Editing Leghemoglobin (section)

== 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> One leghemoglobin protein consists of a heme bound to an iron, and one polypeptide chain (the globin).<ref name=Singh/> Similar to myoglobin and hemoglobin, the iron of heme is found in its [[ferrous]] state in vivo, and is the moiety that binds oxygen.<ref name=Singh/> Despite similarities in the mechanism of oxygen binding between leghemoglobin and animal hemoglobin, and the fact that leghemoglobin and animal hemoglobin evolved from a common ancestor, there is dissimilarity in amino acid sequence between these proteins at about 80% of positions.<ref name=Singh/>
[[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> 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>{{cite journal |vauthors=Wittenberg JB, Appleby CA, Wittenberg BA |date=January 1972 |title=The Kinetics of the Reactions of Leghemoglobin with Oxygen and Carbon Monoxide |journal=Journal of Biological Chemistry |volume=247 |issue=2 |pages=527–531 |doi=10.1016/S0021-9258(19)45734-7 |pmid=4333266 |doi-access=free |url=https://www.jbc.org/article/S0021-9258(19)45734-7/pdf}}</ref> Like myoglobin and hemoglobin, leghemoglobin has a high affinity for carbon monoxide.<ref name=Wittenberg/>

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/> 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>{{cite journal |vauthors=Kawashima K, Suganuma N, Tamaoki M, Kouchi H |title=Two types of pea leghemoglobin genes showing different O2-binding affinities and distinct patterns of spatial expression in nodules |journal=Plant Physiol. |date=2001 |volume=125 |issue=2 |pages=641–651 |doi=10.1104/pp.125.2.641|pmid=11161022 |pmc=64866 }}</ref>