Carbaminohemoglobin

Carbaminohemoglobin (carbaminohaemoglobin BrE) (CO2Hb, also known as carbhemoglobin and carbohemoglobin) is a compound of hemoglobin and carbon dioxide, and is one of the forms in which carbon dioxide exists in the blood.[1] Twenty-three percent of carbon dioxide is carried in blood this way (70% is converted into bicarbonate by carbonic anhydrase and then carried in plasma, 7% carried as free CO2, dissolved in plasma).[2]

Synthesis

When the tissues release carbon dioxide into the bloodstream, around 10% is dissolved into the plasma. The rest of the carbon dioxide is carried either directly or indirectly by hemoglobin. Approximately 10% of the carbon dioxide carried by hemoglobin is in the form of carbaminohemoglobin. This carbaminohemoglobin is formed by the reaction between carbon dioxide and an amino (-NH2) residue from the globin molecule, resulting in the formation of a carbamino residue (-NH.COO−). The rest of the carbon dioxide is transported in the plasma as bicarbonate anions.[3]

Mechanism

When carbon dioxide binds to hemoglobin, carbaminohemoglobin is formed, lowering hemoglobin's affinity for oxygen via the Bohr effect. The reaction is formed between a carbon dioxide molecule and an amino residue.[4] In the absence of oxygen, unbound hemoglobin molecules have a greater chance of becoming carbaminohemoglobin. The Haldane effect relates to the increased affinity of de-oxygenated hemoglobin for H+
: offloading of oxygen to the tissues thus results in increased affinity of the hemoglobin for carbon dioxide, and H+
, which the body needs to get rid of, which can then be transported to the lung for removal. Because the formation of this compound generates hydrogen ions, haemoglobin is needed to buffer it.[4]

Hemoglobin can bind to four molecules of carbon dioxide. The carbon dioxide molecules form a carbamate with the four terminal-amine groups of the four protein chains in the deoxy form of the molecule. Thus, one hemoglobin molecule can transport four carbon dioxide molecules back to the lungs, where they are released when the molecule changes back to the oxyhemoglobin form.

Hydrogen ion and oxygen-carbon dioxide coupling

When carbon dioxide diffuses as a dissolved gas from the tissue capillaries, it binds to the α-amino terminus of the globulin chain, forming Carbaminohemoglobin. Carbaminohemoglobin is able to directly stabilise the T conformation as part of the carbon dioxide Bohr effect. Deoxyhemoglobin in turn subsequently increases the uptake of carbon dioxide in the form of favouring the formation of Bicarbonate as well as Carbaminohemoglobin through the Haldane effect.[5]

See also

References
  1. Betts, J Gordon; Desaix, Peter; Johnson, Eddie; Johnson, Jody E; Korol, Oksana; Kruse, Dean; Poe, Brandon; Wise, James; Womble, Mark D; Young, Kelly A (September 13, 2023). Anatomy & Physiology. Houston: OpenStax CNX. 22.5 Transport of gases. ISBN 978-1-947172-04-3.
  2. Gas Transport in the Blood FIG. 18.11 Carbon dioxide transport
  3. Waterhouse, James; Campbell, Iain (2005-11-01). "Respiration: gas transfer". Anaesthesia & Intensive Care Medicine. Thoracic. 6 (11): 363–366. doi:10.1383/anes.2005.6.11.363. ISSN 1472-0299.
  4. Waterhouse, James; Campbell, Iain (2005-11-01). "Respiration: gas transfer". Anaesthesia & Intensive Care Medicine. Thoracic. 6 (11): 363–366. doi:10.1383/anes.2005.6.11.363. ISSN 1472-0299.
  5. Hsia, Connie C.W. (1998-01-22). "Respiratory Function of Hemoglobin". New England Journal of Medicine. 338 (4): 239–248. doi:10.1056/NEJM199801223380407. ISSN 0028-4793. PMID 9435331.


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