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{{Short description|Oxygen-carrying phytoglobin found in rhizome of leguminous plants}}
{{Infobox protein family
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▲|image=Leghemoglobin 1FSL.png|caption=Leghemoglobin A from [[soybean]] ({{PDB|1FSL}}).
▲|Symbol=3rd Leghaemoglobin_Fe_BS
|PROSITE=PS00208
|InterPro=IPR019824
}}
'''
In plants colonised by ''Rhizobium'', such as [[alfalfa]] or [[soybean]]s, the presence of [[oxygen]] in the root nodules would reduce the activity of the oxygen-sensitive [[nitrogenase]], which is an enzyme responsible for the fixation of atmospheric nitrogen. Leghemoglobin is shown to buffer the concentration of free oxygen in the [[cytoplasm]] of infected plant cells to ensure the proper function of root nodules. That being said, nitrogen fixation is an extremely energetically costly process, so [[aerobic respiration]], which necessitates high oxygen concentration, is necessary in the cells of the root nodule.<ref>{{cite book |last1=Berg
Leghemoglobin falls into the class of '''symbiotic globins''', which also include the root nodules globins of [[actinorhizal plant]]s such as ''[[Casuarina]]''. The ''Casuarina'' symbiotic globin is intermediate between leghemoglobin and nonsymbiotic phytoglobin-2.<ref name="phytoglobin" /><ref name="Hill16">{{cite journal |vauthors=Hill R, Hargrove MS, Arredondo-Peter R |title=Phytoglobin: a novel nomenclature for plant globins accepted by the globin community at the 2014 XVIII conference on Oxygen-Binding and Sensing Proteins |journal=F1000Research |date=2016 |volume=5 |page=212 |doi=10.12688/f1000research.8133.1 |pmid=26998237 |pmc=4792203 |doi-access=free }}</ref>
== Structure ==
Leghemoglobins are monomeric proteins with a mass around 16 kDa, and are structurally similar to [[myoglobin]].<ref name = Singh>Singh S., Varma A. (2017) Structure, Function, and Estimation of Leghemoglobin. In: Hansen A., Choudhary D., Agrawal P., Varma A. (eds) Rhizobium Biology and Biotechnology. Soil Biology, vol 50. Springer, Cham</ref>
[[File:Lba Oxygen Stabilization.png|thumb|379x379px|Oxygen Stabilization of Leghemoglobin A (PDB: 1BIN)]]
Oxygen [[binding affinities]] of leghemoglobins are between 11 and 24 times higher than oxygen binding affinities of sperm whale myoglobin.<ref name="ReferenceA">{{cite journal |title=The structure of deoxy- and oxy-leghaemoglobin from lupin |vauthors=Harutyunyan EH, Safonova TN, Kuranova IP, Popov AN, Teplyakov AV, Obmolova GV, Rusakov AA, Vainshtein BK, Dodson GG, Wilson JC}}{{fcn|date=May 2023}}</ref>
In the primary structure of Leghemoglobin A in soybeans, a valine(F7) is found in place where a serine(F7) is in Myoglobin. Without a hydrogen bond fixing the orientation of the proximal histidine side chain the imidazole ring can occupy a staggered conformation between pyrrole nitrogen atoms and can readily move upward to the heme plane. This greatly increases the reactivity of the iron atom and oxygen affinity. In Leghemoglobin A the distal histidine side chain is also rotated away from the bound ligand by formation of a hydrogen bond with Tyrosine.<ref>{{Cite journal |last1=Smagghe |first1=Benoit J. |last2=Hoy |first2=Julie A. |last3=Percifield |first3=Ryan |last4=Kundu |first4=Suman |last5=Hargrove |first5=Mark S. |last6=Sarath |first6=Gautam |last7=Hilbert |first7=Jean-Louis |last8=Watts |first8=Richard A. |last9=Dennis |first9=Elizabeth S. |last10=Peacock |first10=W. James |last11=Dewilde |first11=Sylvia |last12=Moens |first12=Luc |last13=Blouin |first13=George C. |last14=Olson |first14=John S. |last15=Appleby |first15=Cyril A. |date=December 2009 |title=Review: correlations between oxygen affinity and sequence classifications of plant hemoglobins |journal=Biopolymers |volume=91 |issue=12 |pages=1083–1096 |doi=10.1002/bip.21256 |issn=0006-3525 |pmid=19441024|s2cid=1891302 }}</ref>
Heme groups are the same in all known leghemoglobins, but the amino acid sequence of the globin differs slightly depending on bacterial strain and legume species.<ref name=Singh/>
▲Harutyunyan EH, Safonova TN, Kuranova IP, Popov AN, Teplyakov AV, Obmolova GV, Rusakov AA, Vainshtein BK, Dodson GG, Wilson JC</ref> Differences in the affinities are due to differential rates of association between the two types of proteins.<ref name="ReferenceA"/> One explanation of this phenomenon is that in myoglobin, a bound water molecule is stabilized in a pocket surrounding the heme group. This water group must be displaced in order for oxygen to bind. No such water is bound in the analogous pocket of leghemoglobin, so it is easier for an oxygen molecule to approach the leghemoglobin heme.<ref name=Singh/> Leghemoglobin has a slow oxygen dissociation rate, similar to myoglobin.<ref name=Wittenberg>Wittenberg J. B., Appleby C. A., Wittenberg B. A. (1972) J. Biol. Chem. 247:527–531. https://www.jbc.org/content/247/2/527.short</ref> Like myoglobin and hemoglobin, leghemoglobin has a high affinity for carbon monoxide.<ref name=Wittenberg/>
▲Heme groups are the same in all known leghemoglobins, but the amino acid sequence of the globin differs slightly depending on bacterial strain and legume species.<ref name=Singh/> Even within one leguminous plant, multiple [[Protein isoform|isoforms]] of leghemoglobins can exist. These often differ in oxygen affinity, and help meet the needs of a cell in a particular environment within the nodule.<ref>Kawashima K, Suganuma N, Tamaoki M, Kouchi H. Two types of pea leghemoglobin genes showing different O2-binding affinities and distinct patterns of spatial expression in nodules. Plant Physiol. 2001;125(2):641–651. doi:10.1104/pp.125.2.641</ref>
== Debate on principal function ==
Results of a 1995 study suggested that the low free oxygen concentration in root nodule cells is actually due to the low oxygen permeability of root nodule cells.<ref>{{cite book |
Plants contain both symbiotic and nonsymbiotic hemoglobins. Symbiotic hemoglobins are thought to be important for symbiotic nitrogen fixation (SNF). In legume, SNF takes place in specialized organs called nodules which contain bacteroids, or nitrogen fixing rhizobia. The induction of nodule-specific plant genes, which include those that encode for symbiotic leghemoglobins (Lb), accompany nodule development. Leghemoglobins accumulate to millimolar concentrations in the cytoplasm of infected plant cells prior to nitrogen fixation to buffer free oxygen in the nanomolar range, which can avoid inactivation of oxygen-labile nitrogenase while keeping a high enough oxygen flux for respiration in the cell. The leghemoglobins are required for SNF but are not required for plant growth and development in the presence of an external source of fixed nitrogen. Leghemoglobins make the essential contribution of establishing low free-oxygen concentrations while keep a high energy status in cells. These are the conditions necessary for effective SNF.<ref name=pubmed.15797021/>
== Other plant hemoglobins ==
{{main|Phytoglobin}}
Globins have since been identified as a protein common to many plant taxa, not restricted to symbiotic ones. In light of this discovery, it has been proposed that the term phytoglobins be used for referring to plant globins in general.<ref name="phytoglobin">{{cite journal |last1=Becana |first1=Manuel |last2=Yruela |first2=Inmaculada |last3=Sarath |first3=Gautam |last4=Catalán |first4=Pilar |last5=Hargrove |first5=Mark S. |title=Plant hemoglobins: a journey from unicellular green algae to vascular plants |journal=New Phytologist |date=September 2020 |volume=227 |issue=6 |pages=1618–1635 |doi=10.1111/nph.16444|pmid=31960995 |doi-access=free |hdl=10261/219101 |hdl-access=free }}</ref>
Phytoglobins can be divided into two clades. The 3/3-fold type contains Classes I and II of angiosperm phytoglobins, and is the one common to all eukaryotes ([[horizontal gene transfer|HGT]] of a bacterial flavohemoglobin). The leghemoglobin ''sensu stricto'' is a class II phytoglobin. The 2/2-fold "TrHb2" type contains class III in angiosperm nomenclature, and appears to be acquired from [[Chloroflexota]] (formerly Chloroflexi) by the ancestor of land plants.<ref name="phytoglobin"/>
== Commercial use ==
[[Impossible Foods]] asked the American [[Food and Drug Administration|FDA]] for their approval to use recombinant soy leghemoglobin in foods as an [[Functional analog (chemistry)|analog]] of meat-derived [[hemoglobin]].<ref>{{Cite web |title=GRAS Notice 540 |url=https://www.accessdata.fda.gov/scripts/fdcc/index.cfm?set=GRASNotices&id=540
== See also ==
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== References ==
{{Reflist}}
== Notes ==
{{notelist}}
== Further reading ==
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*[https://www.nytimes.com/2017/08/08/business/impossible-burger-food-meat.html? Impossible Burger’s ‘Secret Sauce’ Highlights Challenges of Food Tech]
*[https://www.fda.gov/food/cfsan-constituent-updates/fda-announces-effective-date-final-rule-adding-soy-leghemoglobin-list-color-additives-exempt Updates FDA Announces Effective Date for Final Rule Adding Soy Leghemoglobin to List of Color Additives Exempt from Certification]
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