The oxygen carrier leghemoglobin
) 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
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 .
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.
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
Artturi Ilmari Virtanen (1948). "Biological Nitrogen Fixation". Annual Review of Microbiology