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From Proteopedia
Leghemoglobin
Introduction
Leghemoglobin is a protein that is required by Legume plants in order to fix nitrogen by participating in the Nitrogen Fixation pathway. It is found in legumes such as cowpea, soybeans, alfalfa, and other different types of beans. Leghemoglobin functions by keeping the oxygen at a specific low concentration to the bacteroids to allow for respiration, but at the same time, this low concentration keeps the oxygen out of the nitrogen cycle so it doesn’t inhibit the nitrogenase activity (Ref 1).
Leghemoglobin is a globin protein, constituted by a heme group(protoporphyrin XI) and a single polypeptide, which binds oxygen for transportation. (Ref 5). It is hypothesized that Leghemoglobin can be formed by one of the two ways:
*# The Rhizobium bacteroid (Ref 2) is thought to synthesize the heme group in the root nodules of the legume, where it then gives the plant the heme to complete the synthesizing of the Lb (Ref 1).
*# The plant itself has also been thought to possibly produce the heme itself, in the mytochondria of the plant cells. It then combines with the peptide sequence to complete the whole protein (Ref 1).
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Function
Structure
The 16 kDa polypeptide leghemoglobin consists of two main subunits, the main globin structure and the iron-encompassing heme (protoporphyrin XI) group. These two subunits form a molecule structurally similar to Myoglobin (Ref 1). The globin fold portion of the molecule is the standard globin secondary structure, a series of 8 alpha helices (Ref 5). Depending on the species and specific type of Leghemoglobin, the primary amino acid sequence can differ some, but overall it is well conserved. The heme group, however, has been found to be largely the same in different proteins and in different species (Ref 1). The main difference is the heme group is considerably larger in Leghemoglobin than its other oxygen-transferring globin counterparts. This makes the oxygen affinity larger than that of Myoglobin and Hemoglobin (Ref 8).
This heme prosthetic group consists of four 5-membered pyrrole rings, forming a cyclic ring around a central iron (Fe) atam. This iron atom is contained within four equatorial nitrogens, in addition to another nitrogen from a close histidine residue and an opposite dioxygen (Ref 9). More specifically, the ligand contact residues are Phe30, His63, and Val67 (Ref 7). ***Add a section about H-bonding of the ligand with the Lb and how the heme group fits into the molecule***
The methods by which various Leghemoglobins were purified, and then analyzed, are as follows:
- Ammonium Sulfate Precipitation
- Concentration by Ultrafiltration
- Electrophoresis
- Anion-exchange Column Chromatography
- Isoelectrofocusing
- Nuclear Magnetic Radiance (NMR)
- X-ray crystallography (Ref 1 and 6)
