His6His6

His6His6 is a sequence of six consecutive histidine residues followed by another six consecutive histidine residues. This repeating motif is sometimes encountered in protein engineering, particularly in the context of affinity tags. Histidine tags are commonly used to facilitate the purification of recombinant proteins. The six-histidine tag, often abbreviated as His-tag or H6, is known for its ability to bind strongly to immobilized metal affinity chromatography (IMAC) resins, such as those containing nickel or cobalt ions.

The His6His6 sequence represents a more extensive histidine-rich region. While a single six-histidine tag is sufficient for many purification purposes, longer or repeated histidine tags can be employed to enhance binding affinity and potentially improve purification yields. The increased number of histidine residues offers a greater number of coordination sites for the metal ions immobilized on the chromatography resin. This can lead to more robust binding, especially for proteins that might have weaker interactions with a standard His-tag, or in situations where higher protein concentrations are being processed.

The implementation of a His6His6 tag in protein expression systems requires careful consideration of potential impacts on protein folding, solubility, and function. While effective for purification, such a long and repetitive sequence could theoretically influence the three-dimensional structure of the target protein or interfere with its biological activity. Researchers must evaluate the trade-offs between enhanced purification capabilities and potential biological consequences. Following purification, the His6His6 tag is often removed using specific proteases to obtain the native protein.