Leghaemoglobin, iron-binding site | |
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Identifiers | |
Symbol | Leghaemoglobin_Fe_BS |
InterPro | IPR019824 |
PROSITE | PS00208 |
Leghemoglobin (also leghaemoglobin or legoglobin) is an oxygen-carrying phytoglobin found in the nitrogen-fixing root nodules of leguminous plants. It is produced by these plants 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 is shown to buffer the concentration of free oxygen in the cytoplasm of infected plant cells to ensure the proper function of root nodules. That being said, nitrogen fixation is an extremely energetically costly process, so aerobic respiration, which necessitates high oxygen concentration, is necessary in the cells of the root nodule. [11] Leghemoglobin maintains a free oxygen concentration that is low enough to allow nitrogenase to function, but a high enough total oxygen concentration (free and bound to leghemoglobin) for aerobic respiration.
Leghemoglobin falls into the class of symbiotic globins, which also include the root nodules globins of actinorhizal plants such as Casuarina . The Casuarina symbiotic globin is intermediate between leghemoglobin and nonsymbiotic phytoglobin-2. [12] [13]
Leghemoglobins are monomeric proteins with a mass around 16 kDa, and are structurally similar to myoglobin. [14] One leghemoglobin protein consists of a heme bound to an iron, and one polypeptide chain (the globin). [14] Similar to myoglobin and hemoglobin, the iron of heme is found in its ferrous state in vivo, and is the moiety that binds oxygen. [14] Despite similarities in the mechanism of oxygen binding between leghemoglobin and animal hemoglobin, and the fact that leghemoglobin and animal hemoglobin evolved from a common ancestor, there is dissimilarity in amino acid sequence between these proteins at about 80% of positions. [14]
Oxygen binding affinities of leghemoglobins are between 11 and 24 times higher than oxygen binding affinities of sperm whale myoglobin. [15] Differences in the affinities are due to differential rates of association between the two types of proteins. [15] One explanation of this phenomenon is that in myoglobin, a bound water molecule is stabilized in a pocket surrounding the heme group. This water group must be displaced in order for oxygen to bind. No such water is bound in the analogous pocket of leghemoglobin, so it is easier for an oxygen molecule to approach the leghemoglobin heme. [14] Leghemoglobin has a slow oxygen dissociation rate, similar to myoglobin. [16] Like myoglobin and hemoglobin, leghemoglobin has a high affinity for carbon monoxide. [16]
In the primary structure of Leghemoglobin A in soybeans, a valine(F7) is found in place where a serine(F7) is in Myoglobin. Without a hydrogen bond fixing the orientation of the proximal histidine side chain the imidazole ring can occupy a staggered conformation between pyrrole nitrogen atoms and can readily move upward to the heme plane. This greatly increases the reactivity of the iron atom and oxygen affinity. In Leghemoglobin A the distal histidine side chain is also rotated away from the bound ligand by formation of a hydrogen bond with Tyrosine. [17]
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. [14] Even within one leguminous plant, multiple isoforms of leghemoglobins can exist. These often differ in oxygen affinity, and help meet the needs of a cell in a particular environment within the nodule. [18]
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. [19] 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. [20] 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. [20]
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. [12]
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. [12]
Impossible Foods asked the American FDA for their approval to use recombinant soy leghemoglobin in meat alternatives as an analog of meat-derived hemoglobin. [21] [22] Approval from the FDA came in July 2019, [23] was challenged, [a] and later upheld, on May 3, 2021, by a San Francisco federal appeals court. [24] [25] It is currently being used in their products to mimic the color, taste, and texture of meat. [26]
Hemoglobin is a protein containing iron that facilitates the transportation of oxygen in red blood cells. Almost all vertebrates contain hemoglobin, with the sole exception of the fish family Channichthyidae. Hemoglobin in the blood carries oxygen from the respiratory organs to the other tissues of the body, where it releases the oxygen to enable aerobic respiration which powers an animal's metabolism. A healthy human has 12 to 20 grams of hemoglobin in every 100 mL of blood. Hemoglobin is a metalloprotein, a chromoprotein, and globulin.
Nitrogen fixation is a chemical process by which molecular dinitrogen is converted into ammonia. It occurs both biologically and abiologically in chemical industries. Biological nitrogen fixation or diazotrophy is catalyzed by enzymes called nitrogenases. These enzyme complexes are encoded by the Nif genes and contain iron, often with a second metal.
Hemoglobinopathy is the medical term for a group of inherited blood disorders involving the hemoglobin, the protein of red blood cells. They are single-gene disorders and, in most cases, they are inherited as autosomal co-dominant traits.
Myoglobin is an iron- and oxygen-binding protein found in the cardiac and skeletal muscle tissue of vertebrates in general and in almost all mammals. Myoglobin is distantly related to hemoglobin. Compared to hemoglobin, myoglobin has a higher affinity for oxygen and does not have cooperative binding with oxygen like hemoglobin does. Myoglobin consists of non-polar amino acids at the core of the globulin, where the heme group is non-covalently bounded with the surrounding polypeptide of myoglobin. In humans, myoglobin is found in the bloodstream only after muscle injury.
A hemeprotein, or heme protein, is a protein that contains a heme prosthetic group. They are a very large class of metalloproteins. The heme group confers functionality, which can include oxygen carrying, oxygen reduction, electron transfer, and other processes. Heme is bound to the protein either covalently or noncovalently or both.
Heme, or haem, is a ring-shaped iron-containing molecular component of hemoglobin, which is necessary to bind oxygen in the bloodstream. It is composed of four pyrrole rings with 2 vinyl and 2 propionic acid side chains. Heme is biosynthesized in both the bone marrow and the liver.
The globins are a superfamily of heme-containing globular proteins, involved in binding and/or transporting oxygen. These proteins all incorporate the globin fold, a series of eight alpha helical segments. Two prominent members include myoglobin and hemoglobin. Both of these proteins reversibly bind oxygen via a heme prosthetic group. They are widely distributed in many organisms.
Rhizobia are diazotrophic bacteria that fix nitrogen after becoming established inside the root nodules of legumes (Fabaceae). To express genes for nitrogen fixation, rhizobia require a plant host; they cannot independently fix nitrogen. In general, they are gram negative, motile, non-sporulating rods.
Diazotrophs are bacteria and archaea that fix atmospheric nitrogen (N2) in the atmosphere into bioavailable forms such as ammonia.
Ensifer meliloti are an aerobic, Gram-negative, and diazotrophic species of bacteria. S. meliloti are motile and possess a cluster of peritrichous flagella. S. meliloti fix atmospheric nitrogen into ammonia for their legume hosts, such as alfalfa. S. meliloti forms a symbiotic relationship with legumes from the genera Medicago, Melilotus and Trigonella, including the model legume Medicago truncatula. This symbiosis promotes the development of a plant organ, termed a root nodule. Because soil often contains a limited amount of nitrogen for plant use, the symbiotic relationship between S. meliloti and their legume hosts has agricultural applications. These techniques reduce the need for inorganic nitrogenous fertilizers.
Root nodules are found on the roots of plants, primarily legumes, that form a symbiosis with nitrogen-fixing bacteria. Under nitrogen-limiting conditions, capable plants form a symbiotic relationship with a host-specific strain of bacteria known as rhizobia. This process has evolved multiple times within the legumes, as well as in other species found within the Rosid clade. Legume crops include beans, peas, and soybeans.
Nod factors, are signaling molecules produced by soil bacteria known as rhizobia in response to flavonoid exudation from plants under nitrogen limited conditions. Nod factors initiate the establishment of a symbiotic relationship between legumes and rhizobia by inducing nodulation. Nod factors produce the differentiation of plant tissue in root hairs into nodules where the bacteria reside and are able to fix nitrogen from the atmosphere for the plant in exchange for photosynthates and the appropriate environment for nitrogen fixation. One of the most important features provided by the plant in this symbiosis is the production of leghemoglobin, which maintains the oxygen concentration low and prevents the inhibition of nitrogenase activity.
Nitrogenases are enzymes (EC 1.18.6.1EC 1.19.6.1) that are produced by certain bacteria, such as cyanobacteria (blue-green bacteria) and rhizobacteria. These enzymes are responsible for the reduction of nitrogen (N2) to ammonia (NH3). Nitrogenases are the only family of enzymes known to catalyze this reaction, which is a step in the process of nitrogen fixation. Nitrogen fixation is required for all forms of life, with nitrogen being essential for the biosynthesis of molecules (nucleotides, amino acids) that create plants, animals and other organisms. They are encoded by the Nif genes or homologs. They are related to protochlorophyllide reductase.
A respiratory pigment is a metalloprotein that serves a variety of important functions, its main being O2 transport. Other functions performed include O2 storage, CO2 transport, and transportation of substances other than respiratory gases. There are four major classifications of respiratory pigment: hemoglobin, hemocyanin, erythrocruorin–chlorocruorin, and hemerythrin. The heme-containing globin is the most commonly-occurring respiratory pigment, occurring in at least 9 different phyla of animals.
Bradyrhizobium is a genus of Gram-negative soil bacteria, many of which fix nitrogen. Nitrogen fixation is an important part of the nitrogen cycle. Plants cannot use atmospheric nitrogen (N2); they must use nitrogen compounds such as nitrates.
Rhizobacteria are root-associated bacteria that can have a detrimental, neutral or beneficial effect on plant growth. The name comes from the Greek rhiza, meaning root. The term usually refers to bacteria that form symbiotic relationships with many plants (mutualism). Rhizobacteria are often referred to as plant growth-promoting rhizobacteria, or PGPRs. The term PGPRs was first used by Joseph W. Kloepper in the late 1970s and has become commonly used in scientific literature.
Nitric oxide dioxygenase (EC 1.14.12.17) is an enzyme that catalyzes the conversion of nitric oxide (NO) to nitrate (NO−
3) . The net reaction for the reaction catalyzed by nitric oxide dioxygenase is shown below:
The nif genes are genes encoding enzymes involved in the fixation of atmospheric nitrogen into a form of nitrogen available to living organisms. The primary enzyme encoded by the nif genes is the nitrogenase complex which is in charge of converting atmospheric nitrogen (N2) to other nitrogen forms such as ammonia which the organism can use for various purposes. Besides the nitrogenase enzyme, the nif genes also encode a number of regulatory proteins involved in nitrogen fixation. The nif genes are found in both free-living nitrogen-fixing bacteria and in symbiotic bacteria associated with various plants. The expression of the nif genes is induced as a response to low concentrations of fixed nitrogen and oxygen concentrations (the low oxygen concentrations are actively maintained in the root environment of host plants). The first Rhizobium genes for nitrogen fixation (nif) and for nodulation (nod) were cloned in the early 1980s by Gary Ruvkun and Sharon R. Long in Frederick M. Ausubel's laboratory.
In enzymology, a leghemoglobin reductase (EC 1.6.2.6) is an enzyme that catalyzes the chemical reaction
Phytoglobins are globular plant proteins classified into the globin superfamily, which contain a heme, i.e. protoporphyrin IX-Fe, prosthetic group. The earliest known phytoglobins are leghemoglobins, discovered in 1939 by Kubo after spectroscopic and chemical analysis of the red pigment of soybean root nodules. A few decades after Kubo's report the crystallization of a lupin phytoglobin by Vainshtein and collaborators revealed that the tertiary structure of this protein and that of the sperm whale myoglobin was remarkably similar, thus indicating that the phytoglobin discovered by Kubo did indeed correspond to a globin.
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