Histidine

Histidine (commonly abbreviated to His or H) ^[1]  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 ^[2].  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 ^[3] . 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^[4].

References

1. ↑ ^ IUPAC-IUBMB Joint Commission on Biochemical Nomenclature. "Nomenclature and Symbolism for Amino Acids and Peptides". Recommendations on Organic and Biochemical Nomenclature, Symbols and Terminology etc. Retrieved 2007-05-17.
2. ↑ http://www.detectingdesign.com/images/Abiogenesis/Amino%20Acid%20Chart.jpg
3. ↑ http://www.biology.arizona.edu/biochemistry/problem_sets/aa/histidine.html
4. ↑ Berg, J. M., Tymoczko, J. L., & Stryer, L. (2002). Biochemistry (5th ed.). New York: W.H. Freeman.