His-tagged recombinant proteins: Core tools and technological evolution of immobilized metal affinity chromatography

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This article focuses on the molecular characteristics and purification principles of His-tagged recombinant proteins, systematically elaborating the structural composition of polyhistidine affinity tags and their application basis in immobilized metal affinity chromatography. It analyzes the technical differences and selection criteria of chelating ligands such as IDA, NTA, and TED, and discusses the effects of His tag position (N-terminal or C-terminal) on protein expression yield, solubility, and purification performance.

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His-Tagged Recombinant Proteins: Core Tools for Immobilized Metal Affinity Chromatography and Their Technical Evolution

Brief Introduction: This article focuses on the molecular characteristics and purification principles of His-tagged recombinant proteins, systematically elaborating on the structural composition of polyhistidine affinity tags and their application basis in immobilized metal affinity chromatography, analyzing the technical differences and selection criteria among chelating ligands such as IDA, NTA, and TED, and discussing the effects of His-tag position (N-terminal or C-terminal) on protein expression rate, solubility, and purification efficiency.

I. Molecular Structure and Technical Positioning of His Tags.

The polyhistidine affinity tag, also known as the His tag or 6His, typically consists of six consecutive histidine residues, though some variations exist. The His tag is currently the most widely used affinity chromatography tag. The core of its molecular design lies in utilizing the coordination between the imidazole side chains of consecutive histidine residues and transition metal ions to achieve selective capture of recombinant fusion proteins. Since the tag itself consists of only six amino acids, it has a small molecular weight, low immunogenicity, and minimal impact on the solubility and activity of the target protein, so in most functional studies it can be used directly without cleavage. This "small tag, high versatility" characteristic has made the His tag one of the most fundamental and widespread tools in the field of recombinant protein preparation.

II. Principles and Development of His-Tag Affinity Chromatography.

His-tag affinity chromatography falls within the concept of immobilized metal affinity chromatography. It is known that transition metals have an affinity for histidine and cysteine, and immobilized metal affinity chromatography is based on this affinity. Porath used iminodiacetic acid as a chelating ligand to immobilize metal ions on agarose, laying the foundation for this technology. In the 1980s, the chelating ligand nitrilotriacetic acid was developed, followed by further advancements. Consecutive histidines are a genetic modification of recombinant proteins on a DNA template; this genetic modification strategy allows the His tag to be co-expressed with the target protein, thereby achieving the convenience of "expression equals tagging."

Different chelating ligands differ in coordination capacity and usage characteristics. IDA is tridentate, NTA is tetradentate, and TED is pentadentate. The main difference is that IDA and NTA have higher binding capacities, but IDA loses more nickel ions during equilibration and washing than NTA, and repeated use of IDA for purifying the same protein also results in decreased purity. TED has only one coordination bond remaining to interact with the His tag, so protein recovery is usually lower. These differences suggest that in purification process design, appropriate chelating ligands should be selected based on the characteristics of the target protein and the purification scale.

III. Position Selection of His Tags and Sequence Environment Effects.

His tags have considerable affinity and specificity, so in most cases, one-step His-tag affinity purification is sufficient to prepare target proteins of a certain purity and can meet the needs of many applications. The tag structure can affect protein production at multiple levels: expression rate, binding capacity to immobilized metal affinity chromatography ligands, protein three-dimensional structure, protein crystal formation, etc. The His tag has minimal impact on protein solubility and activity, but it does have some effect. More important than the tag sequence itself is its position and the amino acid sequence at the N-terminus.

It has been reported that the amino acid adjacent to the N-terminal methionine serves to prevent N-terminal methionine processing and has a positive effect on the expression rate of general E. coli proteins. Lysine or arginine at the second position of the N-terminus has this effect. The high success rate of N-terminal His tags not only derives from the stimulatory effect of the second amino acid but may also be because the tag sequence continuously affects the mRNA structure of the translation initiation region. In most cases, N-terminal tags are more favorable for protein expression. However, C-terminal His tags have a more pronounced effect on protein expression rate and solubility; C-terminal fusion His tags increase protein yield compared to untagged proteins. Placing the tag at the C-terminus can avoid interfering with membrane transport and ensure secretory expression of the protein. In addition, when expressing a certain DNA polymerase in E. coli, it was found that with gene sequences using unoptimized codons, a C-terminal His tag could improve the purity of polymerase affinity purification. The speculated reason may be that incomplete protein sequences "interrupted" by rare codons do not carry the His tag, thereby improving the relative purity of purification. These observations suggest that the choice of tag position should be comprehensively judged in combination with the expression system, target protein properties, and purification objectives.

IV. Technical Advantages and Limitations of His Tags.

Immobilized metal affinity chromatography is one of the most widespread chromatography techniques. Its main advantages lie in ease of use, low cost, and good durability; secondly, the His tag can be applied under denaturing conditions such as 8 M urea or 6 M guanidine hydrochloride, and also under redox conditions; the high affinity allows high capture efficiency even at high protein concentrations; the purification process is scalable; additionally, because the tag is small, it does not affect protein activity as much as other large tags, and it usually does not need to be removed during functional studies.

The limitations of His tags also need to be acknowledged. Chelating agents should be avoided as much as possible, or only used at low concentrations, although some manufacturers have developed chromatography media with high tolerance. Other groups with chelating effects, such as Tris, ammonium salts, and certain amino acids, also need to be avoided. In addition, as a purification tag, the His tag has limited influence on the soluble expression of proteins; when solubilizing expression is required, it often needs to be combined with tags such as MBP, GST, and TrxA. Because natural protein surfaces also have groups that interact with metal ions, or contain consecutive histidine residues—for example, the α subunit of human transcription factor TFIIA has seven consecutive histidine residues exposed on its surface—inevitable binding to immobilized metal affinity chromatography media occurs, so the purity of His-tag purification is relatively low. Understanding these limitations helps in rationally anticipating purification results in experimental design and taking corresponding supplementary purification steps.

V. Conclusion.

As a core tool for immobilized metal affinity chromatography, His-tagged recombinant proteins—by virtue of the coordination principle between consecutive histidine residues and transition metal ions, the technical evolution of chelating ligands such as IDA, NTA, and TED, the differential effects of N-terminal and C-terminal tag positions on expression rate and purification efficiency, and the technical advantages of ease of use, low cost, and tolerance to denaturing conditions—have become the most widely used fundamental tool in the field of recombinant protein preparation. While recognizing their advantages, acknowledging their limitations in chelating agent compatibility, limited solubilization capacity, and interference from natural histidine backgrounds will help researchers design purification schemes more rationally. The DDB1/CRBN Complex, His Tag Protein provided by Ulab provides reliable tool support for the preparation of complex recombinant proteins and targeted protein degradation research, and will continue to facilitate in-depth exploration in related fields.

This article is reviewed and published by the technical expert team of UA

Disclaimer: This article partially utilizes artificial intelligence assistance in its creation. If any content involves copyright or intellectual property issues, please let us know and we promise to verify and remove it as soon as possible.

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