Histidine Amino Acid Code (His, H)

Protein sequences usually use one-letter codes because they are compact. In that notation, Hmeans Histidine. The three-letter code His is easier to read in structure notes, mutation descriptions, and teaching materials.

Codons are different from amino acid letter codes. A codon is a three-base RNA word read during translation, while an amino acid code is the protein shorthand. The codons CAU, CAC all point to Histidinein the standard genetic code.

The Histidine amino acid is classified here as basic. That label is a shortcut for first-pass interpretation, not a complete description of every chemical behavior.

Histidine is often checked because charged side chains can affect protein surface behavior, salt bridges, and local interactions. The exact effect depends on protein context, neighboring residues, pH, solvent exposure, and the folded state of the whole protein.

In a protein sequence, Histidine is usually discussed as an amino acid residue. That means the amino acid has been incorporated into a peptide chain and has lost the atoms removed during peptide bond formation. The one-letter code H and three-letter code His refer to that residue in sequence notation.

The listed molecular weight is for the free amino acid. Protein mass calculations often use residue masses instead, because water is removed when peptide bonds form.

Do not confuse the one-letter code H with a DNA or RNA base. It is an amino acid symbol, not a nucleotide in a codon.

Do not read codons without checking the frame. A triplet that encodes Histidine in one frame may disappear in another frame. For gene design, check organism-specific codon usage after confirming the basic amino acid identity.