Reviewed by: Mary Anne Clark, PhD
Leghemoglobin is an oxygen-binding protein produced by legumes. Although it is not closely related to the hemoglobin found in vertebrates, it does serve a similar function by binding oxygen. However, the job of hemoglobin is to transport oxygen from the lungs or gills to other body tissues, and has to both pick up and let go of its oxygen. The job of leghemoglobin, on the other hand, is similar to that of myoglobin, which serves as an oxygen reservoir in muscle tissue. Binding the oxygen keeps it available for Rhizobium, the symbiotic bacterium living in nodules on the roots of legumes, where they fix nitrogen, i.e. convert atmospheric nitrogen to ammonia. However, it also keeps the oxygen away from the bacterial nitrogenase, to which oxygen is toxic.
The degree of similarity between the plant and animal hemoglobins is shown in this tree produced by the alignment of two leghemoglobins (LGB1) from peas and soybeans with human myglobin (MYG) and beta globin (HBB). Human myoglobin and beta globin are only about 25% identical, and human myoglobin and pea LGB1 are about 19% identical.
Both soybeans and garden peas seem to have copied the leghemoglobin gene and produced multiple versions of it (LGB1, LGB2,etc). This alignment suggests that the gene duplications occurred after the evolutionary divergence of the two species of legumes. Within each species, the LGB sequences shown here are about 75% identical in peas and about 90%.identical in soybeans. On the other hand, the LGB1 sequences of peas and soybeans are only about 64% identical.
Even though the plant leghemoglobins and vertebrate myo/hemoglobins are only about 20% identical, there is evidence that both molecules are descended from a common globin ancestor. First the. leghemoglobins are about the size of the human globins, 148 amino acids in LGB1 and 146 in beta globin, and have a similar folding pattern. But the strongest evidence is from the structure of the genes. The HBA, HBB and myoglobin genes all have three exons and two introns. The two introns of myoglobin and hemoglobin genes precisely match the locations of two of the three introns in the LGB genes.
This ancestry is more remarkable since the ancestral eukaryotic globin gene must have preceded the divergence of the Archaeplastida, the eukaryotic group that contains the plants, from the Opisthokonts, the group that contains the multicellular animals, well over a billion years ago.