Histidine

http://chemistry.about.com/od/imagesclipartstructures/ig/Amino-Acid-Structures/Histidine.htm

General Features
Abbreviation: His or H
Chemical Formula:C₆H₉N₃O₂
Molar Mass: 155.15 g/mol
Histidine’s codons are CAU and CAC.  Histidine is an aromatic.  It contains six pi electrons; two from a nitrogen lone pair and four from double bonds.
Physiological Roles
Histidine has many roles.  It is part of catalytic triads.  In this, the basic nitrogen of histidine removed a proton from serine, threonine, or cysteine to create a nucleophile.  Histidine can also participate in proton shuttle.  Histidine does this by using the basic nitrogen to remove a proton to make a positively charged intermediate.  It then uses another molecule to take the proton from its acidic nitrogen. Histidine also plays a role in signal transduction in the form of histidine kinases (HK).  HKs act as cellular receptors for signaling molecules.  HKs typically have an extracellular domain, transmembrane domain, and intracellular domain.  The intracellular domain contain the enzymatic activity.  The intracellular domain also has a region that can bind to secondary effector molecules that further propagate the signal within the cell.  HKs are part of a two-component signal transducation mechanism.  Below is the mechanism of histidine kinase action.

http://en.wikipedia.org/wiki/File:HK_mechanism.png

HKs transfer a phosphate group from ATP to the histidine residue within the kinase and then to an aspartate residue on the receiver domain on a different protein.  For more information, see Two-Domain Reconstitution of a Functional Protein Histidine Kinase by Heiyoung Park, Soumitra K. Saha, and Masayori Inouye.

http://www.chem.qmul.ac.uk/iubmb/enzyme/reaction/AminoAcid/His1.html

http://www.chem.qmul.ac.uk/iubmb/enzyme/reaction/AminoAcid/His2.html

Synthesis
The synthesis of histidine is complex and its pathway intertwines with nucleic acid synthesis (purine specifically).  The pathway appears to be universal in all organisms that are able to synthesize histidine.  The first steps transform phosphoribosyl pyrophosphate (PRPP) into imadiazoleglycerol phosphate.  This begins with a condensation in which the N1 of the purine ring of ATP bonds to the C1 of ribose.  PRPP provides the five carbons of histidine.  The adenine ring of ATP provides the carbon and nitrogen of the imidazole ring.  Once the imadiazole ring is formed, glutamate donates the α-amino group.  The newly formed anime is oxidized to histidine in the last step.  For more information, see Synthesis of Histidine in Escherichia coli by John Westley and Joseph Ceithaml