His159

His159 refers to the histidine residue at position 159 in a protein sequence or structure. It is a labeling convention used to identify a specific amino acid within a polypeptide chain and does not denote a standalone molecule. The exact identity of the protein and the context determine its role.

Nomenclature and context can affect the meaning of a position number. Residue numbering typically starts from the initiator methionine, but may be offset in mature proteins if signal peptides are removed or if different isoforms are used. Therefore, His159 in one protein may correspond to a different residue number in another, and the same label does not imply identical function across proteins.

Functionally, the significance of His159 is highly context-dependent. Histidine side chains can participate in general acid-base catalysis, metal coordination, or hydrogen bonding networks within the active site or at substrate interfaces. In some proteins, a histidine at position 159 might be directly involved in catalysis, stabilize a transition state, or help coordinate a bound metal ion. In others, it may play a structural or allosteric role rather than a catalytic one.

Experimental investigations often explore His159 through site-directed mutagenesis, such as substituting histidine with alanine (H159A) or other residues to assess effects on activity, binding, or stability. Structural methods (X-ray crystallography, cryo-EM) and sequence comparisons can illuminate how the local environment around His159 shapes its properties.

Because residue numbering is specific to each protein, accurate interpretation of His159 requires information about the particular protein, its organism, and the isoform studied.