Editing Histidine (section)

===Ligand===
[[Image:Succinate Dehygrogenase 1YQ3 Haem group.png|thumb|left|The histidine-bound [[heme]] group of [[succinate dehydrogenase]], an [[electron carrier]] in the [[mitochondria]]l [[electron transfer chain]]. The large semi-transparent sphere indicates the location of the [[iron]] [[ion]]. From {{PDB|1YQ3}}.|205x205px]]
[[Image:Cu3Im8laccase.png|thumb|left|The tricopper site found in many [[laccase]]s, notice that each [[copper]] center is bound to the [[imidazole]] sidechains of histidine (color code: copper is brown, [[nitrogen]] is blue).]]
Histidine forms [[amino acid complex|complexes]] with many metal ions. The imidazole sidechain of the histidine residue commonly serves as a [[ligand]] in [[metalloprotein]]s. 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.<ref>{{Cite book|last1=Bornhorst|first1=J. A.|last2=Falke|first2=J. J.|chapter=Purification of proteins using polyhistidine affinity tags |date=2000|title=Applications of Chimeric Genes and Hybrid Proteins Part A: Gene Expression and Protein Purification|series=Methods in Enzymology|volume=326|pages=245–254|doi=10.1016/s0076-6879(00)26058-8|issn=0076-6879|pmc=2909483|pmid=11036646|isbn=978-0-12-182227-9 }}</ref> Natural poly-histidine peptides, found in the venom of the viper ''[[Atheris squamigera]]'' have been shown to bind Zn(II), Ni(II) and Cu(II) and affect the function of venom metalloproteases.<ref>{{Cite journal|last1=Watly|first1=Joanna|last2=Simonovsky|first2=Eyal|last3=Barbosa|first3=Nuno|last4=Spodzieja|first4=Marta|last5=Wieczorek|first5=Robert|last6=Rodziewicz-Motowidlo|first6=Sylwia|last7=Miller|first7=Yifat|last8=Kozlowski|first8=Henryk|date=2015-08-17|title=African Viper Poly-His Tag Peptide Fragment Efficiently Binds Metal Ions and Is Folded into an α-Helical Structure|url=https://pubmed.ncbi.nlm.nih.gov/26214303|journal=Inorganic Chemistry|volume=54|issue=16|pages=7692–7702|doi=10.1021/acs.inorgchem.5b01029|issn=1520-510X|pmid=26214303}}</ref>

N-terminal histidines are known to function as [[bidentate]] ligands, with a metal (generally copper) bound to both the amine of the [[N-terminus]] and the N<sub>ε</sub> of the histidine; the N<sub>δ</sub> is often methylated.<ref name="Walton2023">{{Cite journal |last1=Walton |first1=Paul H. |last2=Davies |first2=Gideon J. |last3=Diaz |first3=Daniel E. |last4=Franco-Cairo |first4=João P. |date=2023 |title=The histidine brace: nature's copper alternative to haem? |journal=FEBS Letters |language=en |volume=597 |issue=4 |pages=485–494 |doi=10.1002/1873-3468.14579 |issn=1873-3468 |pmc=10952591 |pmid=36660911}}</ref> Although recently discovered,<ref>{{Cite journal |last1=Quinlan |first1=R. Jason |last2=Sweeney |first2=Matt D. |last3=Lo Leggio |first3=Leila |last4=Otten |first4=Harm |last5=Poulsen |first5=Jens-Christian N. |last6=Johansen |first6=Katja Salomon |last7=Krogh |first7=Kristian B. R. M. |last8=Jørgensen |first8=Christian Isak |last9=Tovborg |first9=Morten |last10=Anthonsen |first10=Annika |last11=Tryfona |first11=Theodora |last12=Walter |first12=Clive P. |last13=Dupree |first13=Paul |last14=Xu |first14=Feng |last15=Davies |first15=Gideon J. |date=2011-09-13 |title=Insights into the oxidative degradation of cellulose by a copper metalloenzyme that exploits biomass components |journal=Proceedings of the National Academy of Sciences |volume=108 |issue=37 |pages=15079–15084 |doi=10.1073/pnas.1105776108 |doi-access=free |pmc=3174640 |pmid=21876164}}</ref> 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.<ref>{{Cite journal |last1=Munzone |first1=Alessia |last2=Eijsink |first2=Vincent G. H. |last3=Berrin |first3=Jean-Guy |last4=Bissaro |first4=Bastien |date=February 2024 |title=Expanding the catalytic landscape of metalloenzymes with lytic polysaccharide monooxygenases |url=https://www.nature.com/articles/s41570-023-00565-z |journal=Nature Reviews Chemistry |language=en |volume=8 |issue=2 |pages=106–119 |doi=10.1038/s41570-023-00565-z |pmid=38200220 |issn=2397-3358|url-access=subscription }}</ref> It is proposed that the evolution of these enzymes in fungi corresponds to the first widespread ability to decompose woody plant mass, leading to the end of the [[Carboniferous|Carboniferous era]] and its mass [[Carboniferous#Coal formation|accumulation of coal deposits]].<ref name="Walton2023" />