Histidine is one of the essential amino acids that is needed by the human body. It is abbreviated as His or H. Its IUPAC name is 2-amino-3-(1H-imidazol-4-yl)propanoic acid. Below is the structure of the amino acid in discussion.
Histidine has a carboxylic acid, an ammonium and an imidazole ring in its structure. Of these three functional groups, the imidazole ring is the most significant. It is a common coordinating ligand in metalloproteins, part of catalytic sites in certain enzymes and it is a nucleophile. The ring has 6 pi electrons, 4 from the 2 double bonds and 2 from a nitrogen lone pair. Histidine can exist in 4 forms, depending on the pH of the solution, and it is aromatic at all pH values.
The pKa values of the functional groups of histidine are as follows:
- carboxylic acid pKa = 1.78
- imidazole pKa = 5.97
- ammonium pKa = 8.97
The isoelectric point of the amino acid is at pH = 7.47.
Histidine can be analyzed using UV-visible absorption spectrum. The peaks represented around 220 nm result from an excitation of pi electrons in the ground state molecular orbitals to a low energy state.
In the 15N NMR spectrum, increase in pH to about 8 causes protonation of the imidazole ring to be lost, giving rise to N-1 or N-3 tautomers. The chemical shift of N-1 drops slightly to about 190ppm while N-3 drops considerably to 145ppm. N-1 tautomer is preferred because of hydrogen bonding to neighboring ammonium. At pH 9, the chemical shifts of N-1 and N-3 are 185ppm and 170ppm respectively. Below is the 1H NMR spectrum of histidine.
Two important peptides that contain histidine in their structures that are found in mammalian cells are histidylated oligolysine (HoK) and histidylated polylysine (HpK). They aid in the transfer of nucleic acid.
Sources sited:
http://en.wikipedia.org/wiki/Histidine
http://omlc.ogi.edu/spectra/PhotochemCAD/html/histidine.html
http://www.enovatia.com/services/nmr/capillary-nmr/
Sources sited:
http://en.wikipedia.org/wiki/Histidine
http://omlc.ogi.edu/spectra/PhotochemCAD/html/histidine.html
http://www.enovatia.com/services/nmr/capillary-nmr/
I really enjoyed this blog of yours and I think you did a really good job. I’m also glad you found all of your stuff about histidine because I couldn’t find everything on mine. Even though I think you did a good job explaining yours, I would also talk about how histidine is an essential amino acid that has a positively charged imidazole functional group. I also found that the imidazole makes it a common participant in enzyme catalyzed reactions. The unprotonated imidazole is nucleophilic and can serve as a general base, while the protonated form can serve as a general acid. The residue can also serve a role in stabilizing the folded structures of proteins.1 I found this stuff at a website I listed below. Also viewed some more websites about histidine and found that your amino acid is very interesting. Also attached is a picture I liked that explained some things about histidine.
ReplyDelete1 The Biology Project. Histidine. http://www.biology.arizona.edu/biochemistry/problem_sets/aa/histidine.html (accessed 2003)