Sandbox reserved 457
|
Leghemoglobin (Lb) (also Leghaemoglobin) is an oxygen binding hemeprotein of about 16 kDa in size. Hemeproteins have various functions including oxygen transport, catalysis and active membrane transport among others. It is able to bind oxygen due to the iron containing heme at its center. It occurs in the root nodules of leguminous plants where it facilitates the diffusion of oxygen to ultimately promote nitrogen fixation. It is produced by legumes in response to nitrogen-fixating bacterial infection, rhizobia. Like hemoglobin, Lb is red in color. Bacteroids synthesize the heme, and sometimes mitochondria in situations when the bacteria are defective or when Lb is produced in uninfected cells of nodules. Lb must be in the ferrous, reduced state in order to carry oxygen. Healthy nodules contain about 80% ferrous Lb and 20% oxyferrous Lb. This indicates that a mechanism exists in the nodules that maintains the reduced form of Lb.
MechanismMechanism
Nitrogen fixation by bacteroids provides ammonia, which is integrated by the plant into biomolecules, such as proteins, nucleic acids and alkaloids. The plant then supplies the bacteroids with sugars. The process takes place at the root nodules, which are tumor-like structures formed as a result of the infection of roots by the bacteria. The heme group is synthesized by the bacteroids and the globin is synthesized by the ribosomes of the infected cells. Once this synthesis is complete, the heme and globin are joined in the cytoplasm of the plant cell.
In root nodules the O2 level is regulated by a special hemoglobin - leghemoglobin. The globin protein is encoded by plant genes but the heme cofactor is made by the symbiotic bacteria. This is produced only when the plant is infected with Rhizobium. The plant root cells convert sugar to organic acids which they supply to the bacteroids. In exchange, the plant receives amino-acids (rather than free ammonia).[1]
[2]
FunctionFunction
The ferrous form of Lb has a very high affinity for oxygen. Nitrogenase is the enzyme responsible for nitrogen fixation, however it is inactivated by oxygen. Bacteroids need oxygen for energy in order to reduce nitrogen to ammonia. This process is known as facilitated diffusion. The amount of oxygenated Lb present in the nodules can be estimated using spectroscopy. Deoxygenated ferrous Lb has an absorption maxima at 427 and 555 nm. In comparison, oxygenated ferrous Lb has an absorption maxima at 411, 541 and 575 nm. Ferric Lb exhibits a characteristic peak at 625 nm.[3] In short, leghemoglobin's function is to help provide oxygen to the respiring symbiotic bacterial cells in a way similar to hemoglobin transporting oxygen to respiring tissues in animals. [4]
StructureStructure
The secondary structure contains 2 identical amino acid chains with 143 residues per chain. It also contains 14 alpha helices per chain and no beta sheets. Deoxy-leghemoglobin has a proximal histidine. The imidazole rotates between two different positions, forming an eclipsed position. The proximal histidine increases the steric hindrance, while it has a staggered position in oxy-leghemoglobin. These opposite positions between oxy- and deoxy- leghemoglobin allow for a reduced activation energy for the reaction between leghemoglobin and oxygen. [5]
See AlsoSee Also
ReferencesReferences
- ↑ a-s.clayton.edu/science/Biocomputing/Projects/.../Leghemoglobin.ppt
- ↑ http://www.biosci.ohio-state.edu/~plantbio/osu_pcmb/pcmb_lab_resources/pcmb102_activities/n_fixation/nitrogen_cycle.html
- ↑ http://digital.csic.es/bitstream/10261/4277/1/analesv.21n.3-1995-pp203.pdf
- ↑ Ludwig, R. A.; de Vries, G. E. (1986). "Biochemical physiology of Rhizobium dinitrogen fixation". In Broughton, W. J., & Pühler, S. Nitrogen Fixation, Vol. 4: Molecular Biology. Oxford, UK: Clarendon University Press. pp. 50–69. ISBN 0-19-854575-4.
- ↑ http://www.rcsb.org/pdb/explore/explore.do?structureId=2GDM