Histidine (symbolHis orH)[2] is anessential amino acid that is used in the biosynthesis ofproteins. It contains anα-amino group (which is in theprotonated –NH3+ form underbiological conditions), acarboxylic acid group (which is in the deprotonated –COO− form under biological conditions), and animidazole side chain (which is partially protonated), classifying it as a positively charged amino acid at physiologicalpH. Initially thoughtessential only for infants, it has now been shown in longer-term studies to be essential for adults also.[3] It isencoded by thecodons CAU and CAC.
The conjugate acid (protonated form) of theimidazoleside chain in histidine has apKa of approximately 6.0. Thus, below a pH of 6, the imidazole ring is mostlyprotonated (as described by theHenderson–Hasselbalch equation). The resulting imidazolium ring bears two NH bonds and has a positive charge. The positive charge is equally distributed between bothnitrogens and can be represented with two equally importantresonance structures. Sometimes, the symbolHip is used for this protonated form instead of the usual His.[5][6][7] Above pH 6, one of the two protons is lost. The remaining proton of the imidazole ring can reside on either nitrogen, giving rise to what are known as the N3-H or N1-Htautomers. The N3-H tautomer is shown in the figure above. In the N1-H tautomer, the NH is nearer the backbone. These neutral tautomers, also referred to as Nε (or Nτ) and Nδ (or Nπ), are sometimes referred to with symbolsHie andHid, respectively.[8][5][6][7] The imidazole/imidazolium ring of histidine isaromatic at all pH values.[9] Under certain conditions, all three ion-forming groups of histidine can be charged forming the histidinium cation.[10]
The acid-base properties of the imidazole side chain are relevant to thecatalytic mechanism of manyenzymes.[11] Incatalytic triads, the basic nitrogen of histidine abstracts a proton fromserine,threonine, orcysteine to activate it as anucleophile. In a histidineproton shuttle, histidine is used to quickly shuttle protons. It can do this by abstracting a proton with its basic nitrogen to make a positively charged intermediate and then use another molecule, a buffer, to extract the proton from its acidic nitrogen. Incarbonic anhydrases, a histidine proton shuttle is utilized to rapidly shuttle protons away from azinc-bound water molecule to quickly regenerate the active form of the enzyme. In helices E and F ofhemoglobin, histidine influences binding of dioxygen as well ascarbon monoxide. This interaction enhances the affinity of Fe(II) for O2 but destabilizes the binding of CO, which binds only 200 times stronger in hemoglobin, compared to 20,000 times stronger in freeheme.
The tautomerism and acid-base properties of the imidazole side chain has been characterized by15N NMR spectroscopy. The two15N chemical shifts are similar (about 200 ppm, relative tonitric acid on the sigma scale, on which increased shielding corresponds to increasedchemical shift).NMR spectral measurements shows that the chemical shift of N1-H drops slightly, whereas the chemical shift of N3-H drops considerably (about 190 vs. 145 ppm). This change indicates that the N1-H tautomer is preferred, possibly due to hydrogen bonding to the neighboringammonium. The shielding at N3 is substantially reduced due to the second-orderparamagnetic effect, which involves a symmetry-allowed interaction between the nitrogen lone pair and the excited π* states of thearomatic ring. At pH > 9, the chemical shifts of N1 and N3 are approximately 185 and 170 ppm.[12]
Histidine formscomplexes with many metal ions. The imidazole sidechain of the histidine residue commonly serves as aligand inmetalloproteins. One example is the axial base attached to Fe in myoglobin and hemoglobin. Poly-histidine tags (of six or more consecutive H residues) are utilized for protein purification by binding to columns with nickel or cobalt, with micromolar affinity.[13] Natural poly-histidine peptides, found in the venom of the viperAtheris squamigera have been shown to bind Zn(II), Ni(II) and Cu(II) and affect the function of venom metalloproteases.[14]
N-terminal histidines are known to function asbidentate ligands, with a metal (generally copper) bound to both the amine of theN-terminus and the Nε of the histidine; the Nδ is often methylated.[15] Although recently discovered,[16] this "histidine brace" motif is critical in biogeochemical cycles: it functions as the active site of lytic polysaccharide monooxygenases (LPMOs), which break down unreactive polysaccharides such as cellulose.[17] It is proposed that the evolution of these in fungi corresponds to the first widespread ability to decompose woody plant mass, leading to the end of theCarboniferous era and the massaccumulation of coal deposits.[15]
Histidine Biosynthesis Pathway Eight different enzymes can catalyze ten reactions. In this image, His4 catalyzes four different reactions in the pathway.
l-Histidine is an essential amino acid that is not synthesizedde novo in humans.[18] Humans and other animals must ingest histidine or histidine-containing proteins. The biosynthesis of histidine has been widely studied in prokaryotes such asE. coli. Histidine synthesis inE. coli involves eight gene products (His1, 2, 3, 4, 5, 6, 7, and 8) and it occurs in ten steps. This is possible because a single gene product has the ability to catalyze more than one reaction. For example, as shown in the pathway,His4 catalyzes 4 different steps in the pathway.[19]
Histidine is synthesized fromphosphoribosyl pyrophosphate (PRPP), which is made fromribose-5-phosphate byribose-phosphate diphosphokinase in thepentose phosphate pathway. The first reaction of histidine biosynthesis is the condensation of PRPP andadenosine triphosphate (ATP) by the enzymeATP-phosphoribosyl transferase. ATP-phosphoribosyl transferase is indicated by His1 in the image.[19] His4 gene product then hydrolyzes the product of the condensation, phosphoribosyl-ATP, producing phosphoribosyl-AMP (PRAMP), which is an irreversible step. His4 then catalyzes the formation of phosphoribosylformiminoAICAR-phosphate, which is then converted to phosphoribulosylformimino-AICAR-P by the His6 gene product.[20] His7 splits phosphoribulosylformimino-AICAR-P to formd-erythro-imidazole-glycerol-phosphate. After, His3 forms imidazole acetol-phosphate releasing water. His5 then makesl-histidinol-phosphate, which is then hydrolyzed by His2 makinghistidinol.His4 catalyzes the oxidation ofl-histidinol to forml-histidinal, an amino aldehyde. In the last step,l-histidinal is converted tol-histidine.[20][21]
The histidine biosynthesis pathway has been studied in the fungusNeurospora crassa, and a gene (His-3) encoding amultienzyme complex was found that was similar to theHis4 gene of the bacteriumE. coli.[22] A genetic study ofN. crassa histidinemutants indicated that the individual activities of the multienzyme complex occur in discrete, contiguous sections of theHis-3genetic map, suggesting that the different activities of the multienzyme complex are encoded separately from each other.[22] However, mutants were also found that lacked all three activities simultaneously, suggesting that some mutations cause loss of function of the complex as a whole.
Just like animals and microorganisms, plants need histidine for their growth and development.[11] Microorganisms and plants are similar in that they can synthesize histidine.[23] Both synthesize histidine from the biochemical intermediate phosphoribosyl pyrophosphate. In general, the histidine biosynthesis is very similar in plants and microorganisms.[24]
This pathway requires energy in order to occur therefore, the presence of ATP activates the first enzyme of the pathway, ATP-phosphoribosyl transferase (shown as His1 in the image on the right). ATP-phosphoribosyl transferase is the rate determining enzyme, which is regulated through feedback inhibition meaning that it is inhibited in the presence of the product, histidine.[25]
Histidine is one of the amino acids that can be converted to intermediates of the tricarboxylic acid (TCA) cycle (also known as the citric acid cycle).[26] Histidine, along with other amino acids such as proline and arginine, takes part in deamination, a process in which its amino group is removed. Inprokaryotes, histidine is first converted to urocanate by histidase. Then, urocanase converts urocanate to 4-imidazolone-5-propionate. Imidazolonepropionase catalyzes the reaction to formformiminoglutamate (FIGLU) from 4-imidazolone-5-propionate.[27] The formimino group is transferred totetrahydrofolate, and the remaining five carbons form glutamate.[26] Overall, these reactions result in the formation of glutamate and ammonia.[28] Glutamate can then be deaminated byglutamate dehydrogenase or transaminated to form α-ketoglutarate.[26]
TheFood and Nutrition Board (FNB) of theU.S. Institute of Medicine setRecommended Dietary Allowances (RDAs) foressential amino acids in 2002. For histidine, for adults 19 years and older, 14 mg/kg body weight/day.[33] Supplemental histidine is being investigated for use in a variety of different conditions, including neurological disorders, atopic dermatitis, metabolic syndrome, diabetes, uraemic anaemia, ulcers, inflammatory bowel diseases, malignancies, and muscle performance during strenuous exercise.[34]
'^pros' inIUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025.https://doi.org/10.1351/goldbook.P04890
^Mrozek, Agnieszka; Karolak-Wojciechowska, Janina; Kieć-Kononowicz, Katarzyna (2003). "Five-membered heterocycles. Part III. Aromaticity of 1,3-imidazole in 5+n hetero-bicyclic molecules".Journal of Molecular Structure.655 (3):397–403.Bibcode:2003JMoSt.655..397M.doi:10.1016/S0022-2860(03)00282-5.
^Cheng, Yongsong; Zhou, Yunjiao; Yang, Lei; Zhang, Chenglin; Xu, Qingyang; Xie, Xixian; Chen, Ning (2013-05-01). "Modification of histidine biosynthesis pathway genes and the impact on production of L-histidine in Corynebacterium glutamicum".Biotechnology Letters.35 (5):735–741.doi:10.1007/s10529-013-1138-1.ISSN1573-6776.PMID23355034.S2CID18380727.
^abcBoard review series (BRS)-- Biochemistry, Molecular Biology, and Genetics (fifth edition): Swanson, Kim, Glucksman
