Leghemoglobin

The oxygen carrier leghemoglobin (also "legoglobin") is 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, so-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 Harv|O'Brian et al.|1987 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 Harv|Santana et al.|1998.

Leghemoglobin has a high affinity for oxygen (a K_m of about 0.01 µM), about ten times higher than the β chain of human hemoglobin.

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.

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 (Denison and Harter 1995). 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 (Ludwig and de Vries 1986).

"See also: hemoglobin, myoglobin"

External links

* [http://www-saps.plantsci.cam.ac.uk/records/rec193.htm Chemistry of leghaemoglobin]
*

References

Harvard reference|Surname1=O'Brian|Given1=MR|Surname2=Kirshbom|Given2=PM|Surname3=Maier|Given3=RJ
Year=1987|Title=Bacterial heme synthesis is required for expression of the leghemoglobin holoprotein but not the apoprotein in soybean root nodules|Journal=PNAS|Volume=84|Issue=23|Page=8390-3

Harvard reference|Surname1=Santana|Given1=MA|Surname2=Pihakaski-Maunsbach|Given2=K|Surname3=Sandal|Given3=N
Surname4=Marcker|Given4=KA|Surname5=Smith|Given5=AG
Year=1998|Title=Evidence that the plant host synthesizes the heme moiety of leghemoglobin in root nodules|Journal=Plant Physiol|Volume=116|Issue=4|Page=1259-69

cite journal
title = Biological Nitrogen Fixation
author = Artturi Ilmari Virtanen
journal = Annual Review of Microbiology
volume = 2
year = 1948
pages = 485–506
doi = 10.1146/annurev.mi.02.100148.002413

Citation
last1 = Taiz
first1 = Lincoln
last2 = Zeiger
first2 = Eduardo
edition = 3rd
url = http://4e.plantphys.net/
accessdate = 2008-05-03
pages = 269