Leghemoglobin

Leghemoglobins are monomeric proteins with a mass around 16 kDa, and are structurally similar to myoglobin. One leghemoglobin protein consists of a heme bound to an iron, and one polypeptide chain (the globin). Differences in the affinities are due to differential rates of association between the two types of proteins. Like myoglobin and hemoglobin, leghemoglobin has a high affinity for carbon monoxide. Heme groups are the same in all known leghemoglobins, but the amino acid sequence of the globin differs slightly depending on bacterial strain and legume species. == Debate on principal function ==

Results of a 1995 study suggested that the low free oxygen concentration in root nodule cells is actually due to the low oxygen permeability of root nodule cells. It follows that the main purpose of leghemoglobin is to scavenge the limited free oxygen in the cell and deliver it to mitochondria for respiration. But, scientists of a later 2005 article suggest that leghemoglobin is responsible both for buffering oxygen concentration, and for delivery of oxygen to mitochondria. Their leghemoglobin knockout studies showed that leghemoglobin actually does significantly decrease the free oxygen concentration in root nodule cells, and that nitrogenase expression was eliminated in leghemoglobin knockout mutants, assumably due to the degradation of nitrogenase with high free oxygen concentration. Their study also showed a higher ATP/ADP ratio in wild-type root nodule cells with active leghemoglobin, suggesting that leghemoglobin also assists with delivery of oxygen for respiration. Plants contain both symbiotic and nonsymbiotic hemoglobins. Symbiotic hemoglobins are thought to be important for symbiotic nitrogen fixation (SNF). In legume, SNF takes place in specialized organs called nodules which contain bacteroids, or nitrogen fixing rhizobia. The induction of nodule-specific plant genes, which include those that encode for symbiotic leghemoglobins (Lb), accompany nodule development. Leghemoglobins accumulate to millimolar concentrations in the cytoplasm of infected plant cells prior to nitrogen fixation to buffer free oxygen in the nanomolar range, which can avoid inactivation of oxygen-labile nitrogenase while keeping a high enough oxygen flux for respiration in the cell. The leghemoglobins are required for SNF but are not required for plant growth and development in the presence of an external source of fixed nitrogen. Leghemoglobins make the essential contribution of establishing low free-oxygen concentrations while keep a high energy status in cells. These are the conditions necessary for effective SNF. == Other plant hemoglobins ==

Globins have since been identified as a protein common to many plant taxa, not restricted to symbiotic ones. In light of this discovery, it has been proposed that the term phytoglobins be used for referring to plant globins in general. Phytoglobins can be divided into two clades. The 3/3-fold type contains Classes I and II of angiosperm phytoglobins, and is the one common to all eukaryotes (HGT of a bacterial flavohemoglobin). The leghemoglobin sensu stricto is a class II phytoglobin. The 2/2-fold "TrHb2" type contains class III in angiosperm nomenclature, and appears to be acquired from Chloroflexota (formerly Chloroflexi) by the ancestor of land plants. == Commercial use ==

Impossible Foods asked the American FDA for their approval to use recombinant soy leghemoglobin made by Pichia pastoris in meat alternatives as an analog of meat-derived hemoglobin. Approval from the FDA came in July 2019, was challenged, and later upheld, on May 3, 2021, by a San Francisco federal appeals court. It is currently being used in their products to mimic the color, taste, and texture of meat. == See also ==