Leghemoglobin
Encyclopedia
Leghemoglobin is a nitrogen or oxygen carrier, because naturally occurring oxygen and nitrogen interact similarly with this protein; and a hemoprotein
found in the nitrogen-fixing
root nodules of leguminous plants. But nitrogen is necessary for the cycle to occur. It is produced by legumes in response to the roots being infected by nitrogen-fixing bacteria, termed 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 soybean
s), 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.
Other plants, like Casuarina
sp which is a actinorhizal plant
, produce a hemoglobin in their symbiotic root nodules.
Hemoprotein
A hemeprotein , or heme protein, is a metalloprotein containing a heme prosthetic group- an organic compound that allows a protein to carry out a function that it cannot do alone....
found in the nitrogen-fixing
Nitrogen fixation
Nitrogen fixation is the natural process, either biological or abiotic, by which nitrogen in the atmosphere is converted into ammonia . This process is essential for life because fixed nitrogen is required to biosynthesize the basic building blocks of life, e.g., nucleotides for DNA and RNA and...
root nodules of leguminous plants. But nitrogen is necessary for the cycle to occur. It is produced by legumes in response to the roots being infected by nitrogen-fixing bacteria, termed rhizobia
Rhizobia
Rhizobia are soil bacteria that fix nitrogen after becoming established inside root nodules of legumes . Rhizobia require a plant host; they cannot independently fix nitrogen...
, as part of the symbiotic
Symbiosis
Symbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...
interaction between plant and bacterium: roots uninfected with Rhizobium do not synthesise leghemoglobin. Leghemoglobin has close chemical and structural similarities to hemoglobin
Hemoglobin
Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...
, and, like hemoglobin, is red in colour. The protein was believed to be a product of both plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
and the bacterium in which the apoprotein
Apoprotein
Apoprotein can refer to:*Apoenzyme, the protein part of an enzyme without its characteristic prosthetic group.*Apolipoprotein, a lipid-binding protein that is a constituent of the plasma lipoprotein....
is produced by the plant and the heme
Heme
A heme or haem is a prosthetic group that consists of an iron atom contained in the center of a large heterocyclic organic ring called a porphyrin. Not all porphyrins contain iron, but a substantial fraction of porphyrin-containing metalloproteins have heme as their prosthetic group; these are...
(an iron atom bound in a porphyrin
Porphyrin
Porphyrins are a group of organic compounds, many naturally occurring. One of the best-known porphyrins is heme, the pigment in red blood cells; heme is a cofactor of the protein hemoglobin. Porphyrins are heterocyclic macrocycles composed of four modified pyrrole subunits interconnected at...
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
Alfalfa
Alfalfa is a flowering plant in the pea family Fabaceae cultivated as an important forage crop in the US, Canada, Argentina, France, Australia, the Middle East, South Africa, and many other countries. It is known as lucerne in the UK, France, Australia, South Africa and New Zealand, and known as...
or soybean
Soybean
The soybean or soya bean is a species of legume native to East Asia, widely grown for its edible bean which has numerous uses...
s), the presence of oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
in the root nodules would reduce the activity of the oxygen-sensitive nitrogenase
Nitrogenase
Nitrogenases are enzymes used by some organisms to fix atmospheric nitrogen gas . It is the only known family of enzymes that accomplish this process. Dinitrogen is quite inert because of the strength of its N-N triple bond...
- an enzyme responsible for the fixation of atmospheric nitrogen. Leghemoglobin buffers the concentration of free oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
in the cytoplasm
Cytoplasm
The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within 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
Cellular respiration
Cellular respiration is the set of the metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate , and then release waste products. The reactions involved in respiration are catabolic reactions that involve...
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.
Other plants, like Casuarina
Casuarina
Casuarina is a genus of 17 species in the family Casuarinaceae, native to Australasia, southeast Asia, and islands of the western Pacific Ocean. It was once treated as the sole genus in the family, but has been split into three genera .They are evergreen shrubs and trees growing to 35 m tall...
sp which is a actinorhizal plant
Actinorhizal plant
Actinorhizal plants are a group of angiosperms characterized by their ability to form a symbiosis with the nitrogen fixing actinobacteria Frankia...
, produce a hemoglobin in their symbiotic root nodules.