A major contributor to this article appears to have a close connection with its subject.(April 2017) |
Phytoglobins are globular plant (algae and land 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. [1] A few decades after Kubo's report the crystallization of a lupin phytoglobin (known as leghemoglobin) 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. [2]
One important function of phytoglobin is its nitric oxide dioxygenase activity. [3]
Phytoglobins (abbreviated as Phytogbs) are ubiquitously distributed in both green algae and land plants. They can be classified as follows: [3] [4]
2/2 |
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Phytogb3 |
3/3 |
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Non-legume symbiotic globins (SymPhytogbs) [4] are scattered among Phytogb1 and Phytogb2. [3] Symbiotic globins generally provide oxygen to symbiotic bacteria that perform nitrogen-fixing. In legumes the bacteria are rhizobia, but in some actinorhizal plants actinomycete Frankia do the job instead. [3]
Phytogbs are coded by genes interrupted by 3 introns (although a 4 introns-containing phytogb gene has been detected in the moss Physcomitrella patens ). [5] The first and third intron of the phytogb genes are localized in the same position as that of the myoglobin genes, which suggests that phytogb and myoglobin genes evolved from a common ancestor more than 600 million years ago. [6] The existence of a second intron in the phytogb genes was predicted by Go using theoretical analysis, [7] which was further verified by cloning and sequencing of soybean lb genes by Marker and collaborators. [8]
Phytogbs are monomeric proteins whose molecular mass ranges from ~17 to ~19 kDa. However, at high (more than 1 mM) concentrations Phytogbs can form dimers. Phytogbs polypeptide chain folds into a particular arrangement of 6 to 7 helices (named with letters A to H) known as the globin fold which forms a hydrophobic pocket where heme is located. Two types of the globin fold have been identified in Phytogbs: the 3/3- and 2/2-folding, [9] [10] where helices A, E and F overlap to helices B, G and H and helices B and E overlap to helices G and H, respectively.
Like other globins, heme-Fe in Phytogbs is coordinated at the proximal region by a His amino acid (named as proximal His). The distal region of heme-Fe can be occupied by either a variety of ligands (such as oxygen and nitric oxide) or a distal (frequently a His) amino acid, giving place to penta- or hexa-coordinate Phytogbs, respectively. The Phytogbs2, SymPhytogbs and Lbs are predominantly penta-coordinate whereas Phytogbs1 are predominantly hexa-coordinate and Phytogbs0 and Phytogbs3 are a combination of penta- and hexa-coordinate. [4] Heme-Fe coordination is essential for Phytogb (and other globins) function because it regulates the rate of ligand-binding and –releasing as consequence of the kinetic constants kon and koff, respectively. For example, the affinity of soybean Lb and rice Phytogb1 for O2 (KO2) is moderate and very high because kon is 130 and 68 mM−1 s−1, koff is 5.6 and 0.038 s−1 and KO2 (i.e. the O2-affinity resulting from kon/koff) is 23 and 1800 mM−1, respectively. [11] This indicates that soybean Lb could function as an O2-storage or –transport protein and that the function of rice Phytogb1 (and other hexa-coordinate Phytogbs) could be other than O2-transport because the high affinity of this protein for O2 results from an extremely low koff constant. [12]
Like other globins, penta-coordinate Phytogbs reversibly bind and transport O2. The function of Lbs in nodules was elucidated in 1974 by Wittenberg, Appleby and others. [13] In nodules the concentration of Lbs is very high as they correspond to ~30% of the total soluble proteins. The apparent function of Lbs in nodules is to facilitate the diffusion of O2 to the respiring bacteroids for nitrogen-fixation. At the same time, Lb contributes to maintain low O2-levels (~10 nM) to avoid inactivation of the O2-sensitive nitrogenase that fixes the atmospheric nitrogen. [14]
Furthermore, Phytogbs bind other gaseous ligands, most notably nitric oxide (NO), and exhibit a NO dioxygenase activity. [15] Work by Hill and collaborators during the last ~15 years has shown that levels of endogenous NO varies with the concentration of Phytogbs1 in transgenic maize and alfalfa. [16] Based on these observations, these authors have proposed that a function of oxygenated Phytogbs is to modulate levels of NO via an NO dioxygenase activity and to indirectly regulate a wide variety of cell functions that are modulated by levels of NO. Oxygenated class 1 phytoglobins reacting with NO to produce nitrate represent the main mechanism by which NO is scavenged in plants. The cycle involving nitrate reductase, reduction of nitrite to NO, scavenging NO by phytoglobin was defined as the phytoglobin-NO cycle. [17] Its operation leads to the maintenance of redox and energy status during hypoxia and results in the reduced production of ethanol and lactic acid. [18]
Phytogbs0, 1, 2 and 3 are synthesized at very low concentrations in diverse (embryonic and vegetative) plant organs. [19] [20] [21] However, concentrations of Phytogbs increase in plants subjected to specific stress conditions, such as flooding [22] and light-limitation. [23] Hence, some Phytogbs have been considered as plant stress-responsive proteins.
Parts of this article (those related to Becana phylogeny rejects common origin for Lbs and SymPhytogbs in Phytogb1) need to be updated.(November 2020) |
Work by Arredondo-Peter, Vinogradov and collaborators has elucidated the major events that could have occurred during the evolution of Phytogbs. [24] [25] [26] Although Phytogbs evolved from a common ancestor previous to the origin of eukaryotes (i.e. in the eubacteria domain), in plants two evolutionary lineages have been identified for Phytogbs: the (3/3-) Phytogb0, 1 and 2 and SymPhytogb and Lb lineage and the (2/2-) Phytogb3 lineage. Apparently, Phytogbs0 were the ancestors to Phytogbs1 and 2, and SymPhytogbs and Lbs evolved from a Phytogb1 ancestor as an adaptation to the symbiotic nitrogen-fixation. Comparative analysis of moss Phytogb0 with rice Phytogb1 and soybean Lb structure revealed that the major evolutionary changes that probably occurred during the evolution of Phytogbs0, 1 and 2, SymPhytogbs and Lbs were (i) a hexa-coordinate to penta-coordinate transition at the heme group, (ii) a length decrease at the CD-loop and N- and C-termini, and (iii) the compaction of the protein into a globular structure. Furthermore, the structure of the globin domain from Phytogbs3 is highly similar to that of bacterial truncated globins indicating that the 2/2-folding conserved during the evolution of Phytogbs3. However, globin domain from land plant Phytogbs3 is flanked by extra-amino acid sequences that probably originated in the ancestor to land plant Phytogbs3. The functional and evolutionary significance of the extra-amino acid sequences in the Phytogbs3 structure is still not known.
Phytogbs are potentially useful for a number of biotechnological applications. For example, shortly after the discovery of rice Phytogbs1 an intended application was the use of these proteins as O2-sensors in electronic devices. Also, over-expression of Phytogbs has been proposed as a strategy to increase crop tolerance to specific stress conditions, such as crop flooding, [27] [28] as it couples the early flooding signal ethylene to the low oxygen response of plants. [29] [30] [31] [32] Recently, Phytogbs were considered candidates for developing blood substitutes and as additives in veggie burgers.[ citation needed ]
Hemoglobin is a protein containing iron that facilitates the transport 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 the 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.
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.
Leghemoglobin 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. 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.
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.
Hemerythrin (also spelled haemerythrin; Ancient Greek: αἷμα, romanized: haîma, lit. 'blood', Ancient Greek: ἐρυθρός, romanized: erythrós, lit. 'red') is an oligomeric protein responsible for oxygen (O2) transport in the marine invertebrate phyla of sipunculids, priapulids, brachiopods, and in a single annelid worm genus, Magelona. Myohemerythrin is a monomeric O2-binding protein found in the muscles of marine invertebrates. Hemerythrin and myohemerythrin are essentially colorless when deoxygenated, but turn a violet-pink in the oxygenated state.
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.
Carboxyhemoglobin is a stable complex of carbon monoxide and hemoglobin (Hb) that forms in red blood cells upon contact with carbon monoxide. Carboxyhemoglobin is often mistaken for the compound formed by the combination of carbon dioxide (carboxyl) and hemoglobin, which is actually carbaminohemoglobin. Carboxyhemoglobin terminology emerged when carbon monoxide was known by its historic name, "carbonic oxide", and evolved through Germanic and British English etymological influences; the preferred IUPAC nomenclature is carbonylhemoglobin.
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.
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.
Hemoglobin A (HbA), also known as adult hemoglobin, hemoglobin A1 or α2β2, is the most common human hemoglobin tetramer, accounting for over 97% of the total red blood cell hemoglobin. Hemoglobin is an oxygen-binding protein, found in erythrocytes, which transports oxygen from the lungs to the tissues. Hemoglobin A is the most common adult form of hemoglobin and exists as a tetramer containing two alpha subunits and two beta subunits (α2β2). Hemoglobin A2 (HbA2) is a less common adult form of hemoglobin and is composed of two alpha and two delta-globin subunits. This hemoglobin makes up 1-3% of hemoglobin in adults.
Heme oxygenase, or haem oxygenase, is an enzyme that catalyzes the degradation of heme to produce biliverdin, ferrous iron, and carbon monoxide.
The human β-globin locus is composed of five genes located on a short region of chromosome 11, responsible for the creation of the beta parts of the oxygen transport protein Haemoglobin. This locus contains not only the beta globin gene but also delta, gamma-A, gamma-G, and epsilon globin. Expression of all of these genes is controlled by single locus control region (LCR), and the genes are differentially expressed throughout development.
Hemoglobin subunit beta is a globin protein, coded for by the HBB gene, which along with alpha globin (HBA), makes up the most common form of haemoglobin in adult humans, hemoglobin A (HbA). It is 147 amino acids long and has a molecular weight of 15,867 Da. Normal adult human HbA is a heterotetramer consisting of two alpha chains and two beta chains.
Erythrocruorin, and the similar chlorocruorin, are large oxygen-carrying hemeprotein complexes, which have a molecular mass greater than 3.5 million daltons. Both are sometimes called giant hemoglobin or hexagonal bilayer haemoglobin. They are found in many annelids and arthropods.
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:
In enzymology, a leghemoglobin reductase (EC 1.6.2.6) is an enzyme that catalyzes the chemical reaction
Cytoglobin is the protein product of CYGB, a human and mammalian gene.
Vitreoscilla haemoglobin (VHb) is a type of haemoglobin found in the Gram-negative aerobic bacterium, Vitreoscilla. It is the first haemoglobin discovered from bacteria, but unlike classic haemoglobin it is composed only of a single globin molecule. Like typical haemoglobin, its primary role is binding oxygen, but it also performs other functions including delivery of oxygen to oxygenases, detoxification of nitric oxide, sensing and relaying oxygen concentrations, peroxidase-like activity by eliminating autoxidation-derived H2O2 that prevents haeme degradation and iron release.