View source for Hemoglobin

{{heteropolypeptide
  | heteropolymer = Hemoglobin, human, adult
  | polymer_type = heterotetramer, (αβ)<sub>2</sub>
  | protein_type = [[metalloprotein]], [[globulin]]
  | function = [[oxygen]]-transport
  | cofactors = [[heme]] (4)
  | image = 1GZX Haemoglobin.png 
  | image_source = Structure of human hemoglobin. The protein subunits are in red and blue, and the iron-containing [[heme]] groups in green. From {{PDB|1GZX}} {{Proteopedia|Hemoglobin}}
  | SubunitCount = 3
  | subunit1 = Hb α1
  | gene1 = <span class=plainlinks>[http://www.genenames.org/data/hgnc_data.php?match=HBA1 HBA1]</span>
  | locus1 = [[Chromosome 16]]p13.3
  | subunit2 = Hb α2
  | gene2 = <span class=plainlinks>[http://www.genenames.org/data/hgnc_data.php?match=HBA2 HBA2]</span>
  | locus2 = [[Chromosome 16]]p13.3
  | subunit3 = Hb β
  | gene3 = <span class=plainlinks>[http://www.genenames.org/data/hgnc_data.php?match=HBB HBB]</span> 
  | locus3 = [[Chromosome 11]]p15.5
}}
{{protein
| Name = hemoglobin, alpha 1
| caption = 
| image = 
| width = 
| HGNCid = 4823
| Symbol = HBA1
| AltSymbols = 
| EntrezGene = 3039
| OMIM = 141800
| RefSeq = NM_000558
| UniProt = P69905
| PDB = 
| ECnumber = 
| Chromosome = 16
| Arm = p
| Band = 13.3
| LocusSupplementaryData = 
}}
{{protein
| Name = hemoglobin, alpha 2
| caption = 
| image = 
| width = 
| HGNCid = 4824
| Symbol = HBA2
| AltSymbols = 
| EntrezGene = 3040
| OMIM = 141850
| RefSeq = NM_000517
| UniProt = P69905
| PDB = 
| ECnumber = 
| Chromosome = 16
| Arm = p
| Band = 13.3
| LocusSupplementaryData = 
}}
{{protein
| Name = hemoglobin, beta
| caption = 
| image = 
| width = 
| HGNCid = 4827
| Symbol = HBB
| AltSymbols = 
| EntrezGene = 3043
| OMIM = 141900
| RefSeq = NM_000518
| UniProt = P68871
| PDB = 
| ECnumber = 
| Chromosome = 11
| Arm = p
| Band = 15.5
| LocusSupplementaryData = 
}}
{{SI}}

{{CMG}}



==Overview==
'''Hemoglobin''', also spelled '''haemoglobin''' and abbreviated '''Hb''', is the [[iron]]-containing [[oxygen]]-transport [[metalloprotein]] in the [[red blood cell]]s of the [[blood]] in [[vertebrate]]s 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 [[lung]]s or [[gill]]s to the rest of the body, such as to the [[muscle]]s, where it releases its load of oxygen. 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 through ion-induced dipole forces. The most common type of hemoglobin in mammals contains four such subunits, each with one heme group. In humans, each heme group is able to bind one oxygen molecule, and thus, one hemoglobin molecule can bind four oxygen molecules.

Mutations in the [[genes]] for the hemoglobin [[protein]] in humans result in a group of [[hereditary disease]]s termed the ''[[hemoglobinopathy|hemoglobinopathies]]'', the best known of which is [[sickle-cell disease]]. Historically in human medicine, sickle-cell disease was the first disease to be understood in its mechanism of dysfunction, completely down to the molecular level. However, not all such globin-gene mutations result in illness. These mutations are formally recognized as [[hemoglobin variants]] rather than diseases.<ref>[http://globin.cse.psu.edu/html/huisman/variants/]</ref><ref>[http://www.labtestsonline.org/understanding/analytes/hemoglobin_var/glance-3.html]</ref> A (mostly) separate set of diseases involves underproduction of normal and sometimes abnormal hemoglobins, through problems and mutations in globin gene ''regulation''. These diseases, which also often produce anemia, are called [[thalassemia]]s.<ref>{{cite web |url=http://web2.airmail.net/uthman/hemoglobinopathy/hemoglobinopathy.html |title=Hemoglobinopathies and Thalassemias|last=Uthman, MD |first=Ed}}</ref>

Hemoglobin (Hb) is synthesized in a complex series of steps. The heme portion is synthesized in a series of steps which occur in the [[mitochondria]] and the [[cytosol]] of the immature red blood cell, while the [[globin]] protein portions of the molecule are synthesized by ribosomes in the cytosol.<ref>{{cite web |url=http://sickle.bwh.harvard.edu/hbsynthesis.html |title=Hemoglobin Synthesis|date=14 |year=2002 |month=April |work= }}</ref> 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 [[Cell nucleus|nucleus]] is lost in mammalian red blood cells, but not in birds and many other species. Even after the loss of the nucleus in mammals, residual [[ribosomal RNA]] allows further synthesis of Hb until the reticulocyte loses its RNA soon after entering the [[Circulatory system|vasculature]] (this hemoglobin-synthetic RNA in fact gives the reticulocyte its reticulated appearance and name). 

The chemical empirical formula of the most common human hemoglobin is  C<sub>738</sub>H<sub>1166</sub>N<sub>812</sub>O<sub>203</sub>S<sub>2</sub>Fe, but as noted above, hemoglobins vary widely across species, and even (through common mutations) slightly among subgroups of humans.

== Structure ==
[[Image:Heme svg.png|left|framed|Heme group]]

In most 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]].<ref name="Steinberg_2001_95">{{harvnb|Steinberg|2001|p=95}}.</ref><ref name="Hardison_1996_1">{{harvnb|Hardison|1996|p=1}}.</ref> This folding pattern contains a pocket which strongly binds the heme group.

A heme group consists of an iron (Fe) ion (charged atom) held in a [[heterocyclic compound|heterocyclic]] ring, known as a [[porphyrin]]. The iron ion, which is the site of oxygen binding, bonds with the four [[nitrogen]]s 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 the F8 [[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 ion may either be in the Fe<sup>2+</sup> or Fe<sup>3+</sup> state, but ferrihemoglobin ([[methemoglobin]]) (Fe<sup>3+</sup>) cannot bind oxygen.<ref>{{cite journal |author=Linberg R, Conover CD, Shum KL, Shorr RG |title=Hemoglobin based oxygen carriers: how much methemoglobin is too much? |journal=Artif Cells Blood Substit Immobil Biotechnol |volume=26 |issue=2 |pages=133–48 |year=1998 |pmid=9564432}}</ref> In binding, oxygen temporarily oxidizes Fe to (Fe<sup>3+</sup>), so iron must exist in the +2 oxidation state in order to bind oxygen. The enzyme [[methemoglobin reductase]] reactivates hemoglobin found in the inactive (Fe<sup>3+</sup>) 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 α<sub>2</sub>β<sub>2</sub>. The subunits are structurally similar and about the same size. Each subunit has a molecular weight of about 17,000&nbsp;daltons, for a total [[molecular weight]] of the tetramer of about 68,000&nbsp;daltons. Hemoglobin A is the most intensively studied of the hemoglobin molecules.

The four [[polypeptide chains]] are bound to each other by [[salt bridge (protein)|salt bridge]]s, [[hydrogen bond]]s, and [[hydrophobe|hydrophobic interaction]]s. There are two kinds of contacts between the α and β chains: α<sub>1</sub>β<sub>1</sub> and α<sub>1</sub>β<sub>2</sub>.

''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 [[adenosine triphosphate|ATP]] by the process of [[oxidative phosphorylation]]. It does not, however, help to counteract a decrease in blood pH. [[Ventilation (physiology)|Ventilation]], or breathing, may reverse this condition by removal of [[carbon dioxide]], thus causing a shift up in pH.<ref name="02_calc">{{cite web | author=Baillie/Simpson | title=Online model of the haemoglobin binding and the effects of hyperventilation | url=http://www.altitude.org/calculators/saturationgraph/saturationgraph.htm}}</ref>

''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&nbsp;nm wavelength than deoxyhemoglobin, while at 940&nbsp;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 E<sub>g</sub> antibonding orbitals. Iron(III) has an odd number of electrons and thus 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:

# Low-spin Fe<sup>2+</sup> binds to high-energy singlet oxygen. Both low-spin iron and singlet oxygen are diamagnetic.
# Low-spin Fe<sup>3+</sup> binds to '''.'''O<sub>2</sub><sup>-</sup> (the [[superoxide]] ion) and the two unpaired electrons couple antiferromagnetically, giving diamagnetic properties.
# Low-spin Fe<sup>4+</sup> binds to O<sub>2</sub><sup>2-</sup>. Both are diamagnetic.

X-ray photoelectron spectroscopy suggests 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 '''.'''O<sub>2</sub><sup>-</sup> to "hold" the oxidation electron, is incapable of binding O<sub>2</sub>). 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==

[[Image:Hemoglobin t-r state ani.gif|thumb|250px|left|Another schematic visual model of the same process above, showing all four tetramers and hemes, and protein chains only as diagramatic coils, to facilitate visualization into the molecule. Oxygen is not shown in this model, but binds to the flat heme, as shown in green in the previous model.]]

[[Image:Hb-animation2.gif|thumb|180px|left|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 illustrated 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 to the iron. This causes a tug on the peptide strand which tends to open up heme units in the remainder of the molecule, so that there is more room for oxygen molecules to bind at their heme sites.  

In the tetrameric form of normal adult hemoglobin, the binding of oxygen is thus a [[cooperative binding|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 effects|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 [[Sigmoid function|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 ''di''oxide 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 [[proton]]s:

:CO<sub>2</sub> + H<sub>2</sub>O → H<sub>2</sub>CO<sub>3</sub> → HCO<sub>3</sub><sup>-</sup> + H<sup>+</sup>

[[Image:Hb saturation curve.png|thumb|180px|left|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 [[acid]]ic). 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 reduce 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<sup>-</sup>),  [[sulfur monoxide]] (SO)<!-- what?? -->, [[nitrogen dioxide]] (NO<sub>2</sub>), and [[sulfide]] (S<sup>2-</sup>), including [[hydrogen sulfide]] (H<sub>2</sub>S). 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 (Fe<sup>2+</sup>) oxidation state to support oxygen and other gases' binding and transport. Oxidation to the ferric (Fe<sup>3+</sup>) state converts hemoglobin into hemiglobin or [[methaemoglobin]] (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, α<sub>2</sub>γ<sub>2</sub>), 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 cysteine 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 [[Anion Exchanger 1|AE1]] out of them.<ref>{{cite book |last=Rang |first=H.P. |coauthors=Dale M.M., Ritter J.M., Moore P.K. |title=Pharmacology, Fifth Edition |year=2003 |publisher=Elsevier |isbn=04430727027 }}</ref>

==Types of hemoglobins in humans==
Hemoglobin variants are a part of the normal [[human embryonic development|embryonic]] and [[human fetal development|fetal]] development, but may also be pathologic mutant forms of [[hemoglobin]] in a [[population]] (usually of humans), caused by variations in genetics. Some well-known hemoglobin such variants such as [[sickle-cell anemia]] are responsible for diseases, and are considered hemoglobinopathies. Other variants cause no detectable [[pathology]], and are thus considered non-pathological variants.<ref>[http://www.labtestsonline.org/understanding/analytes/hemoglobin_var/glance-3.html Understanding hemoglobin variants]</ref><ref>[http://globin.cse.psu.edu/html/huisman/variants/ A syllabus of hemoglobin variants]</ref>  

In the  [[embryo]]:
* Gower 1 (ζ<sub>2</sub>ε<sub>2</sub>) 
* Gower 2 (α<sub>2</sub>ε<sub>2</sub>) ({{PDB|1A9W}})
* Hemoglobin Portland (ζ<sub>2</sub>γ<sub>2</sub>)

In the [[foetus]]:
* [[Hemoglobin F]] (α<sub>2</sub>γ<sub>2</sub>) ({{PDB|1FDH}}) 

In adults:
* Hemoglobin A (α<sub>2</sub>β<sub>2</sub>) ({{PDB|1BZ0}}) - The most common with a normal amount over 95%
* Hemoglobin A<sub>2</sub> (α<sub>2</sub>δ<sub>2</sub>) - δ chain synthesis begins late in the third trimester and in adults, it has a normal range of 1.5-3.5%
* [[Hemoglobin F]] (α<sub>2</sub>γ<sub>2</sub>) - In adults Hemoglobin F is restricted to a limited population of red cells called F-cells. However, the level of Hb F can be elevated in persons with sickle-cell disease.

Variant forms which cause disease:
* Hemoglobin S (α<sub>2</sub>β<sup>S</sup><sub>2</sub>) - 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]] (α<sub>2</sub>β<sup>C</sup><sub>2</sub>) - 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 cell]]s 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 [http://hyper.ahajournals.org/cgi/content/full/25/2/166]; 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 [[kidney]]s, 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 (medicine)|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 [[hemoglobinopathy|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 ''[[porphyria]]s'' 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 the terminal valine (an alpha aminoacid) of each β chain.  The resulting molecule is often referred to as [[HbA1c|Hb A<sub>1c</sub>]].  As the concentration of glucose in the blood increases, the percentage of Hb A that turns into Hb A<sub>1c</sub> increases.  In [[diabetes mellitus|diabetics]] whose glucose usually runs high, the percent Hb A<sub>1c</sub> also runs high.  Because of the slow rate of Hb A combination with glucose, the Hb A<sub>1c</sub> 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 test]]s, 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 <ref>[http://ibdcrohns.about.com/od/diagnostictesting/p/testhemo.htm]</ref>
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 hemoglobin is the typical test used for [[blood donation]].  A comparison with the [[hematocrit]] can be made by multiplying the hemoglobin by three.  For example, if the hemoglobin is measured at 17, that compares with a hematocrit of .51.<ref>{{cite web |url=http://www.doctorslounge.com/hematology/labs/hematocrit.htm |title=Hematocrit (HCT) or Packed Cell Volume (PCV)|work=DoctorsLounge.com }}</ref>

[[Blood sugar|Blood glucose levels]] can vary widely throughout a day, so one or only a few samples from a patient analyzed for glucose may not be representative of long-term control of glucose levels. For this reason a blood sample may be analyzed for Hb A<sub>1c</sub> 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<sub>1c</sub> runs 6.0% or less show good longer-term glucose control.  Hb A<sub>1c</sub> values which are more than 7.0% are elevated.  This test is especially useful for diabetics.<ref>This Hb A<sub>1c</sub> level is only useful in individuals who have red blood cells (RBCs) with normal survivals (i.e., normal half-life). In individuals with abnormal RBCs, whether due to abnormal hemoglobin molecules (such as Hemoglobin S in Sickle Cell Anemia) or RBC membrane defects - or other problems, the RBC half-life is frequently shortened.   In these individuals an alternative test called "fructosamine level" can be used.  It measures the degree of glycation (glucose binding) to albumin, the most common blood protein, and reflects average blood glucose levels over the previous 18-21 days, which is the half-life of albumin molecules in the circulation.</ref>

===In radiologic imaging: fMRI and its uses with hemoglobin===

The [[FMRI]] machine may use the signal from oxyhemoglobin as it partially aligns these molecules with the magnetic field. The machine will then send a series of magnetic pulses at the participant's head or other body structure, slowly knocking the molecules out of alignment, and a radio wave is emitted when they come back into alignment. The fMRI machine can then pick up these signals and use them to make scans, which are cross-sectional maps showing blood flow.

==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]], [[protozoa]]ns and [[fungi]] all have hemoglobin-like proteins whose known and predicted roles include the reversible binding of gaseous [[ligand]]s. Since many of these proteins contain globins, and also the heme [[Functional group|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, CO<sub>2</sub>, sulfide compounds, and even O<sub>2</sub> 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.<ref>{{cite journal |author=Weber RE, Vinogradov SN |title=Nonvertebrate hemoglobins: functions and molecular adaptations |journal=Physiol. Rev. |volume=81 |issue=2 |pages=569-628 |year=2001 |pmid=11274340 |doi=}}</ref> 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 H<sub>2</sub>S from the vent and O<sub>2</sub> from the water to produce energy to make food from H<sub>2</sub>O and CO<sub>2</sub>. These organisms end with a deep red fan-like structure ("plume") which extends into the water and which absorbs H<sub>2</sub>S and O<sub>2</sub> for the bacteria, and also absorbs CO<sub>2</sub> 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.<ref>{{cite journal |author=Zal F, Lallier FH, Green BN, Vinogradov SN, Toulmond A |title=The multi-hemoglobin system of the hydrothermal vent tube worm Riftia pachyptila. II. Complete polypeptide chain composition investigated by maximum entropy analysis of mass spectra |journal=J. Biol. Chem. |volume=271 |issue=15 |pages=8875-81 |year=1996 |pmid=8621529 |doi=}}</ref><ref>{{cite journal |author=Minic Z, Hervé G |title=Biochemical and enzymological aspects of the symbiosis between the deep-sea tubeworm Riftia pachyptila and its bacterial endosymbiont |journal=Eur. J. Biochem. |volume=271 |issue=15 |pages=3093-102 |year=2004 |pmid=15265029 |doi=10.1111/j.1432-1033.2004.04248.x}}</ref>

== 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 mollusc ''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 history and art==
[[Image:Mars Hubble.jpg|thumb|left|150px|The planet Mars]]
[[Image:Heart of Steel (Hemoglobin).jpg|thumb|right|150px|''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 months of exposure to the elements.]]

Historically, an association between the color of blood and rust occurs in the association of the planet Mars, with the Roman god of war, since the planet is an orange-red which reminded the ancients of blood. Although the color of the planet is due to iron compounds in combination with oxygen in the Martian soil, it is a common misconception that the iron in hemoglobin and its oxides gives blood its red color. The color is actually due to the [[porphyrin]] [[Functional group|moiety]] of hemoglobin to which the iron is bound, not the iron itself,<ref>{{cite book |title=Pharmacy Practice Manual: A Guide to the Clinical Experience |last=Boh |first=Larry |year=2001 |publisher=Lippincott Williams & Wilkins |isbn=0781725410 }}</ref> although the ligation and redox state of the iron can influence the pi to pi* electronic transitions of the porphyrin and hence its optical characteristics.

Artist Julian Voss-Andreae created a sculpture called "Heart of Steel (Hemoglobin)" in 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.<ref>{{cite journal | first = Constance | last = Holden| year = 2005 | month = 30 September | title = Blood and Steel | journal = Science | volume = 309 | pages = 2160 | url = http://www.sciencemag.org/cgi/reprint/309/5744/2160d?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&fulltext=blood&searchid=1&FIRSTINDEX=0&issue=5744&resourcetype=HWCIT.pdf}}</ref>

== See also ==
* [[Chlorophyll]]
* [[Globin fold]]
* [[Hemocyanin]]
* [[Hemoprotein]]
* [[Sickle-cell disease]]

Hemoglobin variants:
* [[HbA1c|Hb A<sub>1C</sub>]]
* [[Hemoglobin A2]]
* [[Hemoglobin C]]
* [[Hemoglobin F]]

Hemoglobin protein subunits (genes):
* [[HBA1|Alpha globin 1]]
* [[HBB|Beta globin]]
* [[HBD|Delta globin]]

== Citations ==

{{reflist|2}}

== References ==
{{col-begin}} 
{{col-2}}
<div class="references-small">
* {{Harvard reference
 | last1     = Campbell
 | first1    = MK
 | year      = 1999
 | title     = Biochemistry (Third Edition)
 | publisher = Harcourt
 | id        = ISBN 0-03-024426-9
}}.
* {{Harvard reference
 | last1      = Eshaghian
 | first1     = S
 | last2      = Horwich
 | first2     = TB
 | last3      = Fonarow 
 | first3     = GC
 | year       = 2006
 | title      = An unexpected inverse relationship between HbA1c levels and mortality in patients with diabetes and advanced systolic heart failure
 | periodical = Am Heart J
 | volume     = 151(1):91
 | date       = January 2006
}}. PMID 16368297.
* {{harvard reference
 | last1     = Ganong
 | first1    = WF
 | year      = 2003
 | title     = Review of Medical Physiology (Twenty-First Edition)
 | publisher = Lange
 | id        = ISBN 0-07-140236-5
}}.
* {{harvard reference
 | last1     = Hager
 | first1    = T
 | year      = 1995
 | title     = Force of Nature: The Life of Linus Pauling
 | publisher = Simon and Schuster
 | id        = ISBN 0-684-80909-5
}}.
</div>
{{col-2}}
<div class="references-small">
* {{Harvard reference
 | last1      = Hardison
 | first1     = RC
 | Year       = 1996
 | Title      = A brief history of hemoglobins: plant, animal, protist, and bacteria
 | Periodical = Proc Natl Acad Sci USA
 | Date       = June 11, 1996 
 | URL        = http://www.pubmedcentral.gov/articlerender.fcgi?tool=pubmed&pubmedid=8650150
}}. PMID 8650150.
* {{Harvard reference
 | last1      = Kneipp
 | first1     = J
 | last2      = Balakrishnan
 | first2     = G
 | last3      = Chen
 | first3     = R, Shen TJ, Sahu SC, Ho NT, Giovannelli JL, Simplaceanu V, Ho C, Spiro TG
 | year       = 2005
 | title      = Dynamics of allostery in hemoglobin: roles of the penultimate tyrosine H bonds
 | periodical = J Mol Biol
 | date       = November 22, 2005 
}}. PMID 16368110.
* {{Harvard reference
 | last1     = Steinberg
 | first1    = MH
 | year      = 2001
 | title     = Disorders of Hemoglobin: Genetics, Pathophysiology, and Clinical Management 
 | publisher = Cambridge University Press
 | url       = http://books.google.com/books?vid=ISBN0521632668
 | id        = ISBN 0-521-63266-8
}}.
</div>
{{col-end}}

==External links==
* [http://www.ufp.pt/~pedros/anim/2frame-hben.htm Interactive models of hemoglobin] (Requires [http://www.mdl.com/products/framework/chime/ MDL Chime])
* [http://www.altitude.org/calculators/saturationgraph/saturationgraph.htm Interactive hemoglobin saturation curves]
*[http://www.anemia.org/ National Anemia Action Council] - anemia.org
* [http://www.clinicalguard.co.uk/pulse-oximetry-glossary-a-3.html Pulse Oximetry Glossary]

{{Respiratory physiology}}


[[Category:Hematology]]
[[Category:Hemoproteins]]
[[Category:Physiology]]
{{WikiDoc Help Menu}}
{{WikiDoc Sources}}

[[ar:خضاب الدم]]
[[bn:হিমোগ্লোবিন]]
[[bs:Hemoglobin]]
[[bg:Хемоглобин]]
[[ca:Hemoglobina]]
[[cs:Hemoglobin]]
[[da:Hæmoglobin]]
[[de:Hämoglobin]]
[[dv:ހިމަގްލޯބިން]]
[[et:Hemoglobiin]]
[[es:Hemoglobina]]
[[eo:Hemoglobino]]
[[fr:Hémoglobine]]
[[ko:헤모글로빈]]
[[hr:Hemoglobin]]
[[id:Hemoglobin]]
[[ia:Hemoglobina]]
[[it:Emoglobina]]
[[he:המוגלובין]]
[[lt:Hemoglobinas]]
[[mk:Хемоглобин]]
[[ms:Hemoglobin]]
[[nl:Hemoglobine]]
[[ja:ヘモグロビン]]
[[no:Hemoglobin]]
[[nn:Hemoglobin]]
[[pl:Hemoglobina]]
[[pt:Hemoglobina]]
[[ru:Гемоглобин]]
[[sq:Hemoglobina]]
[[simple:Hemoglobin]]
[[sr:Хемоглобин]]
[[sh:Hemoglobin]]
[[su:Hémoglobin]]
[[fi:Hemoglobiini]]
[[sv:Hemoglobin]]
[[vi:Hemoglobin]]
[[tg:Ҳемоглобин]]
[[tr:Hemoglobin]]
[[ur:حمریچہ]]
[[zh:血红蛋白]]