Respiratory pigment

A respiratory pigment is a metalloprotein that serves a variety of important functions, its main being O2 transport.[1] Other functions performed include O2 storage, CO2 transport, and transportation of substances other than respiratory gases. There are four major classifications of respiratory pigment: hemoglobin, hemocyanin, erythrocruorin–chlorocruorin, and hemerythrin. The heme-containing globin[lower-alpha 1] is the most commonly-occurring respiratory pigment, occurring in at least 9 different phyla of animals.[2]

Comparing Respiratory Pigments
Metalloprotein Globins Hemocyanin Hemerythrin
Hemoglobin[lower-alpha 2] Erythrocruorin and chlorocruorin
O2 Binding Material Iron[3] Iron[4] Copper[3] Iron[3]
Location Intracellular[2] Extracellular[5] Extracellular[2] Intracellular[2]
Source Organism Almost all vertebrates[2]
  • Annelids and arthropods
  • Chlorocruorin: 4 families of marine polychaetes[6]
 Arthropoda and Mollusca[2]

Sipuncula, priapulida, some brachiopoda, and a single annelid genus [3]

Oxygenated Color Bright red[1]
  • Erythrocruorin: Bright red
  • Chlorocruorin: Green when diluted, red when concentrated [5]
 Blue[1] Violet[1]
Deoxygenated Color Crimson[1]
  • Erythrocruorin: Dark red
  • Chlorocruorin: Green when diluted, brown-red when concentrated
 Colorless[1] Colorless[1]

Hemoglobin, erythrocruorin, and chlorocruorin are all globins, iron-heme proteins with a common core. Their color comes from the absorption spectra of heme with Fe2+. Erythrocruorin and chlorocruorin are closely related giant globins found used by some invertebrates. Chlorocruorin has a special heme group, giving it different colors.

Any of various coloured conjugated proteins, such as hemoglobin, occur in living organisms and function in oxygen transfer in cellular respiration.

Globins

The globin is thought to be a very ancient molecule, even acting as a molecular clock of sorts. It has even been used to date the separation of vertebrates and invertebrates more than 1 billion years ago. Globin enjoys a large biological distribution, not only occurring among more than 9 different phyla of animals but occurring in some fungi and bacteria as well, even being identified in nitrogen-fixing nodules on the roots of some leguminous plants. The isolation of the globin gene from plant root cells has suggested that the globin genes that were inherited from a common ancestor shared by plants and animals may be present in all plants.[7]

Vertebrate hemoglobin

Vertebrates use a tetrameric hemoglobin, carried in red blood cells, to breathe. There are multiple types of hemoglobin that have been found in the human body alone. Hemoglobin A is the “normal” hemoglobin, the variant of hemoglobin that is most common after birth. Hemoglobin A2 is a minor component of hemoglobin found in red blood cells. Hemoglobin A2 makes up less than 3% of total red blood cell hemoglobin. Hemoglobin F typically is only found in the fetal stage of development. While Hemoglobin F falls dramatically after birth, it is possible for some people to produce some levels of Hemoglobin F throughout their full life.[8]

Other animal hemoglobins
Further information: Hemoglobin § Analogues in non-vertebrate organisms

Animals use a great variety of globins for respiration. By structure, they can be classified as:[9]: Fig. 1 

  • Intracellular Hbs. These globins reside inside a cell, much like the vertebrate Hb.
  • Multi-subunit Hbs. These globins form complexes and work outside a cell.
  • Multi-domain, multisubunit Hbs. These globins form complexes, work outside a cell, and have multiple globin domains per peptide chain.

Erythrocruorin and chlorocruorin belong to the multisubunit Hbs, specifically of the 12-dodecamer type.

Leghemoglobin
See also: phytoglobin

Leghemoglobin is a molecular similar in structure to myoglobin that is currently being used in artificial meat products, such as the Impossible Burger, to simulate both the color and taste of meat.[10] Similar in function to hemoglobin, leghemoglobin contains trace amounts of iron, but it is primarily found in plant roots.[11]

Hemocyanin

Hemocyanin is a respiratory pigment that uses copper as its oxygen-binding molecule, as opposed to iron with hemoglobin. Hemocyanin is found in both arthropods and Mollusca, however it is thought that the molecule independently evolved in both phyla. There are several other molecules that exist in arthropods and Mollusca that are similar in structure to hemocyanin but serve entirely different purposes. For example, there are copper-containing tyrosinases that play significant roles in immune defense, wound healing, and the arthropod's cuticle. Molecules similar to hemocyanin in structure are grouped in under the hemocyanin superfamily.[12]

Notes
  1. also known as "hemoglobin", in a lax sense
  2. in the strict sense, for the tetrameric form

References
  1. Urich, Klaus (1994), Urich, Klaus (ed.), "Respiratory Pigments", Comparative Animal Biochemistry, Berlin, Heidelberg: Springer, pp. 249–287, doi:10.1007/978-3-662-06303-3_7, ISBN 978-3-662-06303-3, retrieved 2020-11-21
  2. Hill, Richard W.; Wyse, Gordon A.; Anderson, Margaret (5 October 2017). Transport of Oxygen and Carbon Dioxide in Body Fluids (with an Introduction to Acid–Base Physiology). ISBN 978-1605357379. Retrieved 10 November 2020. {{cite book}}: |website= ignored (help)
  3. Lamy, Jean; Truchot, J.-P; Gilles, R; International Union of Biological Sciences; Section of Comparative Physiology and Biochemistry; International Congress of Comparative Physiology and Biochemistry, eds. (1985). Respiratory pigments in animals: relation, structure-function. Berlin; New York: Springer-Verlag. ISBN 978-0-387-15629-3. OCLC 12558726.
  4. Fox, H. Munro (1949). "On Chlorocruorin and Haemoglobin". Proceedings of the Royal Society of London. Series B, Biological Sciences. 136 (884): 378–388. Bibcode:1949RSPSB.136..378F. doi:10.1098/rspb.1949.0031. ISSN 0080-4649. JSTOR 82565. PMID 18143368. S2CID 6133526.
  5. Fox, Harold Munro; Gardiner, John Stanley (1932-09-01). "The oxygen affinity of chlorocruorin". Proceedings of the Royal Society of London. Series B, Containing Papers of a Biological Character. 111 (772): 356–363. doi:10.1098/rspb.1932.0060.
  6. Imai, Kiyohiro; Yoshikawa, Shinya (1985). "Oxygen-binding characteristics of Potamilla chlorocruorin". European Journal of Biochemistry. 147 (3): 453–463. doi:10.1111/j.0014-2956.1985.00453.x. ISSN 1432-1033. PMID 3979380.
  7. Glomski, Chester; Tamburlin, Judith (1989). "The phylogenetic odyssey of the erythrocyte. I Hemoglobin: the universal respiratory pigment" (PDF). Histol Histopath. 4 (4): 509–514. PMID 2520483.
  8. "Hemoglobinopathies". sickle.bwh.harvard.edu. Retrieved 2020-11-21.
  9. Weber RE, Vinogradov SN (April 2001). "Nonvertebrate Hemoglobins: Functions and Molecular Adaptations". Physiological Reviews. 81 (2): 569–628. doi:10.1152/physrev.2001.81.2.569. PMID 11274340. S2CID 10863037.
  10. Lee, Hyun Jung; Yong, Hae In; Kim, Minsu; Choi, Yun-Sang; Jo, Cheorun (October 2020). "Status of meat alternatives and their potential role in the future meat market — A review". Asian-Australasian Journal of Animal Sciences. 33 (10): 1533–1543. doi:10.5713/ajas.20.0419. ISSN 1011-2367. PMC 7463075. PMID 32819080.
  11. Seehafer, A., & Bartels, M. (2019). Meat 2.0 the regulatory environment of plant-based and cultured meat. European Food and Feed Law Review (EFFL), 14(4),323-331.
  12. Burmester, Thorsten (2001-02-01). "Molecular Evolution of the Arthropod Hemocyanin Superfamily". Molecular Biology and Evolution. 18 (2): 184–195. doi:10.1093/oxfordjournals.molbev.a003792. ISSN 0737-4038. PMID 11158377.

Further reading
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