Hemoglobin

Structure of hemoglobin. The protein subunits are in red and blue, and the iron-containing heme groups in green. From PDB: 1GZX​.

Hemoglobin (American English) or haemoglobin (British English) is the iron-containing oxygen-transport metalloprotein in the red blood cells of the blood in vertebrates and other animals. In mammals the protein makes up about 97% of the red cell’s dry content, and around 35% of the total content (including water). Hemoglobin transports oxygen from the lungs or gills to the rest of the body, such as to the muscles, where it releases the oxygen load. Hemoglobin also has a variety of other gas-transport and effect-modulation duties, which vary from species to species, and may be quite diverse in invertebrates.

The name hemoglobin is the concatenation of heme and globin, reflecting the fact that each subunit of hemoglobin is a globular protein with an embedded heme (or haem) group; each heme group contains an iron atom, and this is responsible for the binding of oxygen. The most common type of hemoglobin in mammals contains four such subunits, each with one heme group.

Mutations in the genes for the hemoglobin protein in humans result in a group of hereditary diseases termed the hemoglobinopathies, the most common members of which are sickle-cell disease and thalassemia. Historically in human medicine, the hemoglobinopathy of sickle-cell disease was the first disease to be understood in its mechanism of dysfunction, completely down to the molecular level. However, not all of such mutations produce disease states, and are formally recognized as hemoglobin variants (not diseases).

Hemoglobin (Hb) is synthesized in a complex series of steps. The heme portion is sythesized in both the mitochondria and cytosol of the immature red blood cell, while the globin protein portions of the molecule are sythesized by ribosomes in the cytosol . Production of Hb continues in the cell throughout its early development from the proerythroblast to the reticulocyte in the bone marrow. At this point, the nucleus is lost in mammals, but not in birds and many other species. Even after the loss of the nucleus in mammals, however, residual ribosomal RNA allows further synthesis of Hb until the reticulocyte loses its RNA soon after entering the vasculature (this hemoglobin-synthetic RNA in fact gives the reticulocyte its reticulated appearance and name).

The empirical chemical formula of the most common human hemoglobin is C₂₉₅₂H₄₆₆₄N₈₁₂O₈₃₂S₈Fe₄, but as noted above, hemoglobins vary widely across species, and even (through common mutations) slightly among subgroups of humans.

Structure

Heme group

In humans, the hemoglobin molecule is an assembly of four globular protein subunits. Each subunit is composed of a protein chain tightly associated with a non-protein heme group. Each protein chain arranges into a set of alpha-helix structural segments connected together in a globin fold arrangement, so called because this arrangement is the same folding motif used in other heme/globin proteins such as myoglobin. This folding pattern contains a pocket which strongly binds the heme group.

A heme group consists of an iron (Fe) atom held in a heterocyclic ring, known as a porphyrin. The iron atom, which is the site of oxygen binding, bonds with the four nitrogens in the center of the ring, which all lie in one plane. The iron is also bound strongly to the globular protein via the imidazole ring of a histidine residue below the porphyrin ring. A sixth position can reversibly bind oxygen, completing the octahedral group of six ligands. Oxygen binds in an "end-on bent" geometry where one oxygen atom binds Fe and the other protrudes at an angle. When oxygen is not bound, a very weakly bonded water molecule fills the site, forming a distorted octahedron.

The iron atom may either be in the Fe²⁺ or Fe³⁺ state, but ferrihemoglobin (methemoglobin) (Fe³⁺) cannot bind oxygen. In binding, oxygen temporarily oxidizes Fe to (Fe³⁺), so iron must exist in the +2 oxidation state in order to bind oxygen. The body reactivates hemoglobin found in the inactive (Fe³⁺) state by reducing the iron center.

In adult humans, the most common hemoglobin type is a tetramer (which contains 4 subunit proteins) called hemoglobin A, consisting of two α and two β subunits non-covalently bound, each made of 141 and 146 amino acid residues, respectively. This is denoted as α₂β₂. The subunits are structurally similar and about the same size. Each subunit has a molecular weight of about 17,000 daltons, for a total molecular weight of the tetramer of about 68,000 daltons. Hemoglobin A is the most intensively studied of the hemoglobin molecules.

The four polypeptide chains are bound to each other by salt bridges, hydrogen bonds, and hydrophobic interactions. There are two kinds of contacts between the α and β chains: α₁β₁ and α₁β₂.

Oxyhemoglobin is formed during respiration when oxygen binds to the heme component of the protein hemoglobin in red blood cells. This process occurs in the pulmonary capillaries adjacent to the alveoli of the lungs. The oxygen then travels through the blood stream to be dropped off at cells where it is utilized in aerobic glycolysis and in the production of ATP by the process of oxidative phosphorylation. It doesn't however help to counteract a decrease in blood pH. Ventilation, or breathing, may reverse this condition by removal of carbon dioxide, thus causing a shift up in pH.

Deoxyhemoglobin is the form of hemoglobin without the bound oxygen. The absorption spectra of oxyhemoglobin and deoxyhemoglobin differ. The oxyhemoglobin has significantly lower absorption of the 660 nm wavelength than deoxyhemoglobin, while at 940 nm its absorption is slightly higher. This difference is used for measurement of the amount of oxygen in patient's blood by an instrument called pulse oximeter.

Iron's oxidation state in oxyhemoglobin

Assigning oxygenated hemoglobin's oxidation state is difficult because oxyhemoglobin is diamagnetic (no net unpaired electrons), but the low-energy electron configurations in both oxygen and iron are paramagnetic. Triplet oxygen, the lowest energy oxygen species, has two unpaired electrons in antibonding π* molecular orbitals. Iron(II) tends to be in a high-spin configuration where unpaired electrons exist in eg antibonding orbitals. Iron(III) has an odd number of electrons and necessarily has unpaired electrons. All of these molecules are paramagnetic (have unpaired electrons), not diamagnetic, so an unintuitive distribution of electrons must exist to induce diamagnetism.

The three logical possibilities are:

1) Low-spin Fe²⁺ binds to high-energy singlet oxygen. Both low-spin iron and singlet oxygen are diamagnetic.

2) High-spin Fe³⁺ binds to .O₂^- (the superoxide ion) and antiferromagnetism oppositely aligns the two unpaired electrons, giving diamagnetic properties.

3) Low-spin Fe⁴⁺ binds to O₂^2-. Both are diamagnetic.

X-ray photoelectron spectroscopy suggests that iron has an oxidation state of approximately 3.2 and infrared stretching frequencies of the O-O bond suggests a bond length fitting with superoxide. The correct oxidation state of iron is thus the +3 state with oxygen in the -1 state. The diamagnetism in this configuration arises from the unpaired electron on superoxide aligning antiferromagnetically in the opposite direction from the unpaired electron on iron. The second choice being correct is not surprising because singlet oxygen and large separations of charge are both unfavorably high-energy states. Iron's shift to a higher oxidation state decreases the atom's size and allows it into the plane of the porphyrin ring, pulling on the coordinated histidine residue and initiating the allosteric changes seen in the globulins. The assignment of oxidation state, however, is only a formalism so all three models may contribute to some small degree.

Early postulates by bioinorganic chemists claimed that possibility (1) (above) was correct and that iron should exist in oxidation state II (indeed iron oxidation state III as methemoglobin, when not accompanied by superoxide .O₂^- to "hold" the oxidation electron, is incapable of binding O₂). The iron chemistry in this model was elegant, but the presence of singlet oxygen was never explained. It was argued that the binding of an oxygen molecule placed high-spin iron(II) in an octahedral field of strong-field ligands; this change in field would increase the crystal field splitting energy, causing iron's electrons to pair into the diamagnetic low-spin configuration.

Binding of ligands

File:Hb-animation2.gif

Binding and release of ligands induces a conformational (structural) change in hemoglobin. Here, the binding and release of oxygen illustrates the structural differences between oxy- and deoxyhemoglobin, respectively. Only one of the four heme groups is shown.

As discussed above, when oxygen binds to the iron center it causes contraction of the iron atom, and causes it to move back into the center of the porphyrin ring plane (see moving diagram). At the same time, the porphyrin ring plane itself is pushed away from the oxygen and toward the imidizole side chain of the histidine residue interacting at the other pole of the iron. The interaction here forces the ring plane sideways toward the outside of the tetramer, and also induces a strain on the protein helix containing the histidine, as it moves nearer the iron. This causes a tug on this peptide strand which tends to open up heme units in the remainder of the molecule, so that there is more room for oxygen to bind at their heme sites.

In the tetrameric form of normal adult hemoglobin, the binding of oxygen is thus a cooperative process. The binding affinity of hemoglobin for oxygen is increased by the oxygen saturation of the molecule, with the first oxygens bound influencing the shape of the binding sites for the next oxygens, in a way favorable for binding. This positive cooperative binding is achieved through steric conformational changes of the hemoglobin protein complex as discussed above, i.e. when one subunit protein in hemoglobin becomes oxygenated, this induces a conformational or structural change in the whole complex, causing the other subunits to gain an increased affinity for oxygen. As a consequence, the oxygen binding curve of hemoglobin is sigmoidal, or S-shaped, as opposed to the normal hyperbolic curve associated with noncooperative binding.

Hemoglobin's oxygen-binding capacity is decreased in the presence of carbon monoxide because both gases compete for the same binding sites on hemoglobin, carbon monoxide binding preferentially in place of oxygen. Carbon dioxide occupies a different binding site on the hemoglobin. Carbon dioxide is more readily dissolved in deoxygenated blood, facilitating its removal from the body after the oxygen has been released to tissues undergoing metabolism. This increased affinity for carbon dioxide by the venous blood is known as the Haldane effect. Through the enzyme carbonic anhydrase, carbon dioxide reacts with water to give carbonic acid, which decomposes into bicarbonate and protons:

CO₂ + H₂O → H₂CO₃ → HCO₃^- + H⁺

The sigmoidal shape of hemoglobin's oxygen-dissociation curve results from cooperative binding of oxygen to hemoglobin.

Hence blood with high carbon dioxide levels is also lower in pH (more acidic). Hemoglobin can bind protons and carbon dioxide which causes a conformational change in the protein and facilitates the release of oxygen. Protons bind at various places along the protein, and carbon dioxide binds at the alpha-amino group forming carbamate. Conversely, when the carbon dioxide levels in the blood decrease (i.e., in the lung capillaries), carbon dioxide and protons are released from hemoglobin, increasing the oxygen affinity of the protein. This control of hemoglobin's affinity for oxygen by the binding and release of carbon dioxide and acid, is known as the Bohr effect.

The binding of oxygen is affected by molecules such as carbon monoxide (CO) (for example from tobacco smoking, cars and furnaces). CO competes with oxygen at the heme binding site. Hemoglobin binding affinity for CO is 200 times greater than its affinity for oxygen, meaning that small amounts of CO dramatically reduces hemoglobin's ability to transport oxygen. When hemoglobin combines with CO, it forms a very bright red compound called carboxyhemoglobin. When inspired air contains CO levels as low as 0.02%, headache and nausea occur; if the CO concentration is increased to 0.1%, unconsciousness will follow. In heavy smokers, up to 20% of the oxygen-active sites can be blocked by CO.

In similar fashion, hemoglobin also has competitive binding affinity for cyanide (CN^-), sulfur monoxide (SO), nitrogen dioxide (NO₂), and sulfide (S^2-), including hydrogen sulfide (H₂S). All of these bind to iron in heme without changing its oxidation state, but they nevertheless inhibit oxygen-binding, causing grave toxicity.

The iron atom in the heme group must be in the ferrous (Fe²⁺) oxidation state to support oxygen and other gases' binding and transport. Oxidation to the ferric (Fe³⁺) state converts hemoglobin into hemiglobin or methemoglobin (pronounced "MET-hemoglobin"), which cannot bind oxygen. Hemoglobin in normal red blood cells is protected by a reduction system to keep this from happening. Nitrogen dioxide and nitrous oxide are capable of converting a small fraction of hemoglobin to methemoglobin, however this is not usually of medical importance (nitrogen dioxide is poisonous by other mechanisms, and nitrous oxide is routinely used in surgical anesthesia in most people without undue methemoglobin buildup).

In people acclimated to high altitudes, the concentration of 2,3-bisphosphoglycerate (2,3-BPG) in the blood is increased, which allows these individuals to deliver a larger amount of oxygen to tissues under conditions of lower oxygen tension. This phenomenon, where molecule Y affects the binding of molecule X to a transport molecule Z, is called a heterotropic allosteric effect.

A variant hemoglobin, called fetal hemoglobin (HbF, α₂γ₂), is found in the developing fetus, and binds oxygen with greater affinity than adult hemoglobin. This means that the oxygen binding curve for fetal hemoglobin is left-shifted (i.e., a higher percentage of hemoglobin has oxygen bound to it at lower oxygen tension), in comparison to that of adult hemoglobin. As a result, fetal blood in the placenta is able to take oxygen from maternal blood.

Hemoglobin also carries nitric oxide in the globin part of the molecule. This improves oxygen delivery in the periphery and contributes to the control of respiration. NO binds reversibly to a specific cystein residue in globin; the binding depends on the state (R or T) of the hemoglobin. The resulting S-nitrosylated hemoglobin influences various NO-related activities such as the control of vascular resistance, blood pressure and respiration. NO is released not in the cytoplasm of erythrocytes but is transported by an anion exchanger called AE1 out of them.

Types of hemoglobins in humans

hemoglobin, alpha 1
Identifiers
Symbol	HBA1
NCBI gene	3039
HGNC	4823
OMIM	141800
RefSeq	NM_000558
UniProt	P69905
Other data
Locus	Chr. 16 p13.3
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hemoglobin, alpha 2
Identifiers
Symbol	HBA2
NCBI gene	3040
HGNC	4824
OMIM	141850
RefSeq	NM_000517
UniProt	P69905
Other data
Locus	Chr. 16 p13.3
Search for Structures Swiss-model Domains InterPro

hemoglobin, beta
Identifiers
Symbol	HBB
NCBI gene	3043
HGNC	4827
OMIM	141900
RefSeq	NM_000518
UniProt	P68871
Other data
Locus	Chr. 11 p15.5
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In the embryo:

• Gower 1 (ξ₂ε₂)
• Gower 2 (α₂ε₂) (PDB: 1A9W​)
• Hemoglobin Portland (ξ₂γ₂)

In the fetus:

• Hemoglobin F (α₂γ₂) (PDB: 1FDH​)

In adults:

• Hemoglobin A (α₂β₂) (PDB: 1BZ0​) - The most common type.
• Hemaglobin A₂ (α₂δ₂) - δ chain synthesis begins late in the third trimester and in adults, it has a normal level of 2.5%
• Hemoglobin F (α₂γ₂) - In adults Hemoglobin F is restricted to a limited population of red cells called F cells.

Variant forms which cause disease:

• Hemoglobin S (α₂β^S₂) - A variant form of hemoglobin found in people with sickle cell disease. There is a variation in the β-chain gene, causing a change in the properties of hemoblobin which results in sickling of red blood cells.
• Hemoglobin C (α₂β^C₂) - Another variant due to a variation in the β-chain gene. This variant causes a mild chronic hemolytic anemia.

Degradation of hemoglobin in vertebrate animals

When red cells reach the end of their life due to aging or defects, they are broken down, the hemoglobin molecule is broken up and the iron gets recycled. When the porphyrin ring is broken up, the fragments are normally secreted in the bile by the liver. This process also produces one molecule of carbon monoxide for every molecule of heme degraded [4]; this is one of the few natural sources of carbon monoxide production in the human body, and is responsible for the normal blood levels of carbon monoxide even in people breathing pure air. The other major final product of heme degradation is bilirubin. Increased levels of this chemical are detected in the blood if red cells are being destroyed more rapidly than usual. Improperly degraded hemoglobin protein or hemoglobin that has been released from the blood cells too rapidly can clog small blood vessels, especially the delicate blood filtering vessels of the kidneys, causing kidney damage.

Role in disease

Decrease of hemoglobin, with or without an absolute decrease of red blood cells, leads to symptoms of anemia. Anemia has many different causes, although iron deficiency and its resultant iron deficiency anemia are the most common causes in the Western world. As absence of iron decreases heme synthesis, red blood cells in iron deficiency anemia are hypochromic (lacking the red hemoglobin pigment) and microcytic (smaller than normal). Other anemias are rarer. In hemolysis (accelerated breakdown of red blood cells), associated jaundice is caused by the hemoglobin metabolite bilirubin, and the circulating hemoglobin can cause renal failure.

Some mutations in the globin chain are associated with the hemoglobinopathies, such as sickle-cell disease and thalassemia. Other mutations, as discussed at the beginning of the article, are benign and are referred to merely as hemoglobin variants.

There is a group of genetic disorders, known as the porphyrias that are characterized by errors in metabolic pathways of heme synthesis. King George III of the United Kingdom was probably the most famous porphyria sufferer.

To a small extent, hemoglobin A slowly combines with glucose at a certain location in the molecule. The resulting molecule is often referred to as Hb A_1c. As the concentration of glucose in the blood increases, the percentage of Hb A that turns into Hb A_1c increases. In diabetics whose glucose usually runs high, the percent Hb A_1c also runs high. Because of the slow rate of Hb A combination with glucose, the Hb A_1c percentage is representative of glucose level in the blood averaged over a longer time (the half-life of red blood cells, which is typically 50-55 days).

Diagnostic use

Hemoglobin levels are amongst the most commonly performed blood tests, usually as part of a full blood count or complete blood count. Results are reported in g/L, g/dL or mol/L. For conversion, 1 g/dL is 0.621 mmol/L. If the total hemoglobin concentration in the blood falls below a set point, this is called anemia. Normal values for hemoglobin levels are:

• Women: 12.1 to 15.1 g/dl
• Men: 13.8 to 17.2 g/dl
• Children: 11 to 16 g/dl
• Pregnant women: 11 to 12 g/dl [5]

Anemias are further subclassified by the size of the red blood cells, which are the cells which contain hemoglobin in vertebrates. They can be classified as microcytic (small sized red blood cells), normocytic (normal sized red blood cells), or macrocytic (large sized red blood cells). The hemaglobin is the typical test used for blood donation. A comparison with the hematocrit can be made by multiplying the hemaglobin by three. For example, if the hemaglobin is measured at 17, that compares with a hematocrit of .51.[6]

Glucose levels in blood can vary widely each hour, so one or only a few samples from a patient analyzed for glucose may not be representative of glucose control in the long run. For this reason a blood sample may be analyzed for Hb A_1c level, which is more representative of glucose control averaged over a longer time period (determined by the half-life of the individual's red blood cells, which is typically 50-55 days). People whose Hb A_1c runs 6.0% or less show good longer-term glucose control. Hb A_1c values which are more than 7.0% are elevated. This test is especially useful for diabetics.

Hemoglobin in the biological range of life

Hemoglobin is by no means unique to vertebrates; there are a variety of oxygen transport and binding proteins throughout the animal (and plant) kingdom. Other organisms including bacteria, protozoans and fungi all have hemoglobin-like proteins whose known and predicted roles include the reversible binding of gaseous ligands. Since many of these proteins contain globins, and also the heme moiety (iron in a flat porphyrin support), these substances are often simply referred to as hemoglobins, even if their overall tertiary structure is very different from that of vertebrate hemoglobin. In particular, the distinction of “myoglobin” and hemoglobin in lower animals is often impossible, because some of these organisms do not contain muscles. Or they may have a recognizable separate circulatory system, but not one which deals with oxygen transport (for example, many insects and other arthropods). In all these groups, heme/globin containing molecules (even monomeric globin ones) which deal with gas-binding are referred to as hemoglobins. In addition to dealing with transport and sensing of oxygen, these molecules may also deal with NO, CO₂, sulfide compounds, and even O₂ scavenging in environments which must be anaerobic. They may even deal with detoxification of chlorinated materials in a manner analogous to heme-containing P450 enzymes and peroxidases.

The structure of hemoglobins varies across species. Hemoglobin occurs in all kingdoms of organisms, but not in all organisms. Single-globin hemoglobins tend to be found in primitive species such as bacteria, protozoa, algae, and plants. Nematode worms, molluscs and crustaceans, however, many contain very large multisubunit molecules much larger than those in vertebrates. Particularly worth noting are chimeric hemoglobins found in fungi and giant annelids, which may contain both globin and other types of proteins [PMID 11274340]. One of the most striking occurrences and uses of hemoglobin in organisms occurs in the (up to) 2.4 meter giant tube worm (Riftia pachyptila also called Vestimentifera) which populates ocean volcanic vents at the sea floor. These worms have no digestive tract, but instead contain a population of bacteria constituting half the organism’s weight, which react H₂S from the vent and O₂ from the water to produce energy to make food from H₂O and CO₂. These organisms end with a deep red fan-like structure ("plume") which extends into the water and which absorbs H₂S and O₂ for the bacteria, and also absorbs CO₂ for use as synthetic raw material (after the manner of photosynthetic plants). The bright red color of the structures results from several extraordinarily complex hemoglobins found in them which contain up to 144 globin chains (presumably each including associated heme structures). These tube worm hemoglobins are remarkable for being able to carry oxygen in the presence of sulfide, and indeed to also carry sulfide, without being completely "poisoned" or inhibited by this molecule, as hemoglobins in most other species are [PMID 8621529]. See also [PMID 15265029].

Other oxygen-binding proteins

Myoglobin: Found in the muscle tissue of many vertebrates, including humans, it gives muscle tissue a distinct red or dark gray color. It is very similar to hemoglobin in structure and sequence, but is not a tetramer; instead, it is a monomer that lacks cooperative binding. It is used to store oxygen rather than transport it.

Hemocyanin: The second most common oxygen transporting protein found in nature, it is found in the blood of many arthropods and molluscs. Uses copper prosthetic groups instead of iron heme groups and is blue in color when oxygenated.

Hemerythrin: Some marine invertebrates and a few species of annelid use this iron containing non-heme protein to carry oxygen in their blood. Appears pink/violet when oxygenated, clear when not.

Chlorocruorin: Found in many annelids, it is very similar to erythrocruorin, but the heme group is significantly different in structure. Appears green when deoxygenated and red when oxygenated.

Vanabins: Also known as vanadium chromagens, they are found in the blood of sea squirts and are hypothesised to use the rare metal vanadium as its oxygen binding prosthetic group.

Erythrocruorin: Found in many annelids, including earthworms, it is a giant free-floating blood protein containing many dozens—possibly hundreds—of iron- and heme-bearing protein subunits bound together into a single protein complex with a molecular mass greater than 3.5 million daltons.

Pinnaglobin: Only seen in the mollusk Pinna squamosa. Brown manganese-based porphyrin protein.

Leghemoglobin: In leguminous plants, such as alfalfa or soybeans, the nitrogen fixing bacteria in the roots are protected from oxygen by this iron heme containing, oxygen binding protein.

Hemoglobin in fine art

Heart of Steel (Hemoglobin) by Julian Voss-Andreae. The images show the 5' (1.60 m) tall sculpture right after installation, after 10 days, and after several month of exposure to the elements.

Artist Julian Voss-Andreae has created a sculpture called "Heart of Steel (Hemoglobin)" (2005) based on the protein's backbone. The sculpture was made from glass and weathering steel. The intentional rusting of the initially shiny work of art mirrors hemoglobin's fundamental chemical reaction of iron binding to oxygen.

See also

• Chlorophyll
• Globin fold
• Hemocyanin
• Hemoglobin A1C
• Hemoglobin A2
• Hemoglobin C
• Hemoglobin F
• Hemoprotein
• Sickle-cell disease

References

• Template:Harvard reference.
• Template:Harvard reference. PMID 16368297.
• Template:Harvard reference.
• Template:Harvard reference.

• Template:Harvard reference. PMID 8650150.
• Template:Harvard reference. PMID 16368110.
• Template:Harvard reference.

External links

• Interactive models of hemoglobin (Requires MDL Chime)
• Interactive hemoglobin saturation curves
• Information on Haemoglobin

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