page_title: Leghemoglobin
section_title: Structure
hierarchical_section_title: Leghemoglobin / Structure
caption_attribution_description: English: Cartoons of three oxygenated globin molecules- leghemoglobin, hemoglobin, and myoglobin. Oxygen molecules are shown in gray.
context_page_description: Leghemoglobin is an oxygen carrier and hemoprotein found in the nitrogen-fixing root nodules of leguminous plants. It is produced by these plants in response to the roots being colonized by nitrogen-fixing bacteria, termed rhizobia, as part of the symbiotic interaction between plant and bacterium: roots not colonized by Rhizobium do not synthesise leghemoglobin. Leghemoglobin has close chemical and structural similarities to hemoglobin, and, like hemoglobin, is red in colour. It was originally thought that the heme prosthetic group for plant leghemoglobin was provided by the bacterial symbiont within symbiotic root nodules. However, subsequent work shows that the plant host strongly expresses heme biosynthesis genes within nodules, and that activation of those genes correlates with leghemoglobin gene expression in developing nodules.
  In plants colonised by Rhizobium, such as alfalfa or soybeans, 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.
context_section_description: Leghemoglobins are monomeric proteins with a mass around 16 kDa, and are structurally similar to myoglobin.  One leghemoglobin protein consists of a heme bound to an iron, and one polypeptide chain (the globin). Similar to myoglobin and hemoglobin, the iron of heme is found in its ferrous state in vivo, and is the moiety that binds oxygen.  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.
  Oxygen binding affinities of leghemoglobins are between 11 and 24 times higher than oxygen binding affinities of sperm whale myoglobin.  Differences in the affinities are due to differential rates of association between the two types of proteins.  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. Leghemoglobin has a slow oxygen dissociation rate, similar to myoglobin. Like myoglobin and hemoglobin, leghemoglobin has a high affinity for carbon monoxide.
  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.   Even within one leguminous plant, multiple 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.
caption: Cartoons of three oxygenated globin molecules.  Oxygens (shown in gray) bind similarly to heme in each.
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