Leghemoglobin
Leghemoglobin are globins which occur in the nitrogen-fixing root nodules of legumes (legumes). Several paralogous isoforms are formed that contain between 145 and 150 amino acids.
function
If nitrogen-fixing nodule bacteria infect a root of a plant, they stimulate it to produce leghemoglobin. Uninfected parts of the root do not produce the substance. Leghemoglobin is evolutionarily closely related to myoglobin and hemoglobin and has a very similar tertiary structure. Like other globular heme proteins, it has the ability to reversibly bind oxygen . Leghemoglobin ensures that there is a very low concentration of free oxygen in the root nodules. This is essential for a functioning symbiosis with the nodule bacteria. The nitrogenase complex of bacteria is extremely sensitive to oxygen, as this can inactivate the complex and thus the nitrogen fixation even in low concentrations .
Leghemoglobin has an affinity for oxygen approximately ten times that of the β chain of human hemoglobin. As a result, the oxygen concentration in the plant tissue and in the bacteria is low enough not to deactivate the nitrogenase. The bacteria and the plant tissue can still obtain enough oxygen for cell respiration . The functioning of leghemoglobin is thus directly comparable to that of hemoglobin.
It used to be believed that all of the protein was synthesized by the plant. O'Brian et al. were able to show, however, that both plants and bacteria are involved in the synthesis of the protein. The plant produces the apoprotein while the bacterium synthesizes the heme . Other research even suggests that some of the globin is made by the bacteria.
Evolutionary family tree
Analysis of the amino acid sequence of the globins shows the relationship of leghemoglobin to myoglobin and hemoglobin. The first splitting of globins into the two classes leghemoglobins and hemoglobins took place around 800 million years ago.
| Leghemoglobin | |||||||||||||||
| Myoglobin | |||||||||||||||
| Hemoglobin α | |||||||||||||||
| Hemoglobin β | |||||||||||||||
Individual evidence
- ↑ O'Brian MR, Kirshbom PM, Maier RJ, et al : Bacterial heme synthesis is required for expression of the leghemoglobin holoprotein but not the apoprotein in soybean root nodules . In: PNAS . 84, No. 23, 1987, pp. 8390-8393. PMC 299548 (free full text).
- ↑ Santana MA, Pihakaski-Maunsbach K, Sandal N ,: Evidence that the plant host synthesizes the heme moiety of leghemoglobin in root nodules . In: Plant Physiol . 116, No. 4, 1989, pp. 1259-1269. PMC 35032 (free full text).
literature
- Jeremy M. Berg, John L. Tymoczko, Lubert Stryer: Biochemistry. 6th Edition Freeman, New York 2007. ISBN 0-7167-8724-5
