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|Although globins are often thought of as respiratory pigments specific to animals, they are known in all groups of organisms, including bacteria, fungi, and plants.|
Hemoglobins are heme containing proteins which reversibly bind oxygen. They are found in bacteria, fungi, higher plants, most invertebrates and all vertebrates. All of them belong to the same globin gene family, having evolved from a single ancestral protein of about 17 kDa.
Hemoglobin is similar in structure to peroxidase
which removes dangerous forms of oxygen.
In nematodes, hemoglobin can function both as a peroxidase and
to remove NO. Bacterial flavohemoglobin
can remove NO by reacting it with oxygen to form nitrate. When oxygen is not present, flavohemoglobin
removes NO by promoting the conversion of N2O. Thus these molecules offer protection from NO
in both aerobic and anaerobic conditions.
In the ancient earth (and in communities of deep sea vents), NO
is abundant while oxygen is scarce. Mycobacterium tuberculosis uses hemoglobin
to protect it from reactive N molecules of the host’s defenses during
infection. The globins of primitive worms can
protect against reactive forms of oxygen.
Given that hemoglobin
occurs is widespread in both prokaryotes and eukaryotes, it must have
been used early in earth’s history before substantial amounts of oxygen
had accumulated in the atmosphere. Other
globin molecules bind hydrogen sulfide.
The ability to bind oxygen may be a secondarily acquired ability
of this gene family (Hausladen, 2001; Couture, 1999; Lecomte, 2005; Wu, 2003b).
In bacteria, some globins function as sensors which sense the presence of oxygen as a stimulus for determining the movement of the bacteria. Proteins similar to sequences thought to be ancestral for all globins in the last universal common ancestor have been found in archaea. They are very sensitive to oxygen concentration, as would be expected from ancestral molecules which existed with very low amounts of free oxygen. It is possible that the ancestral globin bound oxygen as a way of protecting the cell from dangerous oxygen radicals (Freitas, 2005).
In bacteria and yeast, multi-domain proteins combine hemoglobin with another domain to result in a number of functional differences. These additional domains include dihydropteridine reductase, cytochrome c reductase, kinase, and flavoproteins. HemATs are signal transducers in both eubacteria and archebacteria which are composed of a hemoglobin domain (the N terminus) and a bacterial chemoreceptor domain (the C terminus). These molecules mediate responses of bacteria to environmental oxygen, referred to as the aerotactic response. Globin-coupled sensory molecules are known in both eubacteria and archaea (Freitas, 2003; Hou, 2001; Zhu, 1992). The globin fold functions in eubacterial and archaeal globin-coupled receptors known as protoglobins. Signal transduction may have been an/the original function of the globin fold. The globin fold is also known in the α subunit of eukaryotic initiation factor 2A kinase. In vertebrates, the proteins neuroglobin and cytoglobin seem to function in signal transduction as well (Freitas, 2005).
Plants have a variety of hemoglobin molecules and it is even possible that all plants possess hemoglobins (Zhu, 1992, Anderson, 1996). Some, referred to as the symbiotic hemoglobins, are found in the nodules of nitrogen fixing plants (primarily legumes, but nonlegumes as well) where they transport oxygen to nitrogen fixing bacteria. Nonsymbiotic hemoglobins are a distinct group of hemoglobins; it is possible that they are found in all plants. They are expressed in root tissue but their function is unknown (they seem to bind oxygen too tightly to be involved in simple transport). Two classes of nonsymbiotic hemoglobins are recognized and class 2 nonsymbiotic hemoglobins (such as AHB2) may be ancestral to most symbiotic hemoglobins. Parasponia possesses a bi-functional class 1hemoglobin which is expressed both in root tissue and root nodules (Trevaskis, 1997).
Hemoglobin’s three dimensional structure
includes a conserved “3 on 3” set of alpha helices. “Truncated hemoglobin”, with a “2 on 2” arrangement
is found in algae, protozoa, and some prokaryotes. It also is found in plants—probably due to horizontal
transfer from chloroplast genomes. Parasitic
microbial species can utilize truncated globins to insure adequate oxygen
supplies within their host (Wu, 2003b;
Before oxygen reached appreciable levels in the atmosphere, oxygen and reactive byproducts such as NO and CO could have accumulated in local environments. Animals adopted hemoglobin for oxygen transport (Vinogradov, 2005). There are a variety of hemoglobins in invertebrates.
Some are made of a single polypeptide chain with one heme group
(as in those of dipterans), multi-subunit proteins with two heme groups
per subunit (each subunit is 30-40 kd and the entire protein may be 250-800
kd; known primarily from crustaceans), multi-subunit proteins with multiple
heme groups (8 to 20 per subunit; known in crustaceans and mollusks),
and multi-subunit proteins in which not all subunits contain heme and
subunits can be united by disulfide bones (this group includes the chlorocruorins
of annelids and erythrocruorins). Some hemoglobins in invertebrates function inside
cells and others are extracellular (Kloek, 1992).
is known throughout vertebrates and in some invertebrates as well. It is expressed only in the cytoplasm of neurons
(Hankeln, 2005). This globin seems
to have resulted from a very early duplication in invertebrate globin
genes and is less similar to hemoglobins than some invertebrate globins
(Awenius, 2001). Neuroglobin is
produced in neurons in response to hypoxia and is expressed in the cerebrum,
cerebellum, and diencephalon (Wakasugi, 2004).
Neuroglobin is expressed in both human an
mouse brains in the frontal lobe, subthalamic nucleus and thalamus. It may be relevant that these regions of the
brain are less susceptible to ischemia and Alzheimers that
other regions which have a lower expression of neuroglobin (such
as the hippocampus).
Cytoglobin is the fourth type of globin known in humans, mice, and fish. It is expressed in almost all tissues and appears to be related to vertebrate myoglobin (Hankeln, 2005; DeSantis, 2004).
A unique, eye-specific globin has been discovered in chickens (Kugelstadt, 2004).
MYOGLOBIN AND HEMOGLOBIN
In the gnathostome lineage a duplication of globin gene gave rise to myoglobin and hemoglobin; shortly afterwards a duplication of hemoglobin gene gave rise to alpha and beta globin genes. A duplication in the alpha genes gives rise to some members of the alpha family which are expressed only in the embryo.
In chickens, there is a cluster of b genes which is not derived from the same ancestor of the mammalian cluster; its genes are: ---r---bH---bA----e (one of the two beta genes is expressed in the hatchling, one in the adult). Interestingly, marsupials have an orphan b gene, the w gene, which is more similar to the genes in the chicken cluster than the genes of the mammalian cluster (which marsupials also possess). This suggests that a duplication of a b gene in reptiles which would give rise to different b clusters in different descendants of reptiles
Only two b globin genes occur in marsupials: b is expressed starting on the day of birth while e was expressed in the embryo. Eutherian mammals have additional b genes which are expressed during fetal development. It may be relevant that marsupials complete their fetal development in the pouch are breathing oxygen from air while placental mammal fetuses experience lower concentrations of oxygen in the maternal blood (Cooper, 1993)
The gene for myoglobin is located on chromosome 22q11 and is not part of the two globin clusters. It is expressed in muscle (especially cardiac muscle and slow oxidative skeletal muscle; human cardiac and skeletal muscle is pictured below) where it seems to be involved in oxygen delivery to mitochondria. Mice which make no myoglobin are normal (although their muscles are depigmented), indicating that myoglobin is not essential for mammals to meet their oxygen demands, not even during exercise or pregnancy. Myoglobin’s function is currently under reconsideration. There is evidence that myoglobin functions to bind NO, a gas which inhibits mitochondrial respiration (Wu, 2003b).
|Hemoglobin is the respiratory pigment in vertebrate red blood cells. The following images are of blood smears of a bony fish, frog, turtle, and human. The human slide shows a reticulocyte, the precursor to circulating red blood cells which has not yet ejected its nucleus.|
|ALPHA FAMILY: 16pter—p13.3|
|5’-----zeta-------zeta pseudogene-------alpha pseudogene------alpha 2------alpha 1-----theta----3’|
The ancestral α globin was duplicated twice in ancestral amniotes to produce the ancestral α, α D and ζ genes. Although some lineages have lost the αD gene, it exists in its ancestral spot in humans, between the α3 and ζ genes. A second duplication of the α globin gene occurred prior to the separation of therian mammal lineages to produce α and Ө genes. It appears that a β-like globin was located downstream of the α cluster, ω globin, but that this gene has been deleted from the human region of the α cluster (Cooper, 2006). Duplications in the alpha globin genes can result from recombination errors in meiosis and even mitosis. The number of alpha globin genes in a person’s genome (Lam, 2007).
Zeta hemoglobin is a functional gene in human embryonic development and fetuses suffer a number of problems when it is deleted. If children are homozygous for mutations in the alpha genes, their hemoglobin consists of 2 zeta chains and 2 beta chains but often they are stillborn.
There are 3 copies of zeta genes in some Melanesians and Polynesians.
2) Zeta Pseudogene
Interestingly, a common variation of the zeta pseudogene makes it a functional gene.
3) Alpha pseudogene
There are early termination and splice junction mutations which make this gene nonfunctional.
4-5) ALPHA GENES
In adults, hemoglobin A is composed of 2 alpha chains and 2 beta chains; hemoglobin A2 is made of 2 alpha chains and 2 delta chains. Fetal hemoglobin is composed of 2 alpha chains and 2 gamma chains. Embryonic hemoglobin is composed of 2 alpha chains and 2 epsilon chains. While most adults have 2 alpha hemoglobin genes, common variants include the possession of 1 or 3 alpha genes. Melanesians only have 1 alpha gene and, among African-Americans, chromosomes with one alpha gene are about as common as those with two.
The two alpha genes have identical amino acid sequences in humans, chimps, gorillas, and gibbons (there is one amino acid difference between the two proteins in orangutans). The alpha 2 gene codes for the majority of alpha hemoglobin used in red blood cells and therefore mutations in the alpha 2 gene are potentially more serious than those in the alpha 1 gene.
Mutant alpha hemoglobin can cause a group
of blood disorders known as thalassemias.
The severity of a thalassemia can be modified by different alleles
of beta hemoglobin. Some thalassemias
have positive aspects such as reducing the risk of malaria and other diseases
as well. In
This globin gene was first discovered in orangutans and later found to be in humans as well. It is an old gene, having about as many amino acid differences from alpha hemoglobin as alpha and zeta hemoglobin differ from each other. The promoter differs from the promoters of the other genes in the alpha family.
Its function isn’t known although it is expressed in red blood cell lineages in fetal mammals (including humans at about 5 weeks). There have been cases where it has been deleted but no effects of this are known.
|BETA FAMILY: 11p15.5|
|5’---------epsilon----------gammma G---------gamma A----------eta------------delta----------beta---------3’|
The ancestral β globin gene had duplicated
to produce an proto-eta gene and a proto-beta
gene prior to the separation of the lineages of therian mammals.
Early in the evolution of eutherian mammals, the proto-eta locus
duplicated to produce ε, η,
and γ globins and the proto-β
locus duplicated to form β and δ globins. In the primate lineage, the η globin was
mutated and became a pseudogene and the γ globin gene was duplicated
in the lineage of anthropoid primates.
Two epsilon chains complex with two alpha chains to form embryonic hemoglobin, 4 epsilon chains can also form a tetramer with themselves. The amino acid sequence is similar to that of beta and delta hemoglobin and no mutations are known in humans. A deletion from the DNA region upstream of the epsilon gene in mice causes the expression of the protein in adult mice.
2-3) Gamma Genes
Most people have two gamma genes (gamma G and gamma A) but some people have 3 or even 5 (there was even one family that had 4 copies on another chromosome). Two gamma hemoglobin chains complex with two alpha chains to form fetal hemoglobin. A small amount of this fetal hemoglobin (called hemoglobin F) is detectable in the blood of adults. Large amounts of fetal hemoglobin may be found in cases in which there is abnormal beta hemoglobin or in which there has been mutations in the promoters of the gamma genes.
Eta hemoglobin apparently was an embryonic hemoglobin in the ancestors of eutherian mammals. In artiodactyls (deer, cows, giraffes, etc.) it is still a functional gene. In primates, eta is a nonfunctional pseudogene. Rodents no longer have any trace of the eta hemoglobin gene.
Delta hemoglobin can complex with alpha hemoglobin in adults and it can be considered to be a second beta hemoglobin gene. Thalassemias result from mutations in the delta hemoglobin gene. There are examples of fusion hemoglobin genes in which the delta and beta genes have fused into one gene (and even a case of a delta-beta-delta fusion gene).
In adults, 2 beta chains complex with 2 alpha chains to form the hemoglobin molecules of red blood cells. Some people have two beta genes and delta chains can apparently be substituted for beta chains. Mutations in beta globin genes cause a variety of thalassemias which range from minor asymptomatic cases to severe cases (in which no beta or delta hemoglobin is made) which result in sever anemia, the persistance of fetal hemoglobin, and requires bone marrow transplants.
Sickle cell anemia is a disorder in which
the 6th amino acid (not counting the first methionine which
is removed after translation) is changed from glutamic acid to valine. The resulting change in protein structure changes
the structure of red blood cells so that they form inflexible sickle cells
which can clog small blood vessels. Children
with sickle cell anemia suffer from a host of problems including repeated
infections, anemia, strokes, and many develop functional asplenia.
Heterozygotes (carriers) have a condition known as the sickle cell
trait in which the blood can sickle under certain strenuous conditions.
Heterozygotes have increased resistance to malaria, and for this
reason, the sickle cell allele is so common in Africans (and their African-American
descendants). Another allele, hemoglobin
C (in which the 6th amino acid has been changed to lysine),
also confers resistance to malaria and its effects are not as severe as
the sickle cell allele. Some have
hypothesized that this allele will eventually replace the sickle cell
Comparisons of globin sequences have often been used to study the relationships between eukaryotes (particularly vertebrates). For example, comparisons of noncoding DNA in the globin cluster suggests that the rate of nucleotide substitution slowed in higher primates, suggesting a longer lifespan (Page, 2001)