Leghemoglobin (also leghaemoglobin or legoglobin) is an oxygen carrier and a hemoprotein found in the nitrogen-fixing root nodules of leguminous plants. It is produced by legumes in response to the roots being infected by nitrogen-fixing bacteria, called rhizobia, as part of the symbiotic interaction between plant and bacterium: roots uninfected with Rhizobium do not synthesise leghemoglobin. Leghemoglobin has close chemical and structural similarities to hemoglobin, and, like hemoglobin, is red in colour. The protein was believed to be a product of both plant and the bacterium in which the apoprotein is produced by the plant and the heme (an iron atom bound in a porphyrin ring) is produced by the bacterium. Newer findings however, indicate that the heme moiety is also produced by the plant.
In plants infected with Rhizobium, (such as alfalfa or soybeans), the presence of oxygen in the root nodules would reduce the activity of the oxygen-sensitive nitrogenase - an enzyme responsible for the fixation of atmospheric nitrogen. Leghemoglobin buffers the concentration of free oxygen in the cytoplasm of infected plant cells to ensure the proper function of root nodules. Leghemoglobin has a high affinity for oxygen (a Km of about 0.01 µM), about ten times higher than the β chain of human hemoglobin. This allows an oxygen concentration that is low enough to allow nitrogenase to function but high enough so that it can provide the bacteria with oxygen for respiration.
Although leghemoglobin was once thought to provide a buffer for nodule oxygen, recent studies indicate that it stores only enough oxygen to support nodule respiration for a few seconds. Its function is to help transport oxygen to the respiring symbiotic bacterial cells in a manner analogous to hemoglobin transporting oxygen to respiring tissues in animals.
See also: hemoglobin, myoglobin
Virtanen, A. I. (1948). "Biological Nitrogen Fixation.". Annu. Rev. Microbiol. 2: 485–506. doi:10.1146/annurev.mi.02.100148.002413. PMID 18122253. http://arjournals.annualreviews.org/doi/abs/10.1146%2Fannurev.mi.02.100148.002413.
Taiz, L.; Zeiger, E. (2006). Plant Physiology Online (3rd ed.). Sunderland, MA: Sinauwr Associates, Inc. pp. 269. ISBN 0-87893-856-7. http://4e.plantphys.net/. Retrieved 2008-05-03.
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