and reduces mispairing probability. Redox buffer with optimized ratios of oxidized and reduced glutathione (5:1 to 10:1) is supplemented into culture media. Dissolved oxygen content is properly elevated by increasing shaking speed to strengthen intracellular oxidative conditions.
Genetic Engineering Modification
Targeted modification of amino acid sequences via gene editing optimizes the spatial distribution of cysteine residues, lowers the risk of random disulfide bond mispairing, and guides directional formation of native disulfide bonds.
Co-expression of Auxiliary Factors
Co-expression of protein disulfide isomerase (PDI) corrects mismatched disulfide bonds efficiently. Molecular chaperones are co-expressed simultaneously to alleviate protein misfolding and aggregation.
In Vitro Refolding Optimization
Inclusion body renaturation remains the mainstream process for intracellular expressed recombinant proteins. The redox gradient refolding method is widely applied: gradually reduce the concentration of reducing agents such as DTT and add appropriate oxidized glutathione in renaturation buffer. The buffer system is adjusted to neutral to weakly alkaline pH (7.0–8.5) with low ionic strength represented by Tris-HCl. Addition of 0.4–1 M arginine and 5–10% glycerol effectively inhibits protein aggregation and stabilizes native protein conformation during refolding.
4.2 Characterization and Detection Technologies for Disulfide Bonds
Multiple qualitative and quantitative analytical methods are applied to verify the correctness of disulfide bonds:
SDS-PAGE Electrophoresis: Under non-reducing conditions, intact disulfide bonds compact protein structures and accelerate electrophoretic migration; the difference in band positions between non-reducing and reducing electrophoresis preliminarily judges disulfide bond assembly status.
Mass Spectrometry Analysis: Combined with protease digestion, peptide mapping and high-resolution mass spectrometry accurately identify disulfide bond linkage sites and confirm cysteine pairing patterns.
Ellman’s Assay: Achieves quantitative detection of free sulfhydryl groups; complete disulfide bond formation corresponds to extremely low content of residual free thiols.
Spectroscopic Detection: Circular dichroism spectroscopy monitors changes in protein secondary structure to indirectly evaluate disulfide bond correctness.
High-Resolution Structural Analysis: Nuclear magnetic resonance (NMR) directly visualizes the spatial distribution of disulfide bonds, applicable for high-purity protein samples with high detection cost.
Functional Activity Verification: Since mispaired disulfide bonds inevitably lead to declined or lost protein activity, biological activity assays including enzymatic activity detection and receptor binding affinity analysis indirectly evaluate the functional validity of disulfide bonds.
Conclusion
In the research and industrial production of recombinant proteins, disulfide bonds are indispensable structural anchors for maintaining native protein conformation and physiological functions. Rational disulfide bond pairing guarantees complete protein bioactivity, while abnormal disulfide bonds cause functional defects and cytotoxic protein aggregates. This article systematically elaborates the regulatory mechanism, functional value and influencing factors of disulfide bonds in recombinant proteins, and summarizes mature optimization strategies and comprehensive detection systems covering upstream expression regulation and downstream in vitro renaturation.
With the continuous upgrading of biopharmaceutical technology, disulfide bond regulation and detection technologies will become more precise and efficient, which will further improve the expression yield, structural uniformity and functional stability of recombinant proteins, and provide solid technical support for the innovative research and industrial transformation of biopharmaceutical products.
