Editing Hemoglobin (section)

===Oxygen saturation===
In general, hemoglobin can be saturated with oxygen molecules (oxyhemoglobin), or desaturated with oxygen molecules (deoxyhemoglobin).<ref>[https://www.bio.davidson.edu/Courses/Molbio/MolStudents/spring2005/Heiner/hemoglobin.html "Hemoglobin Home."] {{Webarchive|url=https://web.archive.org/web/20091201035525/https://www.bio.davidson.edu/Courses/Molbio/MolStudents/spring2005/Heiner/hemoglobin.html |date=2009-12-01}} Biology @ Davidson. Web. 12 Oct. 2009.</ref>

====Oxyhemoglobin====
''Oxyhemoglobin'' is formed during [[respiration (physiology)|physiological 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 [[pulmonary alveolus|alveoli]] of the lungs. The oxygen then travels through the blood stream to be dropped off at cells where it is utilized as a terminal electron acceptor 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="altitude.org">{{cite web |publisher=altitude.org |title=Hemoglobin saturation graph |url=https://www.altitude.org/hemoglobin_saturation.php |access-date=2010-07-06 |url-status=dead |archive-url=https://web.archive.org/web/20100831024941/https://www.altitude.org/hemoglobin_saturation.php |archive-date=2010-08-31}}</ref>

Hemoglobin exists in two forms, a ''taut (tense) form'' (T) and a ''relaxed form'' (R). Various factors such as low pH, high CO<sub>2</sub> and high [[2,3 BPG]] at the level of the tissues favor the taut form, which has low oxygen affinity and releases oxygen in the tissues. Conversely, a high pH, low CO<sub>2</sub>, or low 2,3 BPG favors the relaxed form, which can better bind oxygen.<ref name="King-2012">{{cite web |author=King, Michael W |title=The Medical Biochemistry Page – Hemoglobin |url=https://themedicalbiochemistrypage.org/hemoglobin-myoglobin.php#hemoglobin |access-date=2012-03-20 |url-status=live |archive-url=https://web.archive.org/web/20120304095702/https://themedicalbiochemistrypage.org/hemoglobin-myoglobin.php#hemoglobin |archive-date=2012-03-04}}</ref> The partial pressure of the system also affects O<sub>2</sub> affinity where, at high partial pressures of oxygen (such as those present in the alveoli), the relaxed (high affinity, R) state is favoured. Inversely, at low partial pressures (such as those present in respiring tissues), the (low affinity, T) tense state is favoured.<ref>Voet, D. (2008) ''Fundamentals of Biochemistry'', 3rd. ed., Fig. 07_06, John Wiley & Sons. {{ISBN|0470129301}}</ref> Additionally, the binding of oxygen to the iron(II) heme pulls the iron into the plane of the porphyrin ring, causing a slight conformational shift. The shift encourages oxygen to bind to the three remaining heme units within hemoglobin (thus, oxygen binding is cooperative).{{citation needed|date=November 2023}}

Classically, the iron in oxyhemoglobin is seen as existing in the iron(II) oxidation state. However, the complex of oxygen with heme iron is [[diamagnetic]], whereas both oxygen and high-spin iron(II) are [[paramagnetic]]. Experimental evidence strongly suggests heme iron is in the iron(III) oxidation state in oxyhemoglobin, with the oxygen existing as [[superoxide anion]] (O<sub>2</sub><sup>•−</sup>) or in a covalent charge-transfer complex.<ref name="Shikama-2006">{{cite journal |vauthors=Shikama K |title=Nature of the FeO2 bonding in myoglobin and hemoglobin: A new molecular paradigm |journal=Prog Biophys Mol Biol |volume=91 |issue=1–2 |pages=83–162 |year=2006 |pmid=16005052 |doi=10.1016/j.pbiomolbio.2005.04.001 |doi-access=free}}</ref>

====Deoxygenated hemoglobin====
Deoxygenated hemoglobin (deoxyhemoglobin) is the form of hemoglobin without the bound oxygen. The [[absorption spectrum|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 the measurement of the amount of oxygen in a patient's blood by an instrument called a [[pulse oximeter]]. This difference also accounts for the presentation of [[cyanosis]], the blue to purplish color that tissues develop during [[hypoxia (medical)|hypoxia]].<ref>{{cite book |last1=Ahrens |last2=Kimberley |first2=Basham |title=Essentials of Oxygenation: Implication for Clinical Practice |year=1993 |publisher=Jones & Bartlett Learning |page=194 |isbn=978-0-86720-332-5}}</ref>

Deoxygenated hemoglobin is [[paramagnetic]]; it is weakly attracted to [[magnetic field]]s.<ref>
{{cite journal
|year=1993
|last1=Ogawa |first1=S
|title=Functional brain mapping by blood oxygenation level-dependent contrast magnetic resonance imaging. A comparison of signal characteristics with a biophysical model
|journal=Biophysical Journal
|volume=64
|issue=3
|pages=803–12
|last2=Menon |first2=R. S.
|last3=Tank |first3=D. W.
|last4=Kim |first4=S. G.
|last5=Merkle |first5=H
|last6=Ellermann |first6=J. M.
|last7=Ugurbil |first7=K
|bibcode=1993BpJ....64..803O
|pmid=8386018
|doi=10.1016/S0006-3495(93)81441-3
|pmc=1262394
}}</ref><ref name="Bren-2015"/> In contrast, oxygenated hemoglobin exhibits [[diamagnetism]], a weak repulsion from a magnetic field.<ref name="Bren-2015">{{cite journal |vauthors=Bren KL, Eisenberg R, Gray HB |title=Discovery of the magnetic behavior of hemoglobin: A beginning of bioinorganic chemistry |journal=Proc Natl Acad Sci U S A |year=2015 |volume=112 |issue=43 |pages=13123–27 |bibcode=2015PNAS..11213123B |pmid=26508205 |doi=10.1073/pnas.1515704112 |doi-access=free |pmc=4629386}}</ref>