A respiratory pigment is a
metalloprotein
Metalloprotein is a generic term for a protein that contains a metal ion Cofactor (biochemistry), cofactor. A large proportion of all proteins are part of this category. For instance, at least 1000 human proteins (out of ~20,000) contain zinc-bi ...
that serves a variety of important functions, its main being O
2 transport.
Other functions performed include O
2 storage, CO
2 transport, and transportation of substances other than respiratory gases. There are four major classifications of respiratory pigment:
hemoglobin
Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythrocyte ...
,
hemocyanin
Hemocyanins (also spelled haemocyanins and abbreviated Hc) are proteins that transport oxygen throughout the bodies of some invertebrate animals. These metalloproteins contain two copper atoms that reversibly bind a single oxygen molecule (O2) ...
, erythrocruorin–
chlorocruorin, and
hemerythrin
Hemerythrin (also spelled haemerythrin; grc, αἷμα, haîma, blood, grc, ἐρυθρός, erythrós, red) is an oligomeric protein responsible for oxygen (O2) transport in the marine invertebrate phyla of sipunculids, priapulids, brachiop ...
. The heme-containing globin is the most commonly-occurring respiratory pigment, occurring in at least 9 different phyla of animals.
Comparing Respiratory Pigments
Hemoglobin,
erythrocruorin
Erythrocruorin (from Greek ''eruthros'' "red" + Latin ''cruor'' "blood"), and the similar chlorocruorin (from Greek ''khlōros'' "green" + Latin ''cruor'' "blood"), are large oxygen-carrying hemeprotein complexes, which have a molecular mass gr ...
, and chlorocruorin are all
globin
The globins are a superfamily of heme-containing globular proteins, involved in binding and/or transporting oxygen. These proteins all incorporate the globin fold, a series of eight alpha helical segments. Two prominent members include myogl ...
s, iron-heme proteins with a common core. Their color comes from the absorption spectra of
heme
Heme, or haem (pronounced / hi:m/ ), is a precursor to hemoglobin, which is necessary to bind oxygen in the bloodstream. Heme is biosynthesized in both the bone marrow and the liver.
In biochemical terms, heme is a coordination complex "consisti ...
with Fe
2+. Erythrocruorin and chlorocruorin are closely related giant globins found used by some invertebrates. Chlorocruorin has a special heme group, giving it different colors.
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.
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.
Other animal hemoglobins
Animals use a great variety of globins for respiration. By structure, they can be classified as:
* 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
Leghemoglobin
3rd Leghemoglobin (also leghaemoglobin or legoglobin) is an oxygen-carrying phytoglobin found in the nitrogen-fixing root nodules of leguminous plants. It is produced by these plants in response to the roots being colonized by nitrogen-fixing b ...
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 of meat and taste. Similar in function to hemoglobin, leghemoglobin contains trace amounts of iron, but it is primarily found in plant roots.
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
arthropod
Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...
s and
Mollusca
Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is esti ...
, 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.
Notes
References
Further reading
*
External links
* Gene Ontology –
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