H9H

H9H refers to a specific sequence of three amino acids, Histidine-Histidine-Histidine, that can occur in proteins. Histidine is a unique amino acid due to its side chain, which contains an imidazole ring. This ring can act as both a proton donor and acceptor at physiological pH, making histidine residues crucial for many enzymatic reactions and protein functions. The presence of multiple consecutive histidine residues, as in H9H, can significantly enhance these properties.

The H9H motif, or a polyhistidine tag, is often used in molecular biology as a tool for protein purification. By genetically fusing a sequence encoding several histidine residues to the N- or C-terminus of a target protein, researchers can exploit the strong binding affinity of histidine to certain metal ions, particularly nickel and cobalt. This allows for efficient purification of the recombinant protein using immobilized metal affinity chromatography (IMAC). The His-tag is highly effective, readily introduced, and generally considered to be non-disruptive to protein folding and function. However, it can sometimes be a target for antibody recognition or may interfere with protein-protein interactions in certain contexts. The specific placement and length of the polyhistidine tag can be optimized depending on the protein and the purification strategy.