Leghemoglobin

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Leghemoglobin from soybean.

Leghemoglobin (also leghaemoglobin or legoglobin) is an oxygen carrier and hemoprotein found in the nitrogen-fixing root nodules of leguminous plants. It is produced by legumes in response to the roots being colonized by nitrogen-fixing bacteria, termed rhizobia, as part of the symbiotic interaction between plant and bacterium: roots not colonized by Rhizobium do not synthesise leghemoglobin. Leghemoglobin has close chemical and structural similarities to hemoglobin, and, like hemoglobin, is red in colour. It was originally thought that the heme prosthetic group for plant leghemoglobin was provided by the bacterial symbiont within symbiotic root nodules.[1][2] However, subsequent work shows that the plant host strongly expresses heme biosynthesis genes within nodules, and that activation of those genes correlates with leghemoglobin gene expression in developing nodules.[3][4][5][6][7][8][9][10]

In plants colonised by Rhizobium, such as alfalfa or soybeans, the presence of oxygen in the root nodules would reduce the activity of the oxygen-sensitive nitrogenase, which is 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 (Km ~ 0.01 μM), about ten times higher than the β chain of human hemoglobin. This maintains an oxygen concentration that is low enough to allow nitrogenase to function, but high enough 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.[11] Its function is to help provide oxygen to the respiring symbiotic bacterial cells in a manner analogous to hemoglobin transporting oxygen to respiring tissues in animals.[12]

Other plants, like Casuarina spp., which are actinorhizal plants, produce a hemoglobin in their symbiotic root nodules.[13]

Impossible Foods have asked the American FDA for their approval to use it in foods as an analog of meat-derived hemoglobin.[14][15] Approval from the FDA came in July 2019.[16]

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References[edit]

  1. ^ Nadler KD, Avissar YJ (September 1977). "Heme Synthesis in Soybean Root Nodules: I. On the Role of Bacteroid delta-Aminolevulinic Acid Synthase and delta-Aminolevulinic Acid Dehydrase in the Synthesis of the Heme of Leghemoglobin". Plant Physiology. 60 (3): 433–6. doi:10.1104/pp.60.3.433. PMC 542631. PMID 16660108.
  2. ^ O'Brian MR, Kirshbom PM, Maier RJ (December 1987). "Bacterial heme synthesis is required for expression of the leghemoglobin holoprotein but not the apoprotein in soybean root nodules". Proceedings of the National Academy of Sciences of the United States of America. 84 (23): 8390–3. Bibcode:1987PNAS...84.8390O. doi:10.1073/pnas.84.23.8390. PMC 299548. PMID 3479799.
  3. ^ Sangwan I, O'brian MR (March 1991). "Evidence for an inter-organismic heme biosynthetic pathway in symbiotic soybean root nodules". Science. 251 (4998): 1220–2. Bibcode:1991Sci...251.1220S. doi:10.1126/science.251.4998.1220. PMID 17799282.
  4. ^ Sangwan I, O'brian MR (March 1992). "Characterization of delta-Aminolevulinic Acid Formation in Soybean Root Nodules". Plant Physiology. 98 (3): 1074–9. doi:10.1104/pp.98.3.1074. PMC 1080310. PMID 16668729.
  5. ^ Sangwan I, O'Brian MR (July 1993). "Expression of the soybean (Glycine max) glutamate 1-semialdehyde aminotransferase gene in symbiotic root nodules". Plant Physiology. 102 (3): 829–34. doi:10.1104/pp.102.3.829. PMC 158853. PMID 8278535.
  6. ^ Madsen O, Sandal L, Sandal NN, Marcker KA (October 1993). "A soybean coproporphyrinogen oxidase gene is highly expressed in root nodules". Plant Molecular Biology. 23 (1): 35–43. doi:10.1007/BF00021417. PMID 8219054.
  7. ^ Kaczor CM, Smith MW, Sangwan I, O'Brian MR (April 1994). "Plant delta-aminolevulinic acid dehydratase. Expression in soybean root nodules and evidence for a bacterial lineage of the Alad gene". Plant Physiology. 104 (4): 1411–7. doi:10.1104/pp.104.4.1411. PMC 159307. PMID 8016269.
  8. ^ Frustaci JM, Sangwan I, O'Brian MR (March 1995). "gsa1 is a universal tetrapyrrole synthesis gene in soybean and is regulated by a GAGA element". The Journal of Biological Chemistry. 270 (13): 7387–93. doi:10.1074/jbc.270.13.7387. PMID 7706283.
  9. ^ Santana MA, Pihakaski-Maunsbach K, Sandal N, Marcker KA, Smith AG (April 1998). "Evidence that the plant host synthesizes the heme moiety of leghemoglobin in root nodules". Plant Physiology. 116 (4): 1259–69. doi:10.1104/pp.116.4.1259. PMC 35032. PMID 9536042.
  10. ^ Sangwan I, O'Brian MR (February 1999). "Expression of a soybean gene encoding the tetrapyrrole-synthesis enzyme glutamyl-tRNA reductase in symbiotic root nodules". Plant Physiology. 119 (2): 593–8. doi:10.1104/pp.119.2.593. PMC 32136. PMID 9952455.
  11. ^ Denison RF, Harter BL (April 1995). "Nitrate Effects on Nodule Oxygen Permeability and Leghemoglobin (Nodule Oximetry and Computer Modeling)". Plant Physiology. 107 (4): 1355–1364. doi:10.1104/pp.107.4.1355. PMC 157270. PMID 12228439.
  12. ^ Ludwig RA, de Vries GE (1986). "Biochemical physiology of Rhizobium dinitrogen fixation". In Broughton WJ, Pühler S (eds.). Nitrogen Fixation, Vol. 4: Molecular Biology. Oxford, UK: Clarendon University Press. pp. 50–69. ISBN 978-0-19-854575-0.
  13. ^ Jacobsen-Lyon K, Jensen EO, Jørgensen JE, Marcker KA, Peacock WJ, Dennis ES (February 1995). "Symbiotic and nonsymbiotic hemoglobin genes of Casuarina glauca". The Plant Cell. 7 (2): 213–23. doi:10.1105/tpc.7.2.213. PMC 160777. PMID 7756831.
  14. ^ "GRAS Notice 540". www.accessdata.fda.gov. Retrieved 2018-01-21.
  15. ^ "GRAS Notice 737". www.accessdata.fda.gov. Retrieved 2018-08-22.
  16. ^ "Beyond Meat's competitor Impossible Foods plans to launch in grocery stores in September after getting FDA approval". CNBC. 31 July 2019. Retrieved 31 July 2019.

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