Histidine (abbreviated as His or H) is an alpha-amino acid. The L-isomer is one of the 22 proteinogenic amino acids, i.e., the building blocks of proteins. It is classified as a charged, polar because of the hydrophilic nature of the imidazole side chain. L-Histidine is a positively charged (pKa 6.5) aromatic amino acid. Histidine residues are often found in enzyme active sites, where the chemistry of the imidazole ring of histidine makes it a nucleophile and a good acid/base catalyzer. Histidine is special in that its biosynthesis is inherently linked to the pathways of nucleotide formation. In the first step of histidine synthesis, PRPP (5-phosphoribosyl-alpha-pyrophosphate) condenses with ATP to form a purine, N1-5'-phosphoribosyl ATP, in a reaction that is driven by the subsequent hydrolysis of the pyrophosphate that condenses out. Glutamine plays a role as an amino group donor resulting in the formation of 5-aminoamidazole-4-carboximide ribonucleotide (ACAIR), which is an intermediate in purine biosynthesis. Hisitidne is catabolized by the enzyme histidine ammonia-lyase which converts histidine into ammonia and urocanic acid.