Histidine (commonly abbreviated to His or H)  is one of the twenty naturally-occurring amino acids. Of these amino acids it is one of ten polar and one of three positively-charged amino acids . The codons of His are CAU or CAC.
Histidine is an aromatic amino acid, which acts as an acid when protonated. When unprotonated it has a positively charged imidazole functional group and acts as a general base. It plays an important role in stabilising the folding structures of proteins and active sites of enzymes  . This ability is due to its binding properties with metals and the basic properties of the nitrogen atoms in the ring. It is only found as an L- isomer and it plays an important role in growth and repair of tissues in the body.
- ↑ ^ IUPAC-IUBMB Joint Commission on Biochemical Nomenclature. "Nomenclature and Symbolism for Amino Acids and Peptides". Recommendations on Organic &amp;amp;amp;amp; Biochemical Nomenclature, Symbols &amp;amp;amp;amp; Terminology etc. Retrieved 2007-05-17.
- ↑ http://www.detectingdesign.com/images/Abiogenesis/Amino%20Acid%20Chart.jpg
- ↑ http://www.biology.arizona.edu/biochemistry/problem_sets/aa/histidine.html